The mitotic checkpoint complex binds a second CDC20 to inhibit active APC/C

CCDC68 Maintains Mitotic Checkpoint Activation by Promoting CDC20 Integration into the MCC

The spindle assembly checkpoint (SAC) ensures chromosome segregation fidelity by manipulating unattached kinetochore-dependent assembly of the mitotic checkpoint complex (MCC). The MCC binds to and inhibits the anaphase promoting complex/cyclosome (APC/C) to postpone mitotic exit. However, the mechanism by which unattached kinetochores mediate MCC formation is not yet fully understood. Here, it is shown that CCDC68 is an outer kinetochore protein that preferentially localizes to unattached kinetochores. Furthermore, CCDC68 interacts with the SAC factor CDC20 to inhibit its autoubiquitination and MCC disassembly. Therefore, CCDC68 restrains APC/C activation to ensure a robust SAC and allow sufficient time for chromosome alignment, thus ensuring chromosomal stability. Hence, the study reveals that CCDC68 is required for CDC20-dependent MCC stabilization to maintain mitotic checkpoint activation.

The two sides of chromosomal instability: drivers and brakes in cancer

Chromosomal instability (CIN) is a hallmark of cancer and is associated with tumor cell malignancy. CIN triggers a chain reaction in cells leading to chromosomal abnormalities, including deviations from the normal chromosome number or structural changes in chromosomes. CIN arises from errors in DNA replication and chromosome segregation during cell division, leading to the formation of cells with abnormal number and/or structure of chromosomes. Errors in DNA replication result from abnormal replication licensing as well as replication stress, such as double-strand breaks and stalled replication forks; meanwhile, errors in chromosome segregation stem from defects in chromosome segregation machinery, including centrosome amplification, erroneous microtubule-kinetochore attachments, spindle assembly checkpoint, or defective sister chromatids cohesion. In normal cells, CIN is deleterious and is associated with DNA damage, proteotoxic stress, metabolic alteration, cell cycle arrest, and senescence. Paradoxically, despite these negative consequences, CIN is one of the hallmarks of cancer found in over 90% of solid tumors and in blood cancers. Furthermore, CIN could endow tumors with enhanced adaptation capabilities due to increased intratumor heterogeneity, thereby facilitating adaptive resistance to therapies; however, excessive CIN could induce tumor cells death, leading to the "just-right" model for CIN in tumors. Elucidating the complex nature of CIN is crucial for understanding the dynamics of tumorigenesis and for developing effective anti-tumor treatments. This review provides an overview of causes and consequences of CIN, as well as the paradox of CIN, a phenomenon that continues to perplex researchers. Finally, this review explores the potential of CIN-based anti-tumor therapy.

MAPK-dependent control of mitotic progression in S. pombe

BACKGROUND

Mitogen-activated protein kinases (MAPKs) preserve cell homeostasis by transducing physicochemical fluctuations of the environment into multiple adaptive responses. These responses involve transcriptional rewiring and the regulation of cell cycle transitions, among others. However, how stress conditions impinge mitotic progression is largely unknown. The mitotic checkpoint is a surveillance mechanism that inhibits mitotic exit in situations of defective chromosome capture, thus preventing the generation of aneuploidies. In this study, we investigate the role of MAPK Pmk1 in the regulation of mitotic exit upon stress.

RESULTS

We show that Schizosaccharomyces pombe cells lacking Pmk1, the MAP kinase effector of the cell integrity pathway (CIP), are hypersensitive to microtubule damage and defective in maintaining a metaphase arrest. Epistasis analysis suggests that Pmk1 is involved in maintaining spindle assembly checkpoint (SAC) signaling, and its deletion is additive to the lack of core SAC components such as Mad2 and Mad3. Strikingly, pmk1Δ cells show up to twofold increased levels of the anaphase-promoting complex (APC/C) activator Cdc20Slp1 during unperturbed growth. We demonstrate that Pmk1 physically interacts with Cdc20Slp1 N-terminus through a canonical MAPK docking site. Most important, the Cdc20Slp1 pool is rapidly degraded in stressed cells undergoing mitosis through a mechanism that requires MAPK activity, Mad3, and the proteasome, thus resulting in a delayed mitotic exit.

CONCLUSIONS

Our data reveal a novel function of MAPK in preventing mitotic exit and activation of cytokinesis in response to stress. The regulation of Cdc20Slp1 turnover by MAPK Pmk1 provides a key mechanism by which the timing of mitotic exit can be adjusted relative to environmental conditions.

Chromosome Division in Early Embryos-Is Everything under Control? And Is the Cell Size Important?

Chromosome segregation in female germ cells and early embryonic blastomeres is known to be highly prone to errors. The resulting aneuploidy is therefore the most frequent cause of termination of early development and embryo loss in mammals. And in specific cases, when the aneuploidy is actually compatible with embryonic and fetal development, it leads to severe developmental disorders. The main surveillance mechanism, which is essential for the fidelity of chromosome segregation, is the Spindle Assembly Checkpoint (SAC). And although all eukaryotic cells carry genes required for SAC, it is not clear whether this pathway is active in all cell types, including blastomeres of early embryos. In this review, we will summarize and discuss the recent progress in our understanding of the mechanisms controlling chromosome segregation and how they might work in embryos and mammalian embryos in particular. Our conclusion from the current literature is that the early mammalian embryos show limited capabilities to react to chromosome segregation defects, which might, at least partially, explain the widespread problem of aneuploidy during the early development in mammals.

Functional analysis of Cdc20 reveals a critical role of CRY box in mitotic checkpoint signaling

Accurate mitosis is coordinated by the spindle assembly checkpoint (SAC) through the mitotic checkpoint complex (MCC), which inhibits the anaphase-promoting complex or cyclosome (APC/C). As an essential regulator, Cdc20 promotes mitotic exit through activating APC/C and monitors kinetochore-microtubule attachment through activating SAC. Cdc20 requires multiple interactions with APC/C and MCC subunits to elicit these functions. Functionally assessing these interactions within cells requires efficient depletion of endogenous Cdc20, which is highly difficult to achieve by RNA interference (RNAi). Here we generated Cdc20 RNAi-sensitive cell lines which display a penetrant metaphase arrest by a single RNAi treatment. In this null background, we accurately measured the contribution of each known motif of Cdc20 on APC/C and SAC activation. The CRY box, a previously identified degron, was found critical for SAC by promoting MCC formation and its interaction with APC/C. These data reveal additional regulation within the SAC and establish a novel method to interrogate Cdc20.

An unconventional regulatory circuitry involving Aurora B controls anaphase onset and error-free chromosome segregation in trypanosomes

Accurate chromosome segregation during mitosis requires that all chromosomes establish stable bi-oriented attachments with the spindle apparatus. Kinetochores form the interface between chromosomes and spindle microtubules and as such are under tight control by complex regulatory circuitry. As part of the chromosomal passenger complex (CPC), the Aurora B kinase plays a central role within this circuitry by destabilizing improper kinetochore-microtubule attachments and relaying the attachment status to the spindle assembly checkpoint, a feedback control system that delays the onset of anaphase by inhibiting the anaphase-promoting complex/cyclosome. Intriguingly, Aurora B is conserved even in kinetoplastids, an evolutionarily divergent group of eukaryotes, whose kinetochores are composed of a unique set of structural and regulatory proteins. Kinetoplastids do not have a canonical spindle checkpoint and it remains unclear how their kinetochores are regulated to ensure the fidelity and timing of chromosome segregation. Here, we show in Trypanosoma brucei, the kinetoplastid parasite that causes African sleeping sickness, that inhibition of Aurora B using an analogue-sensitive approach arrests cells in metaphase, with a reduction in properly bi-oriented kinetochores. Aurora B phosphorylates several kinetochore proteins in vitro, including the N-terminal region of the divergent Bub1-like protein KKT14. Depletion of KKT14 partially overrides the cell cycle arrest caused by Aurora B inhibition, while overexpression of a non-phosphorylatable KKT14 protein results in a prominent delay in the metaphase-to-anaphase transition. Finally, we demonstrate using a nanobody-based system that re-targeting the catalytic module of the CPC to the outer kinetochore is sufficient to promote mitotic exit but causes massive chromosome mis-segregation in anaphase. Our results indicate that the CPC and KKT14 are involved in an unconventional pathway controlling mitotic exit and error-free chromosome segregation in trypanosomes.

