Molecular mechanism of APC/C activation by mitotic phosphorylation

Hernandezine acts as a CDK4 suppressor inhibiting tumor growth by the CDK4/PKM2/NRF2 axis in colon cancer

BACKGROUND

The cyclin-dependent kinase 4 (CDK4) interacts with its canonical and non-canonical substrates modulating the cell cycle in tumor cells. However, the potential substrates and the beyond-cell-cycle-regulated functions of CDK4 in colon cancer (CC) are still unknown. Hernandezine (HER) is previously verified to induce G0/G1 phase arrest and autophagic cell death in human cancer cells, which implies that HER might target G0/G1 phase-related proteins, including CDK4.

PURPOSE

The present study tried to investigate the glycolytic metabolism and oxidative stress functions of CDK4 in colon cancer. Furthermore, the inhibitory effects and potential binding sites of HER on CDK4, as well as its anti-tumor activity were investigated in CC cells.

METHODS

The mass spectrometry assay was performed to identify potential endogenous substrates of CDK4 and the correlation between glycolytic metabolic rate and CDK4 level in COAD patient tissues. Meanwhile, after inhibiting the activity or the expression of CDK4, the binding capacity of CDK4 to PKM2 and NRF2 and the latter two protein distributions in cytoplasm and nucleus were detected in CC cells. In vitro, the regulatory effects of the CDK4-PKM2-NRF2 axis on glycolysis and oxidative stress were performed by ECAR, OCR, and ROS assay. The inhibitory effect of HER on CDK4 activity was explored in CC cells and the potential binding sites were predicted and testified in vitro. Furthermore, tumor growth inhibition of HER by suppressing the CDK4-PKM2-NRF2 axis was also investigated in vitro and in vivo.

RESULTS

PKM2 and NRF2 were identified as endogenous substrates of CDK4 and, high-expressed CDK4 was associated with low-level glycolysis in COAD. In vitro, inactivated CDK4 facilitated CDK4-PKM2-NRF2 complex formation which resulted in 1) inhibited PKM2 activity and retarded the glycolytic rate; 2) cytoplasm-detained NRF2 failed to transcript anti-oxidative gene expressions and induced oxidant stress. Additionally, as a CDK4 inhibitor, HER developed triple anti-tumor effects including induced G0/G1 phase arrest, suppressed glycolysis, and disrupted the anti-oxidative capacity of CC cells.

CONCLUSION

The results first time revealed that CDK4 modulated glycolytic and anti-oxidative capacity of CC cells via bound to its endogenous substrates, PKM2 and NRF2. Additionally, 140Asp145Asn amino acid sites of CDK4 were potential targets of HER. HER exerts anti-tumor activity by inhibited the activity of CDK4, promoted the CDK4-PKM2-NRF2 complex formation in the CC cells.

The C-terminal disordered loop domain of Apc8 unlocks APC/C mitotic activation

The anaphase-promoting complex/cyclosome (APC/C) is a critical and tightly regulated E3 ligase that orchestrates the cellular life cycle by controlling the degradation of cell cycle regulators. An intriguing feature of this complex is an autoinhibition mechanism: an intrinsically disordered loop domain, Apc1-300L, blocks Cdc20 coactivator binding, yet phosphorylation of Apc1-300L counteracts this autoinhibition. Many such disordered loops within APC/C remain unexplored. Our systematic analysis of loop-deficient APC/C mutants uncovered a pivotal role for Apc8's C-terminal loop (Apc8-L) in mitotic activation. Apc8-L directly recruits the CDK adaptor protein, Xe-p9/Cks2, positioning the Xe-p9-CDK-CycB complex near Apc1-300L. This stimulates the phosphorylation and removal of Apc1-300L, prompting the formation of active APC/CCdc20. Strikingly, without both Apc8-L and Apc3-L, the APC/C is rendered inactive during mitosis, highlighting Apc8-L's synergistic role with other loops and kinases. This study broadens our understanding of the intricate dynamics in APC/C regulation and provides insights on the regulation of macromolecular complexes.

Analysis of nondegradable cyclins reveals distinct roles of the mitotic cyclins in Drosophila meiosis

Meiosis is a complex variant of the mitotic cell cycle, and as such relies on many of the same proteins involved in mitosis, but utilizes these in novel ways. As in mitosis, Cdk1 and its cyclin partners, Cyclin A, B, and B3 are required at multiple steps in meiosis. Here, we study the effect of stabilized forms of the three mitotic cyclins to study the consequences of failure to degrade the cyclins in meiosis. We find that stabilized Cyclin B3 promotes ectopic microtubule polymerization throughout the egg, dependent on APC/C activity and apparently due to the consequent destruction of Cyclin A and Cyclin B. We present data that suggests CycB, and possibly CycA, can also promote APC/C activity at specific stages of meiosis. We also present evidence that in meiosis APC/CCort and APC/CFzy are able to target Cyclin B via a novel degron. Overall, our findings highlight the distinct functions of the three mitotic Cdk-cyclin complexes in meiosis.

CDK1-PP2A-B55 interplay ensures cell cycle oscillation via Apc1-loop300

Cell cycle control relies on a delicate balance of phosphorylation with CDK1 and phosphatases like PP1 and PP2A-B55. Yet, identifying the primary substrate responsible for cell cycle oscillations remains a challenge. We uncover the pivotal role of phospho-regulation in the anaphase-promoting complex/cyclosome (APC/C), particularly through the Apc1-loop300 domain (Apc1-300L), orchestrated by CDK1 and PP2A-B55. Premature activation of PP2A-B55 during mitosis, induced by Greatwall kinase depletion, leads to Apc1-300L dephosphorylation, stalling APC/C activity and delaying Cyclin B degradation. This effect can be counteracted using the B55-specific inhibitor pEnsa or by removing Apc1-300L. We also show Cdc20's dynamic APC/C interaction across cell cycle stages, but dephosphorylation of Apc1-300L specifically inhibits further Cdc20 recruitment. Our study underscores APC/C's central role in cell cycle oscillation, identifying it as a primary substrate regulated by the CDK-PP2A partnership.

