Fcp1 phosphatase controls Greatwall kinase to promote PP2A-B55 activation and mitotic progression

Targeted inhibition of the methyltransferase SETD8 synergizes with the Wee1 inhibitor adavosertib in restraining glioblastoma growth

Despite intense research efforts, glioblastoma remains an incurable brain tumor with a dismal median survival time of 15 months. Thus, identifying new therapeutic targets is an urgent need. Here, we show that the lysine methyltransferase SETD8 is overexpressed in 50% of high-grade gliomas. The small molecule SETD8 inhibitor UNC0379, as well as siRNA-mediated inhibition of SETD8, blocked glioblastoma cell proliferation, by inducing DNA damage and activating cell cycle checkpoints. Specifically, in p53-proficient glioblastoma cells, SETD8 inhibition and DNA damage induced p21 accumulation and G1/S arrest whereas, in p53-deficient glioblastoma cells, DNA damage induced by SETD8 inhibition resulted in G2/M arrest mediated by Chk1 activation. Checkpoint abrogation, by the Wee1 kinase inhibitor adavosertib, induced glioblastoma cell lines and primary cells, DNA-damaged by UNC0379, to progress to mitosis where they died by mitotic catastrophe. Finally, UNC0379 and adavosertib synergized in restraining glioblastoma growth in a murine xenograft model, providing a strong rationale to further explore this novel pharmacological approach for adjuvant glioblastoma treatment.

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.

The Plasmodium falciparum Nuclear Protein Phosphatase NIF4 Is Required for Efficient Merozoite Invasion and Regulates Artemisinin Sensitivity

Artemisinin resistance in Plasmodium falciparum has been associated with a mutation in the NLI-interacting factor-like phosphatase PfNIF4, in addition to the mutations in the Kelch13 protein as the major determinant. We found that PfNIF4 was predominantly expressed at the schizont stage and localized in the nuclei of the parasite. To elucidate the functions of PfNIF4 in P. falciparum, we performed PfNIF4 knockdown (KD) using the inducible ribozyme system. PfNIF4 KD attenuated merozoite invasion and affected gametocytogenesis. PfNIF4 KD parasites also showed significantly increased in vitro susceptibility to artemisinins. Transcriptomic and proteomic analysis revealed that PfNIF4 KD led to the downregulation of gene categories involved in invasion and artemisinin resistance (e.g., mitochondrial function, membrane, and Kelch13 interactome) at the trophozoite and/or schizont stage. Consistent with PfNIF4 being a protein phosphatase, PfNIF4 KD resulted in an overall upregulation of the phosphoproteome of infected erythrocytes. Quantitative phosphoproteomic profiling identified a set of PfNIF4-regulated phosphoproteins with functional similarity to FCP1 substrates, particularly proteins involved in chromatin organization and transcriptional regulation. Specifically, we observed increased phosphorylation of Ser2/5 of the tandem repeats in the C-terminal domain (CTD) of RNA polymerase II (RNAPII) upon PfNIF4 KD. Furthermore, using the TurboID-based proteomic approach, we identified that PfNIF4 interacted with the RNAPII components, AP2-domain transcription factors, and chromatin-modifiers and binders. These findings suggest that PfNIF4 may act as the RNAPII CTD phosphatase, regulating the expression of general and parasite-specific cellular pathways during the blood-stage development.

A Molecular switch for FLOWERING LOCUS C activation determines flowering time in Arabidopsis

Plants have evolved sophisticated mechanisms to ensure flowering in favorable conditions for reproductive success. In the model plant Arabidopsis thaliana, FLOWERING LOCUS C (FLC) acts as a central repressor of flowering and the major determinant for winter cold requirement for flowering. FLC is activated in winter annuals by the FRIGIDA (FRI) activator complex containing FRI, FLC EXPRESSOR (FLX), and FLX-LIKE 4 (FLX4), among which FLX and FLX4 are also essential for establishing basal FLC expression in summer annuals. Here we show that a plant RNA polymerase II C-terminal domain phosphatase, C-TERMINAL DOMAIN PHOSPHATASE-LIKE 3 (CPL3), interacts with and dephosphorylates FLX4 through their scaffold protein FLX to inhibit flowering. CPL3-mediated dephosphorylation of FLX4 serves as a key molecular switch that enables binding of dephosphorylated FLX4 to the FLC locus to promote FLC expression, thus repressing flowering in both winter and summer annuals of Arabidopsis. Our findings reveal a molecular switch underlying the activation of FLC for flowering time control.