Heat stress during male meiosis impairs cytoskeletal organization, spindle assembly and tapetum degeneration in wheat

The significance of heat stress in agriculture is ever-increasing with the progress of global climate changes. Due to a negative effect on the yield of staple crops, including wheat, the impairment of plant reproductive development triggered by high ambient temperature became a restraint in food production. Although the heat sensitivity of male meiosis and the following gamete development in wheat has long been recognized, a detailed structural characterization combined with a comprehensive gene expression analysis has not been done about this phenomenon. We demonstrate here that heat stress severely alters the cytoskeletal configuration, triggers the failure of meiotic division in wheat. Moreover, it changes the expression of genes related to gamete development in male meiocytes and the tapetum layer in a genotype-dependent manner. 'Ellvis', a heat-tolerant winter wheat cultivar, showed high spikelet fertility rate and only scarce structural aberrations upon exposure to high temperature. In addition, heat shock genes and genes involved in scavenging reactive oxygen species were significantly upregulated in 'Ellvis', and the expression of meiosis-specific and major developmental genes showed high stability in this cultivar. In the heat-sensitive 'Mv 17-09', however, genes participating in cytoskeletal fiber nucleation, the spindle assembly checkpoint genes, and tapetum-specific developmental regulators were downregulated. These alterations may be related to the decreased cytoskeleton content, frequent micronuclei formation, and the erroneous persistence of the tapetum layer observed in the sensitive genotype. Our results suggest that understanding the heat-sensitive regulation of these gene functions would be an essential contribution to the development of new, heat-tolerant cultivars.

Signaling protein abundance modulates the strength of the spindle assembly checkpoint

During mitosis, unattached kinetochores in a dividing cell signal to the spindle assembly checkpoint (SAC) to delay anaphase onset and prevent chromosome missegregation.1,2,3,4 The signaling activity of these kinetochores and the likelihood of chromosome missegregation depend on the amount of SAC signaling proteins each kinetochore recruits.5,6,7,8 Therefore, factors that control SAC protein recruitment must be thoroughly understood. Phosphoregulation of kinetochore and SAC signaling proteins due to the concerted action of many kinases and phosphatases is a significant determinant of the SAC protein recruitment to signaling kinetochores.9 Whether the abundance of SAC proteins also influences the recruitment and signaling activity of human kinetochores has not been studied.8,10 Here, we reveal that the low cellular abundance of the SAC signaling protein Bub1 limits its own recruitment and that of BubR1 and restricts the SAC signaling activity of the kinetochore. Conversely, Bub1 overexpression results in higher recruitment of SAC proteins, producing longer delays in anaphase onset. We also find that the number of SAC proteins recruited by a signaling kinetochore is inversely correlated with the total number of signaling kinetochores in the cell. This correlation likely arises from the competition among the signaling kinetochores to recruit from a limited pool of signaling proteins, including Bub1. The inverse correlation may allow the dividing cell to prevent a large number of signaling kinetochores in early prophase from generating an overly large signal while enabling the last unaligned kinetochore in late prometaphase to signal at the maximum strength.

Newfound Coding Potential of Transcripts Unveils Missing Members of Human Protein Communities

Recent proteogenomic approaches have led to the discovery that regions of the transcriptome previously annotated as non-coding regions [i.e., untranslated regions (UTRs), open reading frames overlapping annotated coding sequences in a different reading frame, and non-coding RNAs] frequently encode proteins, termed alternative proteins (altProts). This suggests that previously identified protein-protein interaction (PPI) networks are partially incomplete because altProts are not present in conventional protein databases. Here, we used the proteogenomic resource OpenProt and a combined spectrum- and peptide-centric analysis for the re-analysis of a high-throughput human network proteomics dataset, thereby revealing the presence of 261 altProts in the network. We found 19 genes encoding both an annotated (reference) and an alternative protein interacting with each other. Of the 117 altProts encoded by pseudogenes, 38 are direct interactors of reference proteins encoded by their respective parental genes. Finally, we experimentally validate several interactions involving altProts. These data improve the blueprints of the human PPI network and suggest functional roles for hundreds of altProts.

Alternative CDC20 translational isoforms tune mitotic arrest duration

Mitotic defects activate the spindle-assembly checkpoint, which inhibits the anaphase-promoting complex co-activator CDC20 to induce a prolonged cell cycle arrest1,2. Once errors are corrected, the spindle-assembly checkpoint is silenced, allowing anaphase onset to occur. However, in the presence of persistent unresolvable errors, cells can undergo 'mitotic slippage', exiting mitosis into a tetraploid G1 state and escaping the cell death that results from a prolonged arrest. The molecular logic that enables cells to balance these duelling mitotic arrest and slippage behaviours remains unclear. Here we demonstrate that human cells modulate the duration of their mitotic arrest through the presence of conserved, alternative CDC20 translational isoforms. Downstream translation initiation results in a truncated CDC20 isoform that is resistant to spindle-assembly-checkpoint-mediated inhibition and promotes mitotic exit even in the presence of mitotic perturbations. Our study supports a model in which the relative levels of CDC20 translational isoforms control the duration of mitotic arrest. During a prolonged mitotic arrest, new protein synthesis and differential CDC20 isoform turnover create a timer, with mitotic exit occurring once the truncated Met43 isoform achieves sufficient levels. Targeted molecular changes or naturally occurring cancer mutations that alter CDC20 isoform ratios or its translational control modulate mitotic arrest duration and anti-mitotic drug sensitivity, with potential implications for the diagnosis and treatment of human cancers.

Principles and dynamics of spindle assembly checkpoint signalling

The transmission of a complete set of chromosomes to daughter cells during cell division is vital for development and tissue homeostasis. The spindle assembly checkpoint (SAC) ensures correct segregation by informing the cell cycle machinery of potential errors in the interactions of chromosomes with spindle microtubules prior to anaphase. To do so, the SAC monitors microtubule engagement by specialized structures known as kinetochores and integrates local mechanical and chemical cues such that it can signal in a sensitive, responsive and robust manner. In this Review, we discuss how SAC proteins interact to allow production of the mitotic checkpoint complex (MCC) that halts anaphase progression by inhibiting the anaphase-promoting complex/cyclosome (APC/C). We highlight recent advances aimed at understanding the dynamic signalling properties of the SAC and how it interprets various naturally occurring intermediate attachment states. Further, we discuss SAC signalling in the context of the mammalian multisite kinetochore and address the impact of the fibrous corona. We also identify current challenges in understanding how the SAC ensures high-fidelity chromosome segregation.

The roles of E3 ubiquitin ligases in cancer progression and targeted therapy

Ubiquitination is one of the most important post-translational modifications which plays a significant role in conserving the homeostasis of cellular proteins. In the ubiquitination process, ubiquitin is conjugated to target protein substrates for degradation, translocation or activation, dysregulation of which is linked to several diseases including various types of cancers. E3 ubiquitin ligases are regarded as the most influential ubiquitin enzyme owing to their ability to select, bind and recruit target substrates for ubiquitination. In particular, E3 ligases are pivotal in the cancer hallmarks pathways where they serve as tumour promoters or suppressors. The specificity of E3 ligases coupled with their implication in cancer hallmarks engendered the development of compounds that specifically target E3 ligases for cancer therapy. In this review, we highlight the role of E3 ligases in cancer hallmarks such as sustained proliferation via cell cycle progression, immune evasion and tumour promoting inflammation, and in the evasion of apoptosis. In addition, we summarise the application and the role of small compounds that target E3 ligases for cancer treatment along with the significance of targeting E3 ligases as potential cancer therapy.