Activation of the Anaphase Promoting Complex Restores Impaired Mitotic Progression and Chemosensitivity in Multiple Drug-Resistant Human Breast Cancer

The development of multiple-drug-resistant (MDR) cancer all too often signals the need for toxic alternative therapy or palliative care. Our recent in vivo and in vitro studies using canine MDR lymphoma cancer cells demonstrate that the Anaphase Promoting Complex (APC) is impaired in MDR cells compared to normal canine control and drug-sensitive cancer cells. Here, we sought to establish whether this phenomena is a generalizable mechanism independent of species, malignancy type, or chemotherapy regime. To test the association of blunted APC activity with MDR cancer behavior, we used matched parental and MDR MCF7 human breast cancer cells, and a patient-derived xenograft (PDX) model of human triple-negative breast cancer. We show that APC activating mechanisms, such as APC subunit 1 (APC1) phosphorylation and CDC27/CDC20 protein associations, are reduced in MCF7 MDR cells when compared to chemo-sensitive matched cell lines. Consistent with impaired APC function in MDR cells, APC substrate proteins failed to be effectively degraded. Similar to our previous observations in canine MDR lymphoma cells, chemical activation of the APC using Mad2 Inhibitor-1 (M2I-1) in MCF7 MDR cells enhanced APC substrate degradation and resensitized MDR cells in vitro to the cytotoxic effects of the alkylating chemotherapeutic agent, doxorubicin (DOX). Using cell cycle arrest/release experiments, we show that mitosis is delayed in MDR cells with elevated substrate levels. When pretreated with M2I-1, MDR cells progress through mitosis at a faster rate that coincides with reduced levels of APC substrates. In our PDX model, mice growing a clinically MDR human triple-negative breast cancer tumor show significantly reduced tumor growth when treated with M2I-1, with evidence of increased DNA damage and apoptosis. Thus, our results strongly support the hypothesis that APC impairment is a driver of aggressive tumor development and that targeting the APC for activation has the potential for meaningful clinical benefits in treating recurrent cases of MDR malignancy.

CDK9-55 guides the anaphase-promoting complex/cyclosome (APC/C) in choosing the DNA repair pathway choice

DNA double-strand breaks (DSBs) contribute to genome instability, a key feature of cancer. DSBs are mainly repaired by homologous recombination (HR) and non-homologous end-joining (NHEJ). We investigated the role of an isoform of the multifunctional cyclin-dependent kinase 9, CDK9-55, in DNA repair, by generating CDK9-55-knockout HeLa clones (through CRISPR-Cas9), which showed potential HR dysfunction. A phosphoproteomic screening in these clones treated with camptothecin revealed that CDC23 (cell division cycle 23), a component of the E3-ubiquitin ligase APC/C (anaphase-promoting complex/cyclosome), is a new substrate of CDK9-55, with S588 being its putative phosphorylation site. Mutated non-phosphorylatable CDC23(S588A) affected the repair pathway choice by impairing HR and favouring error-prone NHEJ. This CDK9 role should be considered when designing CDK-inhibitor-based cancer therapies.

Phosphate-binding pocket on cyclin B governs CDK substrate phosphorylation and mitotic timing

Cell cycle progression is governed by complexes of the cyclin-dependent kinases (CDKs) and their regulatory subunits cyclin and Cks1. CDKs phosphorylate hundreds of substrates, often at multiple sites. Multisite phosphorylation depends on Cks1, which binds initial priming phosphorylation sites to promote secondary phosphorylation at other sites. Here, we describe a similar role for a recently discovered phosphate-binding pocket (PP) on B-type cyclins. Mutation of the PP in Clb2, the major mitotic cyclin of budding yeast, alters bud morphology and delays the onset of anaphase. Using phosphoproteomics in vivo and kinase reactions in vitro, we find that mutation of the PP reduces phosphorylation of several CDK substrates, including the Bud6 subunit of the polarisome and the Cdc16 and Cdc27 subunits of the anaphase-promoting complex/cyclosome. We conclude that the cyclin PP, like Cks1, controls the timing of multisite phosphorylation on CDK substrates, thereby helping to establish the robust timing of cell-cycle events.

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.

Identification, characterization and expression profiles of E2 and E3 gene superfamilies during the development of tetrasporophytes in Gracilariopsis lemaneiformis (Rhodophyta)

E2 ubiquitin conjugating enzymes and E3 ubiquitin ligases play important roles in the growth and development of plants and animals. To date, the systematic analysis of E2 and E3 genes in Rhodophyta is limited. In this study, 14 E2 genes and 51 E3 genes were identified in Gracilariopsis lemaneiformis, an economically important red alga. E2 genes were classified into four classes according to the structure of the conserved domain, UBC. E3 genes were classified into 12 subfamilies according to individual conserved domains. A phylogenetic tree of seven algae species showed that functional differentiation of RING-type E3s was the highest, and the similarity between orthologous genes was high except in Chlamydomonas reinhardtii and Chara braunii. RNA-seq data analysis showed significant differential expression levels of E2 and E3 genes under the life stages of tetraspore formation and release, especially GlUBCN and GlAPC3. According to GO and KEGG analysis of two transcriptomes, GlUBCN and GlAPC3 were involved in ubiquitin-mediated proteolysis, and other subunits of the anaphase promoting complex or cyclosome (APC/C) and its activators GlCDC20 and GlCDH1 were also enriched into this process. The CDH1 and CDC20 in 981 were down-regulated during tetraspores formation and release, with the down-regulation of CDH1 being particularly significant; CDH1 and CDC20 in WLP-1, ZC, and WT were up-regulated during tetraspores formation and release, with CDC20 being more significantly up-regulated. Therefore, GlCDH1, rather than GlCDC20, in '981' might play the leading role in the activation of the APC/C, and GlCDC20 might play the leading role rather than GlCDH1 in strains WLP-1, ZC and wild type. The low fertility of cultivar 981 might be highly correlated with the inactivity of activators CDH1 and CDC20. This study provided a basic and comprehensive understanding of characteristic of E2 and E3 genes in Gp. lemaneiformis and set a foundation for further understanding of E2 ubiquitin conjugating enzymes and E3 ubiquitin ligase in regulating tetrasporophytes development of Gp. lemaneiformis.