Phosphoproteome Profiling Revealed the Importance of mTOR Inhibition on CDK1 Activation to Further Regulate Cell Cycle Progression

The mammalian target of rapamycin (mTOR) functions as a critical regulator of cell cycle progression. However, the underlying mechanism by which mTOR regulates cell cycle progression remains elusive. In this study, we used stable isotope labeling of amino acids in cell culture with a two-step strategy for phosphopeptide enrichment and high-throughput quantitative mass spectrometry to perform a global phosphoproteome analysis of mTOR inhibition by rapamycin. By monitoring the phosphoproteome alterations upon rapamycin treatment, downregulation of mTOR signaling pathway was detected and enriched. Further functional analysis of phosphoproteome revealed the involvement of cell cycle events. Specifically, the elevated profile of cell cycle-related substrates was observed, and the activation of CDK1, MAPK1, and MAPK3 kinases was determined. Second, pathway interrogation using kinase inhibitor treatment confirmed that CDK1 activation operated downstream from mTOR inhibition to further regulate cell cycle progression. Third, we found that the activation of CDK1 following 4-12 h of mTOR inhibition was accompanied by the activation of the Greatwall-endosulfine complex. In conclusion, we presented a high-confidence phosphoproteome map inside the cells upon mTOR inhibition by rapamycin. Our data implied that mTOR inhibition could contribute to CDK1 activation for further regulating cell cycle progression, which was mediated by the Greatwall-endosulfine complex.

Ctdp1 deficiency leads to early embryonic lethality in mice and defects in cell cycle progression in MEFs

RNA polymerase II subunit A Carboxy-Terminal Domain Phosphatase 1 (CTDP1), a member of the haloacid dehalogenase superfamily phosphatases, has a defined role in transcriptional regulation, but emerging evidence suggests an expanded functional repertoire in the cell cycle and DNA damage response. In humans, a splice site mutation in CTDP1 gives rise to the rare Congenital Cataracts Facial Dysmorphism and Neuropathy syndrome, and recent evidence from our lab indicates CTDP1 is required for breast cancer growth and proliferation. To explore the physiological function of CTDP1 in a mammalian system, we generated a conditional Ctdp1 knockout mouse model by insertion of loxP sites upstream of exon 3 and downstream of exon 4. Biallelic deletion of Ctdp1 results in lethality before embryonic day 7.5, with morphological features indicating embryo cell death and resorption. However, Ctdp1^+/− mice are haplosufficient for phenotypic traits including body weight, hematological parameters, exploratory and locomotive functions. To investigate the potential mechanisms of the embryonic death caused by biallelic Ctdp1 knockout, mouse embryonic fibroblasts (MEFs) were established from Ctdp1^+/+ and Ctdp1^flox/flox mice. Lentivirus delivered Cre-mediated biallelic deletion of Ctdp1 in MEFs results in cell death preceded by impaired proliferation characterized by an increase in G1- and G2-phase populations and a reduction in the S-phase population. These cell cycle alterations caused by deletion of Ctdp1 are associated with an increase in p27 protein expression and a decrease in phosphorylated RB, phosphorylated Histone H3, and Cyclin B expression. Together, these results reveal that Ctdp1 plays an essential role in early mouse embryo development and cell growth and survival in part by regulating the cell cycle.

Summary: Knockout of Ctdp1 reveals its essential role in mammalian embryogenesis and regulation of the cell cycle.

AKT Regulates Mitotic Progression of Mammalian Cells by Phosphorylating MASTL, Leading to Protein Phosphatase 2A Inactivation

Microtubule-associated serine/threonine kinase like (MASTL), also known as Greatwall (Gwl) kinase, has an important role in the regulation of mitosis. By inhibiting protein phosphatase 2A (PP2A), it plays a crucial role in activating one of the most important mitotic kinases, known as cyclin-dependent kinase 1 (CDK1). MASTL has been seen to be upregulated in various types of cancers and is also involved in tumor recurrence. It is activated by CDK1 through phosphorylations in the activation/T-loop, but the complete mechanism of its activation is still unclear. Here, we report that AKT phosphorylates MASTL at residue T299, which plays a critical role in its activation. Our results suggest that AKT increases CDK1-mediated phosphorylation and hence the activity of MASTL, which, in turn, promotes mitotic progression through PP2A inhibition. We also show that the oncogenic potential of AKT is augmented by MASTL activation, since AKT-mediated proliferation in colorectal cell lines can be attenuated by inhibiting and/or silencing MASTL. In brief, we report that AKT plays an important role in the progression of mitosis in mammalian cells and that it does so through the phosphorylation and activation of MASTL.