Targeting the anaphase-promoting complex/cyclosome (APC/C) enhanced antiproliferative and apoptotic response in bladder cancer

Improving the chemotherapy sensitivity of bladder cancer is a current clinical challenge. It is critical to seek out effective combination therapies that include low doses of cisplatin due to its dose-limiting toxicity. This study aims to investigate the cytotoxic effects of the combination therapy including proTAME, a small molecule inhibitor, targeting Cdc-20 and to determine the expression levels of several APC/C pathway-related genes that may play a role in the chemotherapy response of RT-4 (bladder cancer) and ARPE-19 (normal epithelial) cells. The IC20 and IC50 values were determined by MTS assay. The expression levels of apoptosis-associated (Bax and Bcl-2) and APC/C-associated (Cdc-20, Cyclin-B1, Securin, and Cdh-1) genes were assessed by qRT-PCR. Cell colonization ability and apoptosis were examined by clonogenic survival experiment and Annexin V/PI staining, respectively. Low-dose combination therapy showed a superior inhibition effect on RT-4 cells by increasing cell death and inhibiting colony formation. Triple-agent combination therapy further increased the percentage of late apoptotic and necrotic cells compared to the doublet-therapy with gemcitabine and cisplatin. ProTAME-containing combination therapies resulted in an elevation in Bax/Bcl-2 ratio in RT-4 cells, while a significant decrease was observed in proTAME-treated ARPE-19 cells. Cdc-20 expression in proTAME combined treatment groups were found to be decreased compared to their control groups. Low-dose triple-agent combination induced cytotoxicity and apoptosis in RT-4 cells effectively. It is essential to evaluate the role of APC/C pathway-associated potential biomarkers as therapeutic targets and define new combination therapy regimens to achieve improved tolerability in bladder cancer patients in the future.

The large cytoplasmic volume of oocyte

The study of the size of cells and organelles has a long history, dating back to the 1600s when cells were defined. In particular, various methods have elucidated the size of the nucleus and the mitotic spindle in several species. However, little research has been conducted on oocyte size and organelles in mammals, and many questions remain to be answered. The appropriate size is essential to cell function properly. Oocytes have a very large cytoplasm, which is more than 100 times larger than that of general somatic cells in mammals. In this review, we discuss how oocytes acquire an enormous cytoplasmic size and the adverse effects of a large cytoplasmic size on cellular functions.

The insulin receptor endocytosis

Insulin signaling controls multiple aspects of animal physiology. At the cell surface, insulin binds and activates the insulin receptor (IR), a receptor tyrosine kinase. Insulin promotes a large conformational change of IR and stabilizes the active conformation. The insulin-activated IR triggers signaling cascades, thus controlling metabolism, growth, and proliferation. The activated IR undergoes internalization by clathrin- or caveolae-mediated endocytosis. The IR endocytosis plays important roles in insulin clearance from blood, and distribution and termination of the insulin signaling. Despite decades of extensive studies, the mechanism and regulation of IR endocytosis and its contribution to pathophysiology remain incompletely understood. Here we discuss recent findings that provide insights into the molecular mechanisms and regulatory pathways that mediate the IR endocytosis.

N-Heterocycle based Degraders (PROTACs) Manifesting Anticancer Efficacy: Recent Advances

Proteolysis Targeting Chimeras (PROTACs) technology has emerged as a promising strategy for the treatment of undruggable therapeutic targets. Researchers have invested a great effort in developing druggable PROTACs; however, the problems associated with PROTACs, including poor solubility, metabolic stability, cell permeability, and pharmacokinetic profile, restrict their clinical utility. Thus, there is a pressing need to expand the size of the armory of PROTACs which will escalate the chances of pinpointing new PROTACs with optimum pharmacokinetic and pharmacodynamics properties. N- heterocycle is a class of organic frameworks that have been widely explored to construct new and novel PROTACs. This review provides an overview of recent efforts of medicinal chemists to develop N-heterocycle-based PROTACs as effective cancer therapeutics. Specifically, the recent endeavors centred on the discovery of PROTACs have been delved into various classes based on the E3 ligase they target (MDM2, IAP, CRBN, and other E3 ligases). Mechanistic insights revealed during the biological assessment of recently furnished Nheterocyclic- based PROTACs constructed via the utilization of ligands for various E3 ligases have been discussed.

Kinetochore‐catalyzed MCC formation: A structural perspective

The spindle assembly checkpoint (SAC) is a cellular surveillance mechanism that functions to ensure accurate chromosome segregation during mitosis. Macromolecular complexes known as kinetochores, act as the interface of sister chromatid attachment to spindle microtubules. In response to unattached kinetochores, the SAC activates its effector, the mitotic checkpoint complex (MCC), which delays mitotic exit until all sister chromatid pairs have achieved successful attachment to the bipolar mitotic spindle. Formation of the MCC (composed of Mad2, BubR1, Bub3 and Cdc20) is regulated by an Mps1 kinase-dependent phosphorylation signaling cascade which assembles and repositions components of the MCC onto a catalytic scaffold. This scaffold functions to catalyze the conversion of the HORMA-domain protein Mad2 from an "inactive" open-state (O-Mad2) into an "active" closed-Mad2 (C-Mad2), and simultaneous Cdc20 binding. Here, our current understanding of the molecular mechanisms underlying the kinetic barrier to C-Mad2:Cdc20 formation will be reviewed. Recent progress in elucidating the precise molecular choreography orchestrated by the catalytic scaffold to rapidly assemble the MCC will be examined, and unresolved questions will be highlighted. Ultimately, understanding how the SAC rapidly activates the checkpoint not only provides insights into how cells maintain genomic integrity during mitosis, but also provides a paradigm for how cells can utilize molecular switches, including other HORMA domain-containing proteins, to make rapid changes to a cell's physiological state.

Insight into the physiological and pathological roles of USP44, a potential tumor target (Review)

Ubiquitin-specific peptidase 44 (USP44) is a member of the ubiquitin-specific proteases (USPs) family and its functions in various biological processes have been gradually elucidated in recent years. USP44 targets multiple downstream factors and regulates multiple mechanisms through its deubiquitination activity. Ubiquitination is, in essence, a process in which a single ubiquitin molecule or a multiubiquitin chain binds to a substrate protein to form an isopeptide bond. Deubiquitination is the catalyzing of the isopeptide bonds between ubiquitin and substrate proteins through deubiquitylating enzymes. These two processes serve an important role in the regulation of the expression, conformation, localization and function of substrate proteins by regulating their binding to ubiquitin. Based on existing research, this paper summarized the current state of knowledge about USP44. The physiological roles of USP44 in various cellular events and its pathophysiological roles in different cancer types are evaluated and the therapeutic potential of USP44 for cancer treatment is evaluated.

Juxtaposition of Bub1 and Cdc20 on phosphorylated Mad1 during catalytic mitotic checkpoint complex assembly

In response to improper kinetochore-microtubule attachments in mitosis, the spindle assembly checkpoint (SAC) assembles the mitotic checkpoint complex (MCC) to inhibit the anaphase-promoting complex/cyclosome, thereby delaying entry into anaphase. The MCC comprises Mad2:Cdc20:BubR1:Bub3. Its assembly is catalysed by unattached kinetochores on a Mad1:Mad2 platform. Mad1-bound closed-Mad2 (C-Mad2) recruits open-Mad2 (O-Mad2) through self-dimerization. This interaction, combined with Mps1 kinase-mediated phosphorylation of Bub1 and Mad1, accelerates MCC assembly, in a process that requires O-Mad2 to C-Mad2 conversion and concomitant binding of Cdc20. How Mad1 phosphorylation catalyses MCC assembly is poorly understood. Here, we characterized Mps1 phosphorylation of Mad1 and obtained structural insights into a phosphorylation-specific Mad1:Cdc20 interaction. This interaction, together with the Mps1-phosphorylation dependent association of Bub1 and Mad1, generates a tripartite assembly of Bub1 and Cdc20 onto the C-terminal domain of Mad1 (Mad1CTD). We additionally identify flexibility of Mad1:Mad2 that suggests how the Cdc20:Mad1CTD interaction brings the Mad2-interacting motif (MIM) of Cdc20 near O-Mad2. Thus, Mps1-dependent formation of the MCC-assembly scaffold functions to position and orient Cdc20 MIM near O-Mad2, thereby catalysing formation of C-Mad2:Cdc20.