The diverging role of CDC14B: from mitotic exit in yeast to cell fate control in humans

CDC14, originally identified as crucial mediator of mitotic exit in budding yeast, belongs to the family of dual-specificity phosphatases (DUSPs) that are present in most eukaryotes. Contradicting data have sparked a contentious discussion whether a cell cycle role is conserved in the human paralogs CDC14A and CDC14B but possibly masked due to redundancy. Subsequent studies on CDC14A and CDC14B double knockouts in human and mouse demonstrated that CDC14 activity is dispensable for mitotic progression in higher eukaryotes and instead suggested functional specialization. In this review, we provide a comprehensive overview of our current understanding of how faithful cell division is linked to phosphorylation and dephosphorylation and compare functional similarities and divergences between the mitotic phosphatases CDC14, PP2A, and PP1 from yeast and higher eukaryotes. Furthermore, we review the latest discoveries on CDC14B, which identify this nuclear phosphatase as a key regulator of gene expression and reveal its role in neuronal development. Finally, we discuss CDC14B functions in meiosis and possible implications in other developmental processes.

Cyclin B3 is a dominant fast-acting cyclin that drives rapid early embryonic mitoses

In many species, early embryonic mitoses proceed at a very rapid pace, but how this pace is achieved is not understood. Here we show that in the early C. elegans embryo, cyclin B3 is the dominant driver of rapid embryonic mitoses. Metazoans typically have three cyclin B isoforms that associate with and activate Cdk1 kinase to orchestrate mitotic events: the related cyclins B1 and B2 and the more divergent cyclin B3. We show that whereas embryos expressing cyclins B1 and B2 support slow mitosis (NEBD to Anaphase ~ 600s), the presence of cyclin B3 dominantly drives the ~3-fold faster mitosis observed in wildtype embryos. CYB-1/2-driven mitosis is longer than CYB-3-driven mitosis primarily because the progression of mitotic events itself is slower, rather than delayed anaphase onset due to activation of the spindle checkpoint or inhibitory phosphorylation of the anaphase activator CDC-20. Addition of cyclin B1 to cyclin B3-only mitosis introduces an ~60s delay between the completion of chromosome alignment and anaphase onset, which likely ensures segregation fidelity; this delay is mediated by inhibitory phosphorylation on CDC-20. Thus, the dominance of cyclin B3 in driving mitotic events, coupled to introduction of a short cyclin B1-dependent delay in anaphase onset, sets the rapid pace and ensures fidelity of mitoses in the early C. elegans embryo.

USP39-mediated deubiquitination of Cyclin B1 promotes tumor cell proliferation and glioma progression

BACKGROUND

The elevated Cyclin B1 expression contributes to various tumorigenesis and poor prognosis. Cyclin B1 expression could be regulated by ubiquitination and deubiquitination. However, the mechanism of how Cyclin B1 is deubiquitinated and its roles in human glioma remain unclear.

METHODS

Co-immunoprecipitation and other assays were performed to detect the interacting of Cyclin B1 and USP39. A series of in vitro and in vivo experiments were performed to investigate the effect of USP39 on the tumorigenicity of tumor cells.

RESULTS

USP39 interacts with Cyclin B1 and stabilizes its expression by deubiquitinating Cyclin B1. Notably, USP39 cleaves the K29-linked polyubiquitin chain on Cyclin B1 at Lys242. Additionally, overexpression of Cyclin B1 rescues the arrested cell cycle at G2/M transition and the suppressed proliferation of glioma cells caused by USP39 knockdown in vitro. Furthermore, USP39 promotes the growth of glioma xenograft in subcutaneous and in situ of nude mice. Finally, in human tumor specimens, the expression levels of USP39 and Cyclin B1 are positively relevant.

CONCLUSION

Our data support the evidence that USP39 acts a novel deubiquitinating enzyme of Cyclin B1 and promoted tumor cell proliferation at least in part through Cyclin B1 stabilization, represents a promising therapeutic strategy for tumor patients.

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.

Multiple Roles of PLK1 in Mitosis and Meiosis

Cells are equipped with a diverse network of signaling and regulatory proteins that function as cell cycle regulators and checkpoint proteins to ensure the proper progression of cell division. A key regulator of cell division is polo-like kinase 1 (PLK1), a member of the serine/threonine kinase family that plays an important role in regulating the mitotic and meiotic cell cycle. The phosphorylation of specific substrates mediated by PLK1 controls nuclear envelope breakdown (NEBD), centrosome maturation, proper spindle assembly, chromosome segregation, and cytokinesis. In mammalian oogenesis, PLK1 is essential for resuming meiosis before ovulation and for establishing the meiotic spindle. Among other potential roles, PLK1 regulates the localized translation of spindle-enriched mRNAs by phosphorylating and thereby inhibiting the translational repressor 4E-BP1, a downstream target of the mTOR (mammalian target of rapamycin) pathway. In this review, we summarize the functions of PLK1 in mitosis, meiosis, and cytokinesis and focus on the role of PLK1 in regulating mRNA translation. However, knowledge of the role of PLK1 in the regulation of meiosis remains limited.

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.

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.

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.

Structural, enzymatic and spatiotemporal regulation of PP2A-B55 phosphatase in the control of mitosis

Cells require major physical changes to induce a proper repartition of the DNA. Nuclear envelope breakdown, DNA condensation and spindle formation are promoted at mitotic entry by massive protein phosphorylation and reversed at mitotic exit by the timely and ordered dephosphorylation of mitotic substrates. This phosphorylation results from the balance between the activity of kinases and phosphatases. The role of kinases in the control of mitosis has been largely studied, however, the impact of phosphatases has long been underestimated. Recent data have now established that the regulation of phosphatases is crucial to confer timely and ordered cellular events required for cell division. One major phosphatase involved in this process is the phosphatase holoenzyme PP2A-B55. This review will be focused in the latest structural, biochemical and enzymatic insights provided for PP2A-B55 phosphatase as well as its regulators and mechanisms of action.