Recent advances in understanding the role of Cdk1 in the Spindle Assembly Checkpoint

The goal of mitosis is to form two daughter cells each containing one copy of each mother cell chromosome, replicated in the previous S phase. To achieve this, sister chromatids held together back-to-back at their primary constriction, the centromere, have to interact with microtubules of the mitotic spindle so that each chromatid takes connections with microtubules emanating from opposite spindle poles (we will refer to this condition as bipolar attachment). Only once all replicated chromosomes have reached bipolar attachments can sister chromatids lose cohesion with each other, at the onset of anaphase, and move toward opposite spindle poles, being segregated into what will soon become the daughter cell nucleus. Prevention of errors in chromosome segregation is granted by a safeguard mechanism called Spindle Assembly Checkpoint (SAC). Until all chromosomes are bipolarly oriented at the equator of the mitotic spindle, the SAC prevents loss of sister chromatid cohesion, thus anaphase onset, and maintains the mitotic state by inhibiting inactivation of the major M phase promoting kinase, the cyclin B-cdk1 complex (Cdk1). Here, we review recent mechanistic insights about the circuitry that links Cdk1 to the SAC to ensure correct achievement of the goal of mitosis.

Greatwall-Endosulfine: A Molecular Switch that Regulates PP2A/B55 Protein Phosphatase Activity in Dividing and Quiescent Cells

During the cell cycle, hundreds of proteins become phosphorylated and dephosphorylated, indicating that protein kinases and protein phosphatases play a central role in its regulation. It has been widely recognized that oscillation in cyclin-dependent kinase (CDK) activity promotes DNA replication, during S-phase, and chromosome segregation, during mitosis. Each CDK substrate phosphorylation status is defined by the balance between CDKs and CDK-counteracting phosphatases. In fission yeast and animal cells, PP2A/B55 is the main protein phosphatase that counteracts CDK activity. PP2A/B55 plays a key role in mitotic entry and mitotic exit, and it is regulated by the Greatwall-Endosulfine (ENSA) molecular switch that inactivates PP2A/B55 at the onset of mitosis, allowing maximal CDK activity at metaphase. The Greatwall-ENSA-PP2A/B55 pathway is highly conserved from yeast to animal cells. In yeasts, Greatwall is negatively regulated by nutrients through TORC1 and S6 kinase, and couples cell growth, regulated by TORC1, to cell cycle progression, driven by CDK activity. In animal cells, Greatwall is phosphorylated and activated by Cdk1 at G2/M, generating a bistable molecular switch that results in full activation of Cdk1/CyclinB. Here we review the current knowledge of the Greatwall-ENSA-PP2A/B55 pathway and discuss its role in cell cycle progression and as an integrator of nutritional cues.

Triggering mitosis

Entry into mitosis is triggered by the activation of cyclin-dependent kinase 1 (Cdk1). This simple reaction rapidly and irreversibly sets the cell up for division. Even though the core step in triggering mitosis is so simple, the regulation of this cellular switch is highly complex, involving a large number of interconnected signalling cascades. We do have a detailed knowledge of most of the components of this network, but only a poor understanding of how they work together to create a precise and robust system that ensures that mitosis is triggered at the right time and in an orderly fashion. In this review, we will give an overview of the literature that describes the Cdk1 activation network and then address questions relating to the systems biology of this switch. How is the timing of the trigger controlled? How is mitosis insulated from interphase? What determines the sequence of events, following the initial trigger of Cdk1 activation? Which elements ensure robustness in the timing and execution of the switch? How has this system been adapted to the high levels of replication stress in cancer cells?

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

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

Regulation of Skn7-dependent, oxidative stress-induced genes by the RNA polymerase II-CTD phosphatase, Fcp1, and Mediator kinase subunit, Cdk8, in yeast

Fcp1 is a protein phosphatase that facilitates transcription elongation and termination by dephosphorylating the C-terminal domain of RNA polymerase II. High-throughput genetic screening and gene expression profiling of fcp1 mutants revealed a novel connection to Cdk8, the Mediator complex kinase subunit, and Skn7, a key transcription factor in the oxidative stress response pathway. Briefly, Skn7 was enriched as a regulator of genes whose mRNA levels were altered in fcp1 and cdk8Δ mutants and was required for the suppression of fcp1 mutant growth defects by loss of CDK8 under oxidative stress conditions. Targeted analysis revealed that mutating FCP1 decreased Skn7 mRNA and protein levels as well as its association with target gene promoters but paradoxically increased the mRNA levels of Skn7-dependent oxidative stress-induced genes (TRX2 and TSA1) under basal and induced conditions. The latter was in part recapitulated via chemical inhibition of transcription in WT cells, suggesting that a combination of transcriptional and posttranscriptional effects underscored the increased mRNA levels of TRX2 and TSA1 observed in the fcp1 mutant. Interestingly, loss of CDK8 robustly normalized the mRNA levels of Skn7-dependent genes in the fcp1 mutant background and also increased Skn7 protein levels by preventing its turnover. As such, our work suggested that loss of CDK8 could overcome transcriptional and/or posttranscriptional alterations in the fcp1 mutant through its regulatory effect on Skn7. Furthermore, our work also implicated FCP1 and CDK8 in the broader response to environmental stressors in yeast.