APC/CCdc20-mediated degradation of Clb4 prompts astral microtubule stabilization at anaphase onset

Key for accurate chromosome partitioning to the offspring is the ability of mitotic spindle microtubules to respond to different molecular signals and remodel their dynamics accordingly. Spindle microtubules are conventionally divided into three classes: kinetochore, interpolar, and astral microtubules (kMTs, iMTs, and aMTs, respectively). Among all, aMT regulation remains elusive. Here, we show that aMT dynamics are tightly regulated. aMTs remain unstable up to metaphase and are stabilized at anaphase onset. This switch in aMT dynamics, important for proper spindle orientation, specifically requires the degradation of the mitotic cyclin Clb4 by the Anaphase Promoting Complex bound to its activator subunit Cdc20 (APC/CCdc20). These data highlight a unique role for mitotic cyclin Clb4 in controlling aMT regulating factors, of which Kip2 is a prime candidate, provide a framework to understand aMT regulation in vertebrates, and uncover mechanistic principles of how the APC/CCdc20 choreographs the timing of late mitotic events by sequentially impacting on the three classes of spindle microtubules.

Transcriptomics Using the Enriched Arabidopsis Shoot Apex Reveals Developmental Priming Genes Involved in Plastic Plant Growth under Salt Stress Conditions

In the shoot apical meristem (SAM), the homeostasis of the stem cell population supplying new cells for organ formation is likely a key mechanism of multicellular plant growth and development. As plants are sessile organisms and constantly encounter environmental abiotic stresses, postembryonic development from the shoot stem cell population must be considered with surrounding abiotic stresses for plant adaptation. However, the underlying molecular mechanisms for plant adaptation remain unclear. Previous studies found that the stem-cell-related mutant clv3-2 has the property of salt tolerance without the differential response of typical stress-responsive genes compared to those in WT Ler. Based on these facts, we hypothesized that shoot meristems contain developmental priming genes having comprehensively converged functions involved in abiotic stress response and development. To better understand the biological process of developmental priming genes in the SAM, we performed RNA sequencing (RNA-seq) and transcriptome analysis through comparing genome-wide gene expression profiles between enriched shoot apex and leaf tissues. As a result, 121 putative developmental priming genes differentially expressed in the shoot apex compared to the leaf were identified under normal and salt stress conditions. RNA-seq experiments also revealed the shoot apex-specific responsive genes for salt stress conditions. Based on combinatorial comparisons, 19 developmental priming genes were finally identified, including developmental genes related to cell division and abiotic/biotic-stress-responsive genes. Moreover, some priming genes showed CLV3-dependent responses under salt stress conditions in the clv3-2. These results presumably provide insight into how shoot meristem tissues have relatively high viability against stressful environmental conditions for the developmental plasticity of plants.

Recovery from spindle checkpoint-mediated arrest requires a novel Dnt1-dependent APC/C activation mechanism

The activated spindle assembly checkpoint (SAC) potently inhibits the anaphase-promoting complex/cyclosome (APC/C) to ensure accurate chromosome segregation at anaphase. Early studies have recognized that the SAC should be silenced within minutes to enable rapid APC/C activation and synchronous segregation of chromosomes once all kinetochores are properly attached, but the underlying silencers are still being elucidated. Here, we report that the timely silencing of SAC in fission yeast requires dnt1+, which causes severe thiabendazole (TBZ) sensitivity and increased rate of lagging chromosomes when deleted. The absence of Dnt1 results in prolonged inhibitory binding of mitotic checkpoint complex (MCC) to APC/C and attenuated protein levels of Slp1Cdc20, consequently slows the degradation of cyclin B and securin, and eventually delays anaphase entry in cells released from SAC activation. Interestingly, Dnt1 physically associates with APC/C upon SAC activation. We propose that this association may fend off excessive and prolonged MCC binding to APC/C and help to maintain Slp1Cdc20 stability. This may allow a subset of APC/C to retain activity, which ensures rapid anaphase onset and mitotic exit once SAC is inactivated. Therefore, our study uncovered a new player in dictating the timing and efficacy of APC/C activation, which is actively required for maintaining cell viability upon recovery from the inhibition of APC/C by spindle checkpoint.

Analysis of the potential role of fission yeast PP2A in spindle assembly checkpoint inactivation

As a surveillance mechanism, the activated spindle assembly checkpoint (SAC) potently inhibits the E3 ubiquitin ligase APC/C (anaphase-promoting complex/cyclosome) to ensure accurate chromosome segregation. Although the protein phosphatase 2A (PP2A) has been proposed to be both, directly and indirectly, involved in spindle assembly checkpoint inactivation in mammalian cells, whether it is similarly operating in the fission yeast Schizosaccharomycer pombe has never been demonstrated. Here, we investigated whether fission yeast PP2A is involved in SAC silencing by following the rate of cyclin B (Cdc13) destruction at SPBs during the recovery phase in nda3-KM311 cells released from the inhibition of APC/C by the activated spindle checkpoint. The timing of the SAC inactivation is only slightly delayed when two B56 regulatory subunits (Par1 and Par2) of fission yeast PP2A are absent. Overproduction of individual PP2A subunits either globally in the nda3-KM311 arrest-and-release system or locally in the synthetic spindle checkpoint activation system only slightly suppresses the SAC silencing defects in PP1 deletion (dis2Δ) cells. Our study thus demonstrates that the fission yeast PP2A is not a key regulator actively involved in SAC inactivation.

Activation of the Anaphase Promoting Complex Reverses Multiple Drug Resistant Cancer in a Canine Model of Multiple Drug Resistant Lymphoma

Simple Summary

Multiple drug resistant cancers develop all too soon in patients who received successful cancer treatment. A lack of treatment options often leaves palliative care as the last resort. We tested whether the insulin sensitizer, metformin, known to have anti-cancer activity, could impact canines with drug resistant lymphoma when added to chemotherapy. All canines in the study expressed protein markers of drug resistance and within weeks of receiving metformin, the markers were decreased. A microarray was performed, and from four canines assessed, a common set of 290 elevated genes were discovered in tumor cells compared to control cells. This cluster was enriched with genes that stall the cell cycle, with a large component representing substrates of the Anaphase Promoting Complex (APC), which degrades proteins. One canine entered partial remission. RNAs from this canine showed that APC substrates were decreased during remission and elevated again during relapse, suggesting that the APC was impaired in drug resistant canines and restored when remission occurred. We validated our results in cell lines using APC inhibitors and activators. We conclude that the APC may be a vital guardian of the genome and could delay the onset of multiple drug resistance when activated.

Abstract

Like humans, canine lymphomas are treated by chemotherapy cocktails and frequently develop multiple drug resistance (MDR). Their shortened clinical timelines and tumor accessibility make canines excellent models to study MDR mechanisms. Insulin-sensitizers have been shown to reduce the incidence of cancer in humans prescribed them, and we previously demonstrated that they also reverse and delay MDR development in vitro. Here, we treated canines with MDR lymphoma with metformin to assess clinical and tumoral responses, including changes in MDR biomarkers, and used mRNA microarrays to determine differential gene expression. Metformin reduced MDR protein markers in all canines in the study. Microarrays performed on mRNAs gathered through longitudinal tumor sampling identified a 290 gene set that was enriched in Anaphase Promoting Complex (APC) substrates and additional mRNAs associated with slowed mitotic progression in MDR samples compared to skin controls. mRNAs from a canine that went into remission showed that APC substrate mRNAs were decreased, indicating that the APC was activated during remission. In vitro validation using canine lymphoma cells selected for resistance to chemotherapeutic drugs confirmed that APC activation restored MDR chemosensitivity, and that APC activity was reduced in MDR cells. This supports the idea that rapidly pushing MDR cells that harbor high loads of chromosome instability through mitosis, by activating the APC, contributes to improved survival and disease-free duration.