Upregulation of Cell Division Cycle 20 Expression Alters the Morphology of Neuronal Dendritic Spines in the Nucleus Accumbens by Promoting FMRP Ubiquitination

The nucleus accumbens (NAc) is the key area of the reward circuit, but its heterogeneity has been poorly studied. Using single-cell RNA sequencing, we revealed a subcluster of GABAergic neurons characterized by cell division cycle 20 (Cdc20) mRNA expression in the NAc of adult rats. We studied the coexpression of Cdc20 and Gad1 mRNA in the NAc neurons of adult rats and assessed Cdc20 protein expression in the NAc during rat development. Moreover, we microinjected AAV2/9-hSyn-Cdc20 with or without the dual-AAV system into the bilateral NAc for sparse labelling to observe changes in the synaptic morphology of mature neurons and assessed rat behaviours in open field and elevated plus maze tests. Furthermore, we performed the experiments with a Cdc20 inhibitor, Cdc20 overexpression AAV vector, and Cdc20 conditional knockout primary striatal neurons to understand the ubiquitination-dependent degradation of fragile X mental retardation protein (FMRP) in vitro and in vivo. We confirmed the mRNA expression of Cdc20 in the NAc GABAergic neurons of adult rats, and its protein level was decreased significantly 3 weeks post-birth. Upregulated Cdc20 expression in the bilateral NAc decreased the dendritic spine density in mature neurons and induced anxiety-like behaviour in rats. Cdc20-APC triggered FMRP degradation through K48-linked polyubiquitination in Neuro-2a cells and primary striatal neurons and downregulated FMRP expression in the NAc of adult rats. These data revealed that upregulation of Cdc20 in the bilateral NAc reduced dendritic spine density and led to anxiety-like behaviours, possibly by enhancing FMRP degradation via K48-linked polyubiquitination.

The spindle assembly checkpoint and the spatial activation of Polo kinase determine the duration of cell division and prevent tumor formation

The maintenance of a restricted pool of asymmetrically dividing stem cells is essential for tissue homeostasis. This process requires the control of mitotic progression that ensures the accurate chromosome segregation. In addition, this event is coupled to the asymmetric distribution of cell fate determinants in order to prevent stem cell amplification. How this coupling is regulated remains poorly described. Here, using asymmetrically dividing Drosophila neural stem cells (NSCs), we show that Polo kinase activity levels determine timely Cyclin B degradation and mitotic progression independent of the spindle assembly checkpoint (SAC). This event is mediated by the direct phosphorylation of Polo kinase by Aurora A at spindle poles and Aurora B kinases at centromeres. Furthermore, we show that Aurora A-dependent activation of Polo is the major event that promotes NSC polarization and together with the SAC prevents brain tumor growth. Altogether, our results show that an Aurora/Polo kinase module couples NSC mitotic progression and polarization for tissue homeostasis.

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.

Single-molecule analysis of specificity and multivalency in binding of short linear substrate motifs to the APC/C

Robust regulatory signals in the cell often depend on interactions between short linear motifs (SLiMs) and globular proteins. Many of these interactions are poorly characterized because the binding proteins cannot be produced in the amounts needed for traditional methods. To address this problem, we developed a single-molecule off-rate (SMOR) assay based on microscopy of fluorescent ligand binding to immobilized protein partners. We used it to characterize substrate binding to the Anaphase-Promoting Complex/Cyclosome (APC/C), a ubiquitin ligase that triggers chromosome segregation. We find that SLiMs in APC/C substrates (the D box and KEN box) display distinct affinities and specificities for the substrate-binding subunits of the APC/C, and we show that multiple SLiMs in a substrate generate a high-affinity multivalent interaction. The remarkably adaptable substrate-binding mechanisms of the APC/C have the potential to govern the order of substrate destruction in mitosis.

The APC/C targets the Cep152-Cep63 complex at the centrosome to regulate mitotic spindle assembly

The control of protein abundance is a fundamental regulatory mechanism during mitosis. The anaphase-promoting complex/cyclosome (APC/C) is the main protein ubiquitin ligase responsible for the temporal regulation of mitotic progression. It has been proposed that the APC/C might fulfil other functions, including assembly of the mitotic spindle. Here, we show that the APC/C localizes to centrosomes, the organizers of the eukaryotic microtubule cytoskeleton, specifically during mitosis. Recruitment of the APC/C to spindle poles requires the centrosomal protein Cep152, and we identified Cep152 as both an APC/C interaction partner and an APC/C substrate. Previous studies have shown that Cep152 forms a complex with Cep57 and Cep63. The APC/C-mediated ubiquitylation of Cep152 at the centrosome releases Cep57 from this inhibitory complex and enables its interaction with pericentrin, a critical step in promoting microtubule nucleation. Thus, our study extends the function of the APC/C from being a regulator of mitosis to also acting as a positive governor of spindle assembly. The APC/C thereby integrates control of these two important processes in a temporal manner.

APC7 mediates ubiquitin signaling in constitutive heterochromatin in the developing mammalian brain

Neurodevelopmental cognitive disorders provide insights into mechanisms of human brain development. Here, we report an intellectual disability syndrome caused by the loss of APC7, a core component of the E3 ubiquitin ligase anaphase promoting complex (APC). In mechanistic studies, we uncover a critical role for APC7 during the recruitment and ubiquitination of APC substrates. In proteomics analyses of the brain from mice harboring the patient-specific APC7 mutation, we identify the chromatin-associated protein Ki-67 as an APC7-dependent substrate of the APC in neurons. Conditional knockout of the APC coactivator protein Cdh1, but not Cdc20, leads to the accumulation of Ki-67 protein in neurons in vivo, suggesting that APC7 is required for the function of Cdh1-APC in the brain. Deregulated neuronal Ki-67 upon APC7 loss localizes predominantly to constitutive heterochromatin. Our findings define an essential function for APC7 and Cdh1-APC in neuronal heterochromatin regulation, with implications for understanding human brain development and disease.