Cdc14 activation requires coordinated Cdk1-dependent phosphorylation of Net1 and PP2A-Cdc55 at anaphase onset

Exit from mitosis and completion of cytokinesis require the inactivation of mitotic cyclin-dependent kinase (Cdk) activity. In budding yeast, Cdc14 phosphatase is a key mitotic regulator that is activated in anaphase to counteract Cdk activity. In metaphase, Cdc14 is kept inactive in the nucleolus, where it is sequestered by its inhibitor, Net1. At anaphase onset, downregulation of PP2ACdc55 phosphatase by separase and Zds1 protein promotes Net1 phosphorylation and, consequently, Cdc14 release from the nucleolus. The mechanism by which PP2ACdc55 activity is downregulated during anaphase remains to be elucidated. Here, we demonstrate that Cdc55 regulatory subunit is phosphorylated in anaphase in a Cdk1-Clb2-dependent manner. Interestingly, cdc55-ED phosphomimetic mutant inactivates PP2ACdc55 phosphatase activity towards Net1 and promotes Cdc14 activation. Separase and Zds1 facilitate Cdk-dependent Net1 phosphorylation and Cdc14 release from the nucleolus by modulating PP2ACdc55 activity via Cdc55 phosphorylation. In addition, human Cdk1-CyclinB1 phosphorylates human B55, indicating that the mechanism is conserved in higher eukaryotes.

Wee1 Rather Than Plk1 Is Inhibited by AZD1775 at Therapeutically Relevant Concentrations

Wee1 kinase is an inhibitor of cyclin-dependent kinase (cdk)s, crucial cell cycle progression drivers. By phosphorylating cdk1 at tyrosine 15, Wee1 inhibits activation of cyclin B-cdk1 (Cdk1), preventing cells from entering mitosis with incompletely replicated or damaged DNA. Thus, inhibiting Wee1, alone or in combination with DNA damaging agents, can kill cancer cells by mitotic catastrophe, a tumor suppressive response that follows mitosis onset in the presence of under-replicated or damaged DNA. AZD1775, an orally available Wee1 inhibitor, has entered clinical trials for cancer treatment following this strategy, with promising results. Recently, however, AZD1775 has been shown to inhibit also the polo-like kinase homolog Plk1 in vitro, casting doubts on its mechanism of action. Here we asked whether, in the clinically relevant concentration range, AZD1775 inhibited Wee1 or Plk1 in transformed and non-transformed human cells. We found that in the clinically relevant, nanomolar, concentration range AZD1775 inhibited Wee1 rather than Plk1. In addition, AZD1775 treatment accelerated mitosis onset overriding the DNA replication checkpoint and hastened Plk1-dependent phosphorylation. On the contrary selective Plk1 inhibition exerted opposite effects. Thus, at therapeutic concentrations, AZD1775 inhibited Wee1 rather than Plk1. This information will help to better interpret results obtained by using AZD1775 both in the clinical and experimental settings and provide a stronger rationale for combination therapies.

The Oncogenic Functions of MASTL Kinase

MASTL kinase is a master regulator of mitosis, essential for ensuring that mitotic substrate phosphorylation is correctly maintained. It achieves this through the phosphorylation of alpha-endosulfine and subsequent inhibition of the tumor suppressor PP2A-B55 phosphatase. In recent years MASTL has also emerged as a novel oncogenic kinase that is upregulated in a number of cancer types, correlating with chromosome instability and poor patient survival. While the chromosome instability is likely directly linked to MASTL's control of mitotic phosphorylation, several new studies indicated that MASTL has additional effects outside of mitosis and beyond regulation of PP2A-B55. These include control of normal DNA replication timing, and regulation of AKT/mTOR and Wnt/β-catenin oncogenic kinase signaling. In this review, we will examine the phenotypes and mechanisms for how MASTL, ENSA, and PP2A-B55 deregulation drives tumor progression and metastasis. Finally, we will explore the rationale for the future development of MASTL inhibitors as new cancer therapeutics.