Role of phosphorylation of Cdc20 in the regulation of the action of APC/C in mitosis

The ubiquitin ligase APC/C (anaphase-promoting complex/cyclosome) is essential for the control of mitosis, and its activity is subject to tight regulation. In early mitosis, APC/C is inhibited by the mitotic checkpoint system, but subsequently it regains activity and promotes metaphase-anaphase transition by targeting cyclin B and securin for degradation. The phosphorylation of APC/C by the mitotic protein kinase Cdk1-cyclin B facilitates its interaction with its coactivator Cdc20, while the phosphorylation of Cdc20 inhibits its binding to APC/C. This raises the question of how Cdc20 binds to APC/C under conditions of high Cdk1 activity. It seemed possible that the opposing action of protein phosphatases produces a fraction of unphosphorylated Cdc20 that binds to APC/C. We found, however, that while inhibitors of protein phosphatases PP2A and PP1 increased the overall phosphorylation of Cdc20 in anaphase extracts from Xenopus eggs, they did not decrease the levels of Cdc20 bound to APC/C. Searching for alternative mechanisms, we found that following the binding of Cdc20 to APC/C, it became significantly protected against phosphorylation by Cdk1. Protection was mainly at threonine sites at the N-terminal region of Cdc20, known to affect its interaction with APC/C. A model is proposed according to which a pool of unphosphorylated Cdc20, originating from initial stages of mitosis or from phosphatase action, combines with phosphorylated APC/C and thus becomes stabilized against further phosphorylation, possibly by steric hindrance of Cdk1 action. This pool of APC^Cdc20 appears to be required for the regulation of APC/C activity at different stages of mitosis.

Biochemical, biophysical, and functional characterisation of the E3 ubiquitin ligase APC/C regulator CDC20 from Arabidopsis thaliana

The Anaphase Promoting Complex (APC/C), a large cullin-RING E3-type ubiquitin ligase, constitutes the ultimate target of the Spindle Assembly Checkpoint (SAC), an intricate regulatory circuit that ensures the high fidelity of chromosome segregation in eukaryotic organisms by delaying the onset of anaphase until each chromosome is properly bi-oriented on the mitotic spindle. Cell-division cycle protein 20 homologue (CDC20) is a key regulator of APC/C function in mitosis. The formation of the APC/C^CDC20 complex is required for the ubiquitination and degradation of select substrates, which is necessary to maintain the mitotic state. In contrast to the roles of CDC20 in animal species, little is known about CDC20 roles in the regulation of chromosome segregation in plants. Here we address this gap in knowledge and report the expression in insect cells; the biochemical and biophysical characterisation of Arabidopsis thaliana (AtCDC20) WD40 domain; and the nuclear and cytoplasmic distribution of full-length AtCDC20 when transiently expressed in tobacco plants. We also show that most AtCDC20 degrons share a high sequence similarity to other eukaryotes, arguing in favour of conserved degron functions in AtCDC20. However, important exceptions were noted such as the lack of a canonical MAD1 binding motif; a fully conserved RRY-box in all six AtCDC20 isoforms instead of a CRY-box motif, and low conservation of key residues known to be phosphorylated by BUB1 and PLK1 in other species to ensure a robust SAC response. Taken together, our studies provide insights into AtCDC20 structure and function and the evolution of SAC signalling in plants.

The Role of the APC/C and Its Coactivators Cdh1 and Cdc20 in Cancer Development and Therapy

To sustain genomic stability by correct DNA replication and mitosis, cell cycle progression is tightly controlled by the cyclic activity of cyclin-dependent kinases, their binding to cyclins in the respective phase and the regulation of cyclin levels by ubiquitin-dependent proteolysis. The spindle assembly checkpoint plays an important role at the metaphase-anaphase transition to ensure a correct separation of sister chromatids before cytokinesis and to initiate mitotic exit, as an incorrect chromosome distribution may lead to genetically unstable cells and tumorigenesis. The ubiquitin ligase anaphase-promoting complex or cyclosome (APC/C) is essential for these processes by mediating the proteasomal destruction of cyclins and other important cell cycle regulators. To this end, it interacts with the two regulatory subunits Cdh1 and Cdc20. Both play a role in tumorigenesis with Cdh1 being a tumor suppressor and Cdc20 an oncogene. In this review, we summarize the current knowledge about the APC/C-regulators Cdh1 and Cdc20 in tumorigenesis and potential targeted therapeutic approaches.

Cell cycle involvement in cancer therapy; WEE1 kinase, a potential target as therapeutic strategy

Mitosis is the process of cell division and is regulated by checkpoints in the cell cycle. G1-S, S, and G2-M are the three main checkpoints that prevent initiation of the next phase of the cell cycle phase until previous phase has completed. DNA damage leads to activation of the G2-M checkpoint, which can trigger a downstream DNA damage response (DDR) pathway to induce cell cycle arrest while the damage is repaired. If the DNA damage cannot be repaired, the replication stress response (RSR) pathway finally leads to cell death by apoptosis, in this case called mitotic catastrophe. Many cancer treatments (chemotherapy and radiotherapy) cause DNA damages based on SSBs (single strand breaks) or DSBs (double strand breaks), which cause cell death through mitotic catastrophe. However, damaged cells can activate WEE1 kinase (as a part of the DDR and RSR pathways), which prevents apoptosis and cell death by inducing cell cycle arrest at G2 phase. Therefore, inhibition of WEE1 kinase could sensitize cancer cells to chemotherapeutic drugs. This review focuses on the role of WEE1 kinase (as a biological macromolecule which has a molecular mass of 96 kDa) in the cell cycle, and its interactions with other regulatory pathways. In addition, we discuss the potential of WEE1 inhibition as a new therapeutic approach in the treatment of various cancers, such as melanoma, breast cancer, pancreatic cancer, cervical cancer, etc.

Rice Cell Division Cycle 20s are required for faithful chromosome segregation and cytokinesis during meiosis

Chromosome segregation must be under strict regulation to maintain chromosome euploidy and stability. Cell Division Cycle 20 (CDC20) is an essential cell cycle regulator that promotes the metaphase-to-anaphase transition and functions in the spindle assembly checkpoint, a surveillance pathway that ensures the fidelity of chromosome segregation. Plant CDC20 genes are present in multiple copies, and whether CDC20s have the same functions in plants as in yeast and animals is unclear, given the potential for divergence or redundancy among the multiple copies. Here, we studied all three CDC20 genes in rice (Oryza sativa) and constructed two triple mutants by clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9-mediated genome editing to explore their roles in development. Knocking out all three CDC20 genes led to total sterility but did not affect vegetative development. Loss of the three CDC20 proteins did not alter mitotic division but severely disrupted meiosis as a result of asynchronous and unequal chromosome segregation, chromosome lagging, and premature separation of chromatids. Immunofluorescence of tubulin revealed malformed meiotic spindles in microsporocytes of the triple mutants. Furthermore, cytokinesis of meiosis I was absent or abnormal, and cytokinesis II was completely prevented in all mutant microsporocytes; thus, no tetrads or pollen formed in either cdc20 triple mutant. Finally, the subcellular structures and functions of the tapetum were disturbed by the lack of CDC20 proteins. These findings demonstrate that the three rice CDC20s play redundant roles but are indispensable for faithful meiotic chromosome segregation and cytokinesis, which are required for the production of fertile microspores.

Discovery of the First Examples of Threonine Tyrosine Kinase PROTAC Degraders

The first examples of threonine tyrosine kinase (TTK) PROTACs were designed and synthesized. Two of the most potent molecules, 8e and 8j, demonstrated strong TTK degradation in COLO-205 human colorectal cancer cells with DC₅₀ values of 1.7 and 3.1 nM, respectively. Proteasome-mediated degradation by the compounds could last for approximately 8 h after washout. The degraders 8e and 8j demonstrated improved antiproliferative activities comparing with the structurally similar inhibitor counterparts 8q and 8r. Degraders 8e and 8j also demonstrated reasonable PK profiles and exhibited potent target degradation and in vivo anticancer efficacy in a xenograft mouse model of COLO-205 human colorectal cancer cells upon i.p. administration.

LncRNA THRIL is involved in the proliferation, migration, and invasion of rheumatoid fibroblast-like synoviocytes

Background

Fibroblast-like synoviocytes (FLSs), which can migrate and directly invade the cartilage and the bone, are crucial players in joint damage in rheumatoid arthritis (RA). Nevertheless, the detailed mechanisms underlying the aberrant activation of RA FLSs remain unclear. Several studies have attempted to explore the relationship between long non-coding RNAs (lncRNAs) and RA pathology; however, the role of lncRNAs in RA is unknown. The present study aimed to determine the functions of tumor necrosis factor-α and heterogeneous nuclear ribonucleoprotein L-related immunoregulatory lincRNA (THRIL) in RA FLSs migration and invasion.