Deficiency of GPI Glycan Modification by Ethanolamine Phosphate Results in Increased Adhesion and Immune Resistance of Aspergillus fumigatus

Glycosylphosphatidylinositol (GPI)-anchored proteins play important roles in maintaining the function of the cell wall and participating in pathogenic processes. The addition and removal of phosphoethanolamine (EtN-P) on the second mannose residue in the GPI anchor are vital for maturation and sorting of GPI-anchored proteins. Previously, we have shown that deletion of the gpi7, the gene that encodes an EtN-P transferase responsible for the addition of EtN-P to the second mannose residue of the GPI anchor, leads to the mislocalization of GPI-anchored proteins, abnormal polarity, reduced conidiation, and fast germination in Aspergillus fumigatus. In this report, the adherence and virulence of the A. fumigatus gpi7 deletion mutant were further investigated. The germinating conidia of the mutant exhibited an increased adhesion and a higher exposure of cell wall polysaccharides. Although the virulence was not affected, an increased adherence and a stronger inflammation response of the mutant were documented in an immunocompromised mouse model. An in vitro assay confirmed that the Δgpi7 mutant induced a stronger immune response and was more resistant to killing. Our findings, for the first time, demonstrate that in A. fumigatus, GPI anchoring is required for proper organization of the conidial cell wall. The lack of Gpi7 leads to fast germination, stronger immune response, and resistance to macrophage killing.

Reconstitution and use of highly active human CDK1:Cyclin-B:CKS1 complexes

As dividing cells transition into mitosis, hundreds of proteins are phosphorylated by a complex of cyclin‐dependent kinase 1 (CDK1) and Cyclin‐B, often at multiple sites. CDK1:Cyclin‐B phosphorylation patterns alter conformations, interaction partners, and enzymatic activities of target proteins and need to be recapitulated in vitro for the structural and functional characterization of the mitotic protein machinery. This requires a pure and active recombinant kinase complex. The kinase activity of CDK1 critically depends on the phosphorylation of a Threonine residue in its activation loop by a CDK1‐activating kinase (CAK). We developed protocols to activate CDK1:Cyclin‐B either in vitro with purified CAKs or in insect cells through CDK‐CAK co‐expression. To boost kinase processivity, we reconstituted a ternary complex consisting of CDK1, Cyclin‐B, and CKS1. In this work, we provide and compare detailed protocols to obtain and use highly active CDK1:Cyclin‐B (CC) and CDK1:Cyclin‐B:CKS1 (CCC).

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.

Dynamic regulation of mitotic ubiquitin ligase APC/C by coordinated Plx1 kinase and PP2A phosphatase action on a flexible Apc1 loop

The anaphase-promoting complex/cyclosome (APC/C), a multi-subunit ubiquitin ligase essential for cell cycle control, is regulated by reversible phosphorylation. APC/C phosphorylation by cyclin-dependent kinase 1 (Cdk1) promotes Cdc20 co-activator loading in mitosis to form active APC/C-Cdc20. However, detailed phospho-regulation of APC/C dynamics through other kinases and phosphatases is still poorly understood. Here, we show that an interplay between polo-like kinase (Plx1) and PP2A-B56 phosphatase on a flexible loop domain of the subunit Apc1 (Apc1-loop500 ) controls APC/C activity and mitotic progression. Plx1 directly binds to the Apc1-loop500 in a phosphorylation-dependent manner and promotes the formation of APC/C-Cdc20 via Apc3 phosphorylation. Upon phosphorylation of loop residue T532, PP2A-B56 is recruited to the Apc1-loop500 and differentially promotes dissociation of Plx1 and PP2A-B56 through dephosphorylation of Plx1-binding sites. Stable Plx1 binding, which prevents PP2A-B56 recruitment, prematurely activates the APC/C and delays APC/C dephosphorylation during mitotic exit. Furthermore, the phosphorylation status of the Apc1-loop500 is controlled by distant Apc3-loop phosphorylation. Our study suggests that phosphorylation-dependent feedback regulation through flexible loop domains within a macromolecular complex coordinates the activity and dynamics of the APC/C during the cell cycle.

Identification of novel microtubule inhibitors effective in fission yeast and human cells and their effects on breast cancer cell lines

Microtubules are critical for a variety of cellular processes such as chromosome segregation, intracellular transport and cell shape. Drugs against microtubules have been widely used in cancer chemotherapies, though the acquisition of drug resistance has been a significant issue for their use. To identify novel small molecules that inhibit microtubule organization, we conducted sequential phenotypic screening of fission yeast and human cells. From a library of diverse 10 371 chemicals, we identified 11 compounds that inhibit proper mitotic progression both in fission yeast and in HeLa cells. An in vitro assay revealed that five of these compounds are strong inhibitors of tubulin polymerization. These compounds directly bind tubulin and destabilize the structures of tubulin dimers. We showed that one of the compounds, L1, binds to the colchicine-binding site of microtubules and exhibits a preferential potency against a panel of human breast cancer cell lines compared with a control non-cancer cell line. In addition, L1 overcomes cellular drug resistance mediated by βIII tubulin overexpression and has a strong synergistic effect when combined with the Plk1 inhibitor BI2536. Thus, we have established an economically effective drug screening strategy to target mitosis and microtubules, and have identified a candidate compound for cancer chemotherapy.

Transcriptome sequencing and screening of genes related to glucose availability in Schizosaccharomyces pombe by RNA-seq analysis

While calorie restriction is the most used experimental intervention to increase lifespan in numerous model organisms, increasing evidence suggests that excess glucose leads to decreased lifespan in various organisms. To fully understand the molecular basis of the pro-aging effect of glucose, it is still important to discover genetic interactions, gene expression patterns, and molecular responses depending on glucose availability. Here, we compared the gene expression profiles in Schizosaccharomyces pombe mid-log-phase cells grown in three different Synthetic Dextrose media with 3%, 5%, and 8% glucose, using the RNA sequencing method. Expression patterns of genes that function in carbohydrate metabolism were downregulated as expected, and these genes were downregulated in line with the increase in glucose content. Significant and consistent changes in the expression were observed such as genes that encoding retrotransposable elements, heat shock proteins, glutathione S-transferase, cell agglutination protein, and conserved fungal proteins. We group some genes that function together in the transcription process and mitotic regulation, which have recently been associated with glucose availability. Our results shed light on the relationship between excess glucose, diverse cellular processes, and aging.

Structural basis of human separase regulation by securin and CDK1-cyclin B1

In early mitosis, the duplicated chromosomes are held together by the ring-shaped cohesin complex1. Separation of chromosomes during anaphase is triggered by separase-a large cysteine endopeptidase that cleaves the cohesin subunit SCC1 (also known as RAD212-4). Separase is activated by degradation of its inhibitors, securin5 and cyclin B6, 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 elegans7 and yeast8, 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 CDK19,10. Another autoinhibitory loop blocks substrate docking in a cleft adjacent to the separase catalytic site. A third separase loop contains a phosphoserine6 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.