Methods

Small interfering RNA targeting THRIL or lentivirus overexpressing THRIL was used to knockdown or overexpress THRIL. Quantitative reverse transcription polymerase chain reaction (PCR) was employed for the detection of RNA expression. The proliferation rate of RA FLSs was measured using a 5-ethynyl-2'-deoxyuridine (EdU) incorporation assay. Migration and invasion were detected using a transwell chamber. Downstream targets were identified using a human cell cycle real-time PCR array and a human cell motility real-time PCR array.

Results

A significant decrease in THRIL expression was found in RA FLSs compared with cells from healthy control (HC)patients. THRIL is mainly localized in the nucleus. Knockdown of THRIL increased the proliferation, migration, and invasion of RA FLSs. In contrast, THRIL overexpression had the opposite effect. THRIL knockdown increased interleukin-1β (IL-1β)-triggered expression of matrix metalloproteinase (MMP)-1, MMP-3, and MMP-13. THRIL overexpression led to a significant decrease in MMP-13 expression in response to stimulation with IL-1β. Furthermore, we observed that the expression levels of cyclin-dependent kinase 1 (CDK1) and G2 and S phase-expressed-1 (GTSE1), both of which are associated with cellular mobility and proliferation, were downregulated with THRIL overexpression.

Conclusions

Reduced expression of lncRNA THRIL represses the proliferation, migration, and invasion of RA FLSs, suggesting that lncRNA THRIL might be a potential target for RA therapy.

CCT4 suppression inhibits tumor growth in hepatocellular carcinoma by interacting with Cdc20

BACKGROUND

The chaperonin containing t-complex (CCT) proteins play an important role in cell cycle-related protein degradation in yeast and mammals. The role of the chaperonin containing t-complex 4 (CCT4), one subtype of CCT proteins, in the progress of hepatocellular carcinoma (HCC) was not fully elucidated. Here, we aimed to explore the mechanisms of CCT4 in HCC.

METHODS

In this study, we used the UALCAN platform to analyze the relationship between CCT4 and HCC, and the association of CCT4 with the overall survival (OS) of HCC patients was also analyzed. CCT4 expression in HCC tumor tissues and normal tissues was also determined by western blot (WB) assay. Lentivirus vector was used to knock down the CCT4 expression, and quantitative polymerase chain reaction and WB were used to determine the level of CCT4 in HCC cell lines. Cell counting kit-8 (CCK-8) and 5-ethynyl-2'-deoxyuridine (EdU) assays were used to detect the cell proliferation, and flow cytometry (FCM) was performed to evaluate the effect of CCT4 on the apoptosis of HCC cells. Co-immunoprecipitation (co-IP) assay and WB were used to explore the mechanisms of CCT4 regulating the growth of HCC. Data were calculated from at least three replicate experiments and expressed as mean ± standard deviation. Student's t test, paired t test, and Kaplan-Meier analysis were used to compare across different groups.

RESULTS

We found CCT4 was upregulated in HCC tissues compared with normal tissues, and its high expression was associated with poor prognosis (P < 0.001). CCT4 was significantly increased in HCC tumor tissues compared with normal tissues (0.98 ± 0.12 vs. 0.23 ± 0.05, t = 7.73, P < 0.001). After being transfected with CCT4 short-hairpin RNA (shRNA), CCT4 was decreased in mRNA level and protein level in both Huh7 (mRNA level: 0.41 ± 0.07 vs. 1.01 ± 0.11, t = 8.09, P = 0.001; protein level: 0.61 ± 0.03 vs. 0.93 ± 0.07, t = 7.19, P = 0.002) and Hep3b cells (mRNA level: 0.55 ± 0.11 vs. 1.04 ± 0.15, t = 4.51, P = 0.011; protein level: 0.64 ± 0.10 vs. 0.95 ± 0.08, t = 4.32, P = 0.012). CCK8 assay indicated that CCT4 knockdown inhibited cell proliferation in both Huh7 (OD value of 3 days: 0.60 ± 0.14 vs. 0.97 ± 0.16, t = 3.13, P = 0.036; OD value of 4 days: 1.03 ± 0.07 vs. 1.50 ± 0.12, t = 5.97, P = 0.004) and Hep3b (OD value of 3 days: 0.69 ± 0.14 vs. 1.10 ± 0.11, t = 3.91, P = 0.017; OD value of 4 days: 1.12 ± 0.12 vs. 1.48 ± 0.13, t = 3.55, P = 0.024) cells. EdU assay showed that CCT4 knockdown inhibited the cell proliferation in both Huh7 (EdU positive rate: [31.25 ± 3.41]% vs. [58.72 ± 3.78]%, t = 9.34, P = 0.001) and Hep3b cells (EdU positive rate: [44.13 ± 7.02]% vs. [61.79 ± 3.96]%, t = 3.79, P = 0.019). FCM assay suggested that CCT4 knockdown induced apoptosis in HCC cells (apoptosis rate of Huh7: [9.10 ± 0.80]% vs. [3.66 ± 0.64]%, t = -9.18, P = 0.001; apoptosis rate of Hep3b: [6.69 ± 0.72]% vs. [4.20 ± 0.86]%, t = -3.84, P = 0.018). We also found that CCT4 could regulate anaphase-promoting complex (APC)Cdc20 activity via interacting with Cdc20. Furthermore, CCT4 knockdown induced securin (0.65 ± 0.06 vs. 0.44 ± 0.05, t = -4.69, P = 0.009) and B-cell lymphoma-2 (Bcl-2) interacting mediator of cell death (Bim; 0.96 ± 0.06 vs. 0.61 ± 0.09, t = -5.65, P = 0.005) accumulation. The upregulation of securin inhibited cell growth by downregulating cyclin D1 (0.65 ± 0.05 vs. 1.04 ± 0.07, t = 8.12, P = 0.001), and the accumulation of Bim inhibited Bcl-2 (0.77 ± 0.04 vs. 0.87 ± 0.04, t = 3.00, P = 0.040) and activated caspase 9 (caspase 9: 0.77 ± 0.04 vs. 0.84 ± 0.05, t = 1.81, P = 0.145; cleaved caspase 9: 0.64 ± 0.06 vs. 0.16 ± 0.07, t = 1.81, P = 0.001), which led to elevated apoptosis.

CONCLUSIONS

Overall, these results showed that CCT4 played an important role in HCC pathogenesis through, at least partly, interacting with Cdc20.

CBX8 interacts with chromatin PTEN and is involved in regulating mitotic progression

OBJECTIVES

Besides its role in regulating phosphatidylinositol-3 kinase (PI3K) signalling in the cytosol, PTEN also has a nuclear function. In this study, we attempted to understand the mechanism of chromatin PTEN in suppressing chromosomal instability during cell division.

MATERIALS AND METHODS

Immunocoprecipitation, ectopic expression, and deletional analyses were used to identify the physical interaction between Chromobox Homolog protein 8 (CBX8) and PTEN, as well as the functional domain(s) of PTEN mediating the interaction. Cell synchronization followed by immunoblotting was employed to study cell cycle regulation of CBX8 and the functional interaction between chromatin PTEN and CBX8. Small interfering RNAs (siRNAs) were used to study the role of PTEN and CBX8 in modulating histone epigenetic markers during the cell cycle.

RESULTS

Polycomb group (PcG) proteins including CBXs function to repress gene expression in a wide range of organisms including mammals. We recently showed that PTEN interacted with CBX8, a component of Polycomb Repressing Complex 1 (PRC1), and that CBX8 co-localized with PTEN in the nucleus. CBX8 levels were high, coinciding with its phosphorylation in mitosis. Phosphorylation of CBX8 was associated with monoubiquitinated PTEN and phosphorylated-BubR1 on chromatin. Moreover, CBX8 played an important role in cell proliferation and mitotic progression. Significantly, downregulation of either PTEN or CBX8 induced H3K27Me3 epigenetic marker in mitotic cells.

CONCLUSION

CBX8 is a new component that physically interacts with chromatin PTEN, playing an important role in regulating mitotic progression.

NF2 Gene Participates in Regulation of the Cell Cycle of Meningiomas by Restoring Spindle Assembly Checkpoint Function and Inhibiting the Binding of Cdc20 Protein to Anaphase Promoting Complex/Cyclosome

BACKGROUND

The neurofibromatosis type 2 (NF2) gene mutation is the leading genetic event in meningiomas, usually accompanied by malignant features. Dysfunction of the spindle assembly checkpoint (SAC) induces tumorigenesis. However, the crosstalk between NF2 and SAC in meningiomas remains unclear.

METHODS

Cell proliferation, invasion, apoptosis, and cell cycle of meningiomas were determined by cell counting kit-8 assay, transwell assay, and flow cytometry, respectively. The expression of SAC in meningioma cells was detected by quantitative real-time polymerase chain reaction and Western blot. The interaction between anaphase promoting complex/cyclosome (APC/C) and cell division cycle 20 (Cdc20) protein in meningioma cells was further explored by co-immunoprecipitation.

RESULTS

We found that the expression of NF2/merlin was low or absent in malignant meningiomas. Overexpression of NF2 suppressed the proliferation and invasion of meningioma cells, prolonged the G₂/M phase, and elevated the expression of SAC proteins at posttranscription. Furthermore, the interaction between APC/C and Cdc20 was inhibited by NF2.

CONCLUSIONS

Our findings suggested that NF2 might restore SAC function by impairing the binding of APC/C and Cdc20, thereby limiting the mitotic rate and inhibiting proliferation of meningiomas.

Spindle assembly checkpoint activation and silencing at kinetochores

The spindle assembly checkpoint (SAC) is a surveillance mechanism that promotes accurate chromosome segregation in mitosis. The checkpoint senses the attachment state of kinetochores, the proteinaceous structures that assemble onto chromosomes in mitosis in order to mediate their interaction with spindle microtubules. When unattached, kinetochores generate a diffusible inhibitor that blocks the activity of the anaphase-promoting complex/cyclosome (APC/C), an E3 ubiquitin ligase required for sister chromatid separation and exit from mitosis. Work from the past decade has greatly illuminated our understanding of the mechanisms by which the diffusible inhibitor is assembled and how it inhibits the APC/C. However, less is understood about how SAC proteins are recruited to kinetochores in the absence of microtubule attachment, how the kinetochore catalyzes formation of the diffusible inhibitor, and how attachments silence the SAC at the kinetochore. Here, we summarize current understanding of the mechanisms that activate and silence the SAC at kinetochores and highlight open questions for future investigation.

Evolutionary Dynamics of the Spindle Assembly Checkpoint in Eukaryotes

The tremendous diversity in eukaryotic life forms can ultimately be traced back to evolutionary modifications at the level of molecular networks. Deep understanding of these modifications will not only explain cellular diversity, but will also uncover different ways to execute similar processes and expose the evolutionary 'rules' that shape the molecular networks. Here, we review the evolutionary dynamics of the spindle assembly checkpoint (SAC), a signaling network that guards fidelity of chromosome segregation. We illustrate how the interpretation of divergent SAC systems in eukaryotic species is facilitated by combining detailed molecular knowledge of the SAC and extensive comparative genome analyses. Ultimately, expanding this to other core cellular systems and experimentally interrogating such systems in organisms from all major lineages may start outlining the routes to and eventual manifestation of the cellular diversity of eukaryotic life.

Molecular mechanism of Mad1 kinetochore targeting by phosphorylated Bub1

During metaphase, in response to improper kinetochore-microtubule attachments, the spindle assembly checkpoint (SAC) activates the mitotic checkpoint complex (MCC), an inhibitor of the anaphase-promoting complex/cyclosome (APC/C). This process is orchestrated by the kinase Mps1, which initiates the assembly of the MCC onto kinetochores through a sequential phosphorylation-dependent signalling cascade. The Mad1-Mad2 complex, which is required to catalyse MCC formation, is targeted to kinetochores through a direct interaction with the phosphorylated conserved domain 1 (CD1) of Bub1. Here, we present the crystal structure of the C-terminal domain of Mad1 (Mad1CTD ) bound to two phosphorylated Bub1CD1 peptides at 1.75 Å resolution. This interaction is mediated by phosphorylated Bub1 Thr461, which not only directly interacts with Arg617 of the Mad1 RLK (Arg-Leu-Lys) motif, but also directly acts as an N-terminal cap to the CD1 α-helix dipole. Surprisingly, only one Bub1CD1 peptide binds to the Mad1 homodimer in solution. We suggest that this stoichiometry is due to inherent asymmetry in the coiled-coil of Mad1CTD and has implications for how the Mad1-Bub1 complex at kinetochores promotes efficient MCC assembly.

Intricate Regulatory Mechanisms of the Anaphase-Promoting Complex/Cyclosome and Its Role in Chromatin Regulation

The ubiquitin (Ub)-proteasome system is vital to nearly every biological process in eukaryotes. Specifically, the conjugation of Ub to target proteins by Ub ligases, such as the Anaphase-Promoting Complex/Cyclosome (APC/C), is paramount for cell cycle transitions as it leads to the irreversible destruction of cell cycle regulators by the proteasome. Through this activity, the RING Ub ligase APC/C governs mitosis, G1, and numerous aspects of neurobiology. Pioneering cryo-EM, biochemical reconstitution, and cell-based studies have illuminated many aspects of the conformational dynamics of this large, multi-subunit complex and the sophisticated regulation of APC/C function. More recent studies have revealed new mechanisms that selectively dictate APC/C activity and explore additional pathways that are controlled by APC/C-mediated ubiquitination, including an intimate relationship with chromatin regulation. These tasks go beyond the traditional cell cycle role historically ascribed to the APC/C. Here, we review these novel findings, examine the mechanistic implications of APC/C regulation, and discuss the role of the APC/C in previously unappreciated signaling pathways.

Coupling of Cdc20 inhibition and activation by BubR1

Tight regulation of the APC/C-Cdc20 ubiquitin ligase that targets cyclin B1 for degradation is important for mitotic fidelity. The spindle assembly checkpoint (SAC) inhibits Cdc20 through the mitotic checkpoint complex (MCC). In addition, phosphorylation of Cdc20 by cyclin B1–Cdk1 independently inhibits APC/C–Cdc20 activation. This creates a conundrum for how Cdc20 is activated before cyclin B1 degradation. Here, we show that the MCC component BubR1 harbors both Cdc20 inhibition and activation activities, allowing for cross-talk between the two Cdc20 inhibition pathways. Specifically, BubR1 acts as a substrate specifier for PP2A-B56 to enable efficient Cdc20 dephosphorylation in the MCC. A mutant Cdc20 mimicking the dephosphorylated state escapes a mitotic checkpoint arrest, arguing that restricting Cdc20 dephosphorylation to the MCC is important. Collectively, our work reveals how Cdc20 can be dephosphorylated in the presence of cyclin B1-Cdk1 activity without causing premature anaphase onset.

Antagonizing the spindle assembly checkpoint silencing enhances paclitaxel and Navitoclax-mediated apoptosis with distinct mechanistic

Antimitotic drugs arrest cells in mitosis through chronic activation of the spindle assembly checkpoint (SAC), leading to cell death. However, drug-treated cancer cells can escape death by undergoing mitotic slippage, due to premature mitotic exit. Therefore, overcoming slippage issue is a promising chemotherapeutic strategy to improve the effectiveness of antimitotics. Here, we antagonized SAC silencing by knocking down the MAD2-binding protein p31^comet, to delay mitotic slippage, and tracked cancer cells treated with the antimitotic drug paclitaxel, over 3 days live-cell time-lapse analysis. We found that in the absence of p31^comet, the duration of mitotic block was increased in cells challenged with nanomolar concentrations of paclitaxel, leading to an additive effects in terms of cell death which was predominantly anticipated during the first mitosis. As accumulation of an apoptotic signal was suggested to prevent mitotic slippage, when we challenged p31^comet-depleted mitotic-arrested cells with the apoptosis potentiator Navitoclax (previously called ABT-263), cell fate was shifted to accelerated post-mitotic death. We conclude that inhibition of SAC silencing is critical for enhancing the lethality of antimitotic drugs as well as that of therapeutic apoptosis-inducing small molecules, with distinct mechanisms. The study highlights the potential of p31^comet as a target for antimitotic therapies.