Cell cycle regulation by complex nanomachines

The cell cycle is the essential biological process where one cell replicates its genome and segregates the resulting two copies into the daughter cells during mitosis. Several aspects of this process have fascinated humans since the nineteenth century. Today, the cell cycle is exhaustively investigated because of its profound connections with human diseases and cancer. At the heart of the molecular network controlling the cell cycle, we find the cyclin-dependent kinases (CDKs) acting as an oscillator to impose an orderly and highly regulated progression through the different cell cycle phases. This oscillator integrates both internal and external signals via a multitude of signalling pathways involving posttranslational modifications including phosphorylation, protein ubiquitination and mechanisms of transcriptional regulation. These tasks are specifically performed by multi-subunit complexes, which are intensively studied both biochemically and structurally with the aim to unveil mechanistic insights into their molecular function. The scope of this review is to summarise the structural biology of the cell cycle machinery, with specific focus on the core cell cycle machinery involving the CDK-cyclin oscillator. We highlight the contribution of cryo-electron microscopy, which has started to revolutionise our understanding of the molecular function and dynamics of the key players of the cell cycle.

CDC20 and PTTG1 are Important Biomarkers and Potential Therapeutic Targets for Metastatic Prostate Cancer

INTRODUCTION

Metastatic prostate cancer (mPCa) is responsible for most prostate cancer (PCa) deaths worldwide. The present study aims to explore the molecular differences between mPCa and PCa.

METHODS

The authors downloaded GSE6752, GSE6919, and GSE32269 from the Gene Expression Omnibus and employed integrated analysis to identify differentially expressed genes (DEGs) between mPCa and PCa. Functional and pathway-enrichment analyses were performed, and a protein-protein interaction (PPI) network and modules were constructed. Clinical mPCa specimens were collected to verify the results by performing RT-qPCR. The Cancer Genome Atlas database was used to conduct a survival analysis, and an immunohistochemical assay was performed. The invasion ability of PCa cells was verified by Transwell assay.

RESULTS

One-hundred six consistently DEGs were found in mPCa compared with PCa. DEGs significantly enriched the positive regulation of cell proliferation, cell division, and cell adhesion in small cell lung cancer and PCa. Cell division, nucleoplasm, and cell cycle were selected from the PPI network, and the top 10 hub genes were selected. CDC20 and PTTG1 with genetic alterations were significantly associated with poorer disease-free survival. Immunohistochemical assay results showed that the expression levels of CDC20 and PTTG1 in mPCa were higher than those in PCa. The results of the migration assay indicated that CDC20 and PTTG1 could enhance the migration ability of PCa cells.

CONCLUSION

The present study revealed that CDC20 and PTTG1 contribute more to migration, progression, and poorer prognoses in mPCa compared with PCa. CDC20 and PTTG1 could represent therapeutic targets in mPCa medical research and clinical studies.

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.

Molecular mechanisms of APC/C release from spindle assembly checkpoint inhibition by APC/C SUMOylation

The anaphase-promoting complex/cyclosome (APC/C) is an E3 ubiquitin ligase that controls cell cycle transitions. Its regulation by the spindle assembly checkpoint (SAC) is coordinated with the attachment of sister chromatids to the mitotic spindle. APC/C SUMOylation on APC4 ensures timely anaphase onset and chromosome segregation. To understand the structural and functional consequences of APC/C SUMOylation, we reconstituted SUMOylated APC/C for electron cryo-microscopy and biochemical analyses. SUMOylation of the APC/C causes a substantial rearrangement of the WHB domain of APC/C's cullin subunit (APC2WHB). Although APC/CCdc20 SUMOylation results in a modest impact on normal APC/CCdc20 activity, repositioning APC2WHB reduces the affinity of APC/CCdc20 for the mitotic checkpoint complex (MCC), the effector of the SAC. This attenuates MCC-mediated suppression of APC/CCdc20 activity, allowing for more efficient ubiquitination of APC/CCdc20 substrates in the presence of the MCC. Thus, SUMOylation stimulates the reactivation of APC/CCdc20 when the SAC is silenced, contributing to timely anaphase onset.

A proteomics approach for the identification of cullin-9 (CUL9) related signaling pathways in induced pluripotent stem cell models

CUL9 is a non-canonical and poorly characterized member of the largest family of E3 ubiquitin ligases known as the Cullin RING ligases (CRLs). Most CRLs play a critical role in developmental processes, however, the role of CUL9 in neuronal development remains elusive. We determined that deletion or depletion of CUL9 protein causes aberrant formation of neural rosettes, an in vitro model of early neuralization. In this study, we applied mass spectrometric approaches in human pluripotent stem cells (hPSCs) and neural progenitor cells (hNPCs) to identify CUL9 related signaling pathways that may contribute to this phenotype. Through LC-MS/MS analysis of immunoprecipitated endogenous CUL9, we identified several subunits of the APC/C, a major cell cycle regulator, as potential CUL9 interacting proteins. Knockdown of the APC/C adapter protein FZR1 resulted in a significant increase in CUL9 protein levels, however, CUL9 does not appear to affect protein abundance of APC/C subunits and adapters or alter cell cycle progression. Quantitative proteomic analysis of CUL9 KO hPSCs and hNPCs identified protein networks related to metabolic, ubiquitin degradation, and transcriptional regulation pathways that are disrupted by CUL9 deletion in both hPSCs. No significant changes in oxygen consumption rates or ATP production were detected in either cell type. The results of our study build on current evidence that CUL9 may have unique functions in different cell types and that compensatory mechanisms may contribute to the difficulty of identifying CUL9 substrates.

Mechanisms of signalling-memory governing progression through the eukaryotic cell cycle

As cells pass through each replication-division cycle, they must be able to postpone further progression if they detect any threats to genome integrity, such as DNA damage or misaligned chromosomes. Once a 'decision' is made to proceed, the cell unequivocally enters into a qualitatively different biochemical state, which makes the transitions from one cell cycle phase to the next switch-like and irreversible. Each transition is governed by a unique signalling network; nonetheless, they share a common characteristic of bistable behaviour, a hallmark of molecular memory devices. Comparing the cell cycle signalling mechanisms acting at the restriction point, G1/S, G2/M and meta-to-anaphase transitions, we deduce a generic network motif of coupled positive and negative feedback loops underlying each transition.

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.

Bistable, Biphasic Regulation of PP2A-B55 Accounts for the Dynamics of Mitotic Substrate Phosphorylation

The phosphorylation of mitotic proteins is bistable, which contributes to the decisiveness of the transitions into and out of M phase. The bistability in substrate phosphorylation has been attributed to bistability in the activation of the cyclin-dependent kinase Cdk1. However, more recently it has been suggested that bistability also arises from positive feedback in the regulation of the Cdk1-counteracting phosphatase PP2A-B55. Here, we demonstrate biochemically using Xenopus laevis egg extracts that the Cdk1-counter-acting phosphatase PP2A-B55 functions as a bistable switch, even when the bistability of Cdk1 activation is suppressed. In addition, Cdk1 regulates PP2A-B55 in a biphasic manner; low concentrations of Cdk1 activate PP2A-B55 and high concentrations inactivate it. As a consequence of this incoherent feedforward regulation, PP2A-B55 activity rises concurrently with Cdk1 activity during interphase and suppresses substrate phosphorylation. PP2A-B55 activity is then sharply downregulated at the onset of mitosis. During mitotic exit, Cdk1 activity initially falls with no obvious change in substrate phosphorylation; dephosphorylation then commences once PP2A-B55 spikes in activity. These findings suggest that changes in Cdk1 activity are permissive for mitotic entry and exit but that the changes in PP2A-B55 activity are the ultimate trigger.

Mitotic transitions are accompanied by drastic changes in the phosphorylation state of proteins. Kamenz et al. demonstrate biochemically that the major mitotic phosphatase PP2A-B55 is regulated by incoherent feedforward and double-negative feedback loops to promote rapid and switch-like mitotic entry and exit.

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.

Cdc20 induces the radioresistance of bladder cancer cells by targeting FoxO1 degradation

Ionizing radiation is a conventional therapy for cancer patients, but patients often experience distant metastasis and recurrence, which lead to a poor prognosis after the implementation of this treatment. Moreover, the underlying mechanisms by which radioresistance contributes to metastatic potential is still elusive. Here, we explored the molecular mechanisms that contribute to radioresistance in bladder cancer. To achieve this, we established two irradiation-resistant (IR) cell lines, T24R and 5637R, which were derived from parental bladder cancer cell lines. Cell viability was detected by CCK-8 assay, while migration and invasion abilities were examined by wound healing and Transwell chamber assays, respectively. Furthermore, the role of Cdc20 in the regulation of epithelial to mesenchymal transition (EMT) in IR cells was explored by Western blotting, immunoprecipitation and immunofluorescence staining. The IR cells exhibited EMT properties, and our data showed that Cdc20 expression was significantly elevated in IR cells. Remarkably, Cdc20 silencing reversed the EMT phenotype in IR cells. Mechanistically, Cdc20 governed IR-mediated EMT in part by governing forkhead box O1 (FoxO1) degradation. Taken together, our findings showed that the inactivation of Cdc20 or the activation of FoxO1 might be a potential strategy to overcome radioresistance in bladder cancer.

Cyclin B3 activates the Anaphase-Promoting Complex/Cyclosome in meiosis and mitosis

In mitosis and meiosis, chromosome segregation is triggered by the Anaphase-Promoting Complex/Cyclosome (APC/C), a multi-subunit ubiquitin ligase that targets proteins for degradation, leading to the separation of chromatids. APC/C activation requires phosphorylation of its APC3 and APC1 subunits, which allows the APC/C to bind its co-activator Cdc20. The identity of the kinase(s) responsible for APC/C activation in vivo is unclear. Cyclin B3 (CycB3) is an activator of the Cyclin-Dependent Kinase 1 (Cdk1) that is required for meiotic anaphase in flies, worms and vertebrates. It has been hypothesized that CycB3-Cdk1 may be responsible for APC/C activation in meiosis but this remains to be determined. Using Drosophila, we found that mutations in CycB3 genetically enhance mutations in tws, which encodes the B55 regulatory subunit of Protein Phosphatase 2A (PP2A) known to promote mitotic exit. Females heterozygous for CycB3 and tws loss-of-function alleles lay embryos that arrest in mitotic metaphase in a maternal effect, indicating that CycB3 promotes anaphase in mitosis in addition to meiosis. This metaphase arrest is not due to the Spindle Assembly Checkpoint (SAC) because mutation of mad2 that inactivates the SAC does not rescue the development of embryos from CycB3-/+, tws-/+ females. Moreover, we found that CycB3 promotes APC/C activity and anaphase in cells in culture. We show that CycB3 physically associates with the APC/C, is required for phosphorylation of APC3, and promotes APC/C association with its Cdc20 co-activators Fizzy and Cortex. Our results strongly suggest that CycB3-Cdk1 directly activates the APC/C to promote anaphase in both meiosis and mitosis.

Ubiquitin signaling in cell cycle control and tumorigenesis

Cell cycle progression is a tightly regulated process by which DNA replicates and cell reproduces. The major driving force underlying cell cycle progression is the sequential activation of cyclin-dependent kinases (CDKs), which is achieved in part by the ubiquitin-mediated proteolysis of their cyclin partners and kinase inhibitors (CKIs). In eukaryotic cells, two families of E3 ubiquitin ligases, anaphase-promoting complex/cyclosome and Skp1-Cul1-F-box protein complex, are responsible for ubiquitination and proteasomal degradation of many of these CDK regulators, ensuring cell cycle progresses in a timely and precisely regulated manner. In the past couple of decades, accumulating evidence have demonstrated that the dysregulated cell cycle transition caused by inefficient proteolytic control leads to uncontrolled cell proliferation and finally results in tumorigenesis. Based upon this notion, targeting the E3 ubiquitin ligases involved in cell cycle regulation is expected to provide novel therapeutic strategies for cancer treatment. Thus, a better understanding of the diversity and complexity of ubiquitin signaling in cell cycle regulation will shed new light on the precise control of the cell cycle progression and guide anticancer drug development.