CDC20 assists its catalytic incorporation in the mitotic checkpoint complex

Open (O) and closed (C) topologies of HORMA-domain proteins are respectively associated with inactive and active states of fundamental cellular pathways. The HORMA protein O-MAD2 converts to C-MAD2 upon binding CDC20. This is rate limiting for assembly of the mitotic checkpoint complex (MCC), the effector of a checkpoint required for mitotic fidelity. A catalyst assembled at kinetochores accelerates MAD2:CDC20 association through a poorly understood mechanism. Using a reconstituted SAC system, we discovered that CDC20 is an impervious substrate for which access to MAD2 requires simultaneous docking on several sites of the catalytic complex. Our analysis indicates that the checkpoint catalyst is substrate assisted and promotes MCC assembly through spatially and temporally coordinated conformational changes in both MAD2 and CDC20. This may define a paradigm for other HORMA-controlled systems.

Real-Time Monitoring of APC /C-Mediated Substrate Degradation Using Xenopus laevis Egg Extracts

The anaphase promoting complex/cyclosome (APC/C), a large E3 ubiquitin ligase, is a key regulator of mitotic progression. Upon activation in mitosis, the APC/C targets its two essential substrates, securin and cyclin B, for proteasomal destruction. Cyclin B is the activator of cyclin-dependent kinase 1 (Cdk1), the major mitotic kinase, and both cyclin B and securin are safeguards of sister chromatid cohesion. Conversely, the degradation of securin and cyclin B promotes sister chromatid separation and mitotic exit. The negative feedback loop between Cdk1 and APC/C-Cdk1 activating the APC/C and the APC/C inactivating Cdk1-constitutes the core of the biochemical cell cycle oscillator.Since its discovery three decades ago, the mechanisms of APC /C regulation have been intensively studied, and several in vitro assays exist to measure the activity of the APC /C in different activation states. However, most of these assays require the purification of numerous recombinant enzymes involved in the ubiquitylation process (e.g., ubiquitin, the E1 and E2 ubiquitin ligases, and the APC /C) and/or the use of radioactive isotopes. In this chapter, we describe an easy-to-implement method to continuously measure APC /C activity in Xenopus laevis egg extracts using APC /C substrates fused to fluorescent proteins and a fluorescence plate reader. Because the egg extract provides all important enzymes and proteins for the reaction, this method can be used largely without the need for recombinant protein purification. It can also easily be adapted to test the activity of APC /C mutants or investigate other mechanisms of APC /C regulation.

Functional reciprocity of proteins involved in mitosis and endocytosis

Mitosis and endocytosis are two fundamental cellular processes essential for maintaining a eukaryotic life. Mitosis partitions duplicated chromatin enveloped in the nuclear membrane into two new cells, whereas endocytosis takes in extracellular substances through membrane invagination. These two processes are spatiotemporally separated and seemingly unrelated. However, recent studies have uncovered that endocytic proteins have moonlighting functions in mitosis, and mitotic complexes manifest additional roles in endocytosis. In this review, we summarize important proteins or protein complexes that participate in both processes, compare their mechanism of action, and discuss the rationale behind this multifunctionality. We also speculate on the possible origin of the functional reciprocity from an evolutionary perspective.

Tank Binding Kinase 1 modulates spindle assembly checkpoint components to regulate mitosis in breast and lung cancer cells

Error-free progression through mitosis is critical for proper cell division and accurate distribution of the genetic material. The anaphase-promoting complex/cyclosome (APC/C) ubiquitin ligase regulates the progression from metaphase to anaphase and its activation is controlled by the cofactors Cdc20 and Cdh1. Additionally, genome stability is maintained by the spindle assembly checkpoint (SAC), which monitors proper attachment of chromosomes to spindle microtubules prior to cell division. We had shown a role for Tank Binding Kinase 1 (TBK1) in microtubule dynamics and mitosis and here we describe a novel role of TBK1 in regulating SAC in breast and lung cancer cells. TBK1 interacts with and phosphorylates Cdc20 and Cdh1 and depletion of TBK1 elevates SAC components. TBK1 inhibition increases the association of Cdc20 with APC/C and BubR1 indicating inactivation of APC/C; similarly, interaction of Cdh1 with APC/C is also enhanced. TBK1 and TTK inhibition reduces cell viability and enhances centrosome amplification and micronucleation. These results indicate that alterations in TBK1 will impede mitotic progression and combining TBK1 inhibitors with other regulators of mitosis might be effective in eliminating cancer cells.

SAC during early cell divisions: Sacrificing fidelity over timely division, regulated differently across organisms: Chromosome alignment and segregation are left unsupervised from the onset of development until checkpoint activity is acquired, varying from species to species

Early embryogenesis is marked by a frail Spindle Assembly Checkpoint (SAC). The time of SAC acquisition varies depending on the species, cell size or a yet to be uncovered developmental timer. This means that for a specific number of divisions, biorientation of sister chromatids occurs unsupervised. When error-prone segregation is an issue, an aneuploidy-selective apoptosis system can come into play to eliminate chromosomally unbalanced cells resulting in healthy newborns. However, aneuploidy content can be too great to overcome, endangering viability. SAC generates a diffusible signal to lengthen time spent in mitosis if needed, ensuring correct chromosome segregation, a fundamental factor in the generation of euploid cells. Thus, it remains puzzling what benefit could come from delaying SAC acquisition till later in the development. In this review, we describe what is known on SAC acquisition in distinct species and highlight pending research as well as potential applications for such knowledge.

Premature aging syndrome showing random chromosome number instabilities with CDC20 mutation

Damage to the genome can accelerate aging. The percentage of aneuploid cells, that is, cells with an abnormal number of chromosomes, increases during aging; however, it is not clear whether increased aneuploidy accelerates aging. Here, we report an individual showing premature aging phenotypes of various organs including early hair loss, atrophic skin, and loss of hematopoietic stem cells; instability of chromosome numbers known as mosaic variegated aneuploidy (MVA); and spindle assembly checkpoint (SAC) failure. Exome sequencing identified a de novo heterozygous germline missense mutation of c.856C>A (p.R286S) in the mitotic activator CDC20. The mutant CDC20 showed lower binding affinity to BUBR1 during the formation of the mitotic checkpoint complex (MCC), but not during the interaction between MCC and the anaphase‐promoting complex/cyclosome (APC/C)–CDC20 complex. While heterozygous knockout of CDC20 did not induce SAC failure, knock‐in of the mutant CDC20 induced SAC failure and random aneuploidy in cultured cells, indicating that the particular missense mutation is pathogenic probably via the resultant imbalance between MCC and APC/C‐CDC20 complex. We postulate that accelerated chromosome number instability induces premature aging in humans, which may be associated with early loss of stem cells. These findings could form the basis of a novel disease model of the aging of the body and organs.

BUBR1 Pseudokinase Domain Promotes Kinetochore PP2A-B56 Recruitment, Spindle Checkpoint Silencing, and Chromosome Alignment

The balance of phospho-signaling at the outer kinetochore is critical for forming accurate attachments between kinetochores and the mitotic spindle and timely exit from mitosis. A major player in determining this balance is the PP2A-B56 phosphatase, which is recruited to the kinase attachment regulatory domain (KARD) of budding uninhibited by benzimidazole 1-related 1 (BUBR1) in a phospho-dependent manner. This unleashes a rapid, switch-like phosphatase relay that reverses mitotic phosphorylation at the kinetochore, extinguishing the checkpoint and promoting anaphase. Here, we demonstrate that the C-terminal pseudokinase domain of human BUBR1 is required to promote KARD phosphorylation. Mutation or removal of the pseudokinase domain results in decreased PP2A-B56 recruitment to the outer kinetochore attenuated checkpoint silencing and errors in chromosome alignment as a result of imbalance in Aurora B activity. Our data, therefore, elucidate a function for the BUBR1 pseudokinase domain in ensuring accurate and timely exit from mitosis.