PP1 promotes cyclin B destruction and the metaphase-anaphase transition by dephosphorylating CDC20

Ubiquitin-dependent proteolysis of cyclin B and securin initiates sister chromatid segregation and anaphase. The anaphase-promoting complex/cyclosome and its coactivator CDC20 (APC/CCDC20) form the main ubiquitin E3 ligase for these two proteins. APC/CCDC20 is regulated by CDK1-cyclin B and counteracting PP1 and PP2A family phosphatases through modulation of both activating and inhibitory phosphorylation. Here, we report that PP1 promotes cyclin B destruction at the onset of anaphase by removing specific inhibitory phosphorylation in the N-terminus of CDC20. Depletion or chemical inhibition of PP1 stabilizes cyclin B and results in a pronounced delay at the metaphase-to-anaphase transition after chromosome alignment. This requirement for PP1 is lost in cells expressing CDK1 phosphorylation-defective CDC206A mutants. These CDC206A cells show a normal spindle checkpoint response and rapidly destroy cyclin B once all chromosomes have aligned and enter into anaphase in the absence of PP1 activity. PP1 therefore facilitates the metaphase-to-anaphase transition by promoting APC/CCDC20-dependent destruction of cyclin B in human cells.

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

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

The Nuclear Lamina: Protein Accumulation and Disease

Cellular health is reliant on proteostasis-the maintenance of protein levels regulated through multiple pathways modulating protein synthesis, degradation and clearance. Loss of proteostasis results in serious disease and is associated with aging. One proteinaceous structure underlying the nuclear envelope-the nuclear lamina-coordinates essential processes including DNA repair, genome organization and epigenetic and transcriptional regulation. Loss of proteostasis within the nuclear lamina results in the accumulation of proteins, disrupting these essential functions, either via direct interactions of protein aggregates within the lamina or by altering systems that maintain lamina structure. Here we discuss the links between proteostasis and disease of the nuclear lamina, as well as how manipulating specific proteostatic pathways involved in protein clearance could improve cellular health and prevent/reverse disease.

Comparative analysis of transcriptomes revealed the molecular mechanism of development of Tricholoma matsutake at different stages of fruiting bodies

The purpose of the study is to investigate the molecular mechanisms of development of Tricholoma matsutake fruiting body at the primordial stage (TM-1), the intermediate stage (TM-2) and the mature stage (TM-3) using RNA-Seq sequencing technology. The analysis of gene expression level revealed that the Spn2 and Eef1a1 gene were the key genes in the primordial stage of T. matsutake by regulating cytokinesis, protein synthesis, and cell growth. And the Ubc, Atp6, Cytb, and Pth2 gene were the key genes in the mature stage of T. matsutake by regulating energy metabolism and protein synthesis. Differential expression genes (DEGs) analysis results showed that Cdc28, Rad53, Dun1, Pho85 and Pho81 were the key DEGs regulating cell cycle genes of T. matsutake from primordial stage to intermediate stage. And APC, Cyr1, Cdc45, Spo11 and Rec8 genes were the key DEGs for the meiosis and sporogenesis of T. matsutake from the intermediate stage to the mature stage.

A unique binding mode of Nek2A to the APC/C allows its ubiquitination during prometaphase

The anaphase-promoting complex (APC/C) is the key E3 ubiquitin ligase which directs mitotic progression and exit by catalysing the sequential ubiquitination of specific substrates. The activity of the APC/C in mitosis is restrained by the spindle assembly checkpoint (SAC), which coordinates chromosome segregation with the assembly of the mitotic spindle. The SAC effector is the mitotic checkpoint complex (MCC), which binds and inhibits the APC/C. It is incompletely understood how the APC/C switches substrate specificity in a cell cycle-specific manner. For instance, it is unclear how in prometaphase, when APC/C activity towards cyclin B and securin is repressed by the MCC, the kinase Nek2A is ubiquitinated. Here, we combine biochemical and structural analysis with functional studies in cells to show that Nek2A is a conformational-specific binder of the APC/C-MCC complex (APC/CMCC ) and that, in contrast to cyclin A, Nek2A can be ubiquitinated efficiently by the APC/C in conjunction with both the E2 enzymes UbcH10 and UbcH5. We propose that these special features of Nek2A allow its prometaphase-specific ubiquitination.

Cyclin B1-Cdk1 facilitates MAD1 release from the nuclear pore to ensure a robust spindle checkpoint

How the cell rapidly and completely reorganizes its architecture when it divides is a problem that has fascinated researchers for almost 150 yr. We now know that the core regulatory machinery is highly conserved in eukaryotes, but how these multiple protein kinases, protein phosphatases, and ubiquitin ligases are coordinated in space and time to remodel the cell in a matter of minutes remains a major question. Cyclin B1-Cdk is the primary kinase that drives mitotic remodeling; here we show that it is targeted to the nuclear pore complex (NPC) by binding an acidic face of the kinetochore checkpoint protein, MAD1, where it coordinates NPC disassembly with kinetochore assembly. Localized cyclin B1-Cdk1 is needed for the proper release of MAD1 from the embrace of TPR at the nuclear pore so that it can be recruited to kinetochores before nuclear envelope breakdown to maintain genomic stability.

Ubiquitin chain-elongating enzyme UBE2S activates the RING E3 ligase APC/C for substrate priming

The interplay between E2 and E3 enzymes regulates the polyubiquitination of substrates in eukaryotes. Among the several RING-domain E3 ligases in humans, many utilize two distinct E2s for polyubiquitination. For example, the cell cycle regulatory E3, human Anaphase-Promoting Complex/Cyclosome (APC/C), relies on UBE2C to prime substrates with ubiquitin (Ub) and UBE2S to extend polyubiquitin chains. However, the potential coordination between these steps in ubiquitin chain formation remains undefined. While numerous studies have unveiled how RING E3s stimulate individual E2s for Ub transfer, here we change perspective to describe a case where the chain-elongating E2 UBE2S feeds back and directly stimulates the E3 APC/C to promote substrate priming and subsequent multiubiquitination by UBE2C. Our work reveals an unexpected paradigm for the mechanisms of RING E3-dependent ubiquitination and for the diverse and complex interrelationship between components of the ubiquitination cascade.