Greatwall kinase at a glance

Mitosis is controlled by a subtle balance between kinase and phosphatase activities that involve the master mitotic kinase cyclin-B-Cdk1 and its antagonizing protein phosphatase 2A-B55 (PP2A-B55). Importantly, the Greatwall (Gwl; known as Mastl in mammals, Rim15 in budding yeast and Ppk18 in fission yeast) kinase pathway regulates PP2A-B55 activity by phosphorylating two proteins, cAMP-regulated phosphoprotein 19 (Arpp19) and α-endosulfine (ENSA). This phosphorylation turns these proteins into potent inhibitors of PP2A-B55, thereby promoting a correct timing and progression of mitosis. In this Cell Science at a Glance article and the accompanying poster, we discuss how Gwl is regulated in space and time, and how the Gwl-Arpp19-ENSA-PP2A-B55 pathway plays an essential role in the control of M and S phases from yeast to human. We also summarize how Gwl modulates oncogenic properties of cells and how nutrient deprivation influences Gwl activity.

Mitosis inhibitors in anticancer therapy: When blocking the exit becomes a solution

Current microtubule-targeting agents (MTAs) remain amongst the most important antimitotic drugs used against a broad range of malignancies. By perturbing spindle assembly, MTAs activate the spindle assembly checkpoint (SAC), which induces mitotic arrest and subsequent apoptosis. However, besides toxic side effects and resistance, mitotic slippage and failure in triggering apoptosis in various cancer cells are limiting factors of MTAs efficacy. Alternative strategies to target mitosis without affecting microtubules have, thus, led to the identification of small molecules, such as those that target spindle Kinesins, Aurora and Polo-like kinases. Unfortunately, these so-called second-generation of antimitotics, encompassing mitotic blockers and mitotic drivers, have failed in clinical trials. Our recent understanding regarding the mechanisms of cell death during a mitotic arrest pointed out apoptosis as the main variable, providing an opportunity to control the cell fates and influence the effectiveness of antimitotics. Here, we provide an overview on the second-generation of antimitotics, and discuss possible strategies that exploit SAC activity, mitotic slippage/exit and apoptosis induction, in order to improve the efficacy of anticancer strategies that target mitosis.

Silencing Arabidopsis CARBOXYL-TERMINAL DOMAIN PHOSPHATASE-LIKE 4 induces cytokinin-oversensitive de novo shoot organogenesis

De novo shoot organogenesis (DNSO) is a post-embryonic development programme that has been widely exploited by plant biotechnology. DNSO is a hormonally regulated process in which auxin and cytokinin (CK) coordinate suites of genes encoding transcription factors, general transcription factors, and RNA metabolism machinery. Here we report that silencing Arabidopsis thaliana carboxyl-terminal domain (CTD) phosphatase-like 4 (CPL4_RNAi ) resulted in increased phosphorylation levels of RNA polymerase II (pol II) CTD and altered lateral root development and DNSO efficiency of the host plants. Under standard growth conditions, CPL4_RNAi lines produced no or few lateral roots. When induced by high concentrations of auxin, CPL4_RNAi lines failed to produce focused auxin maxima at the meristem of lateral root primordia, and produced fasciated lateral roots. In contrast, root explants of CPL4_RNAi lines were highly competent for DNSO. Efficient DNSO of CPL4_RNAi lines was observed even under 10 times less the CK required for the wild-type explants. Transcriptome analysis showed that CPL4_RNAi , but not wild-type explants, expressed high levels of shoot meristem-related genes even during priming on medium with a high auxin/CK ratio, and during subsequent shoot induction with a lower auxin/CK ratio. Conversely, CPL4_RNAi enhanced the inhibitory phenotype of the shoot redifferentiation defective2-1 mutation, which affected snRNA biogenesis and formation of the auxin gradient. These results indicated that CPL4 functions in multiple regulatory pathways that positively and negatively affect DNSO.

Coordination of Protein Kinase and Phosphoprotein Phosphatase Activities in Mitosis

Dynamic changes in protein phosphorylation govern the transitions between different phases of the cell division cycle. A "tug of war" between highly conserved protein kinases and the family of phosphoprotein phosphatases (PPP) establishes the phosphorylation state of proteins, which controls their function. More than three-quarters of all proteins are phosphorylated at one or more sites in human cells, with the highest occupancy of phosphorylation sites seen in mitosis. Spatial and temporal regulation of opposing kinase and PPP activities is crucial for accurate execution of the mitotic program. The role of mitotic kinases has been the focus of many studies, while the contribution of PPPs was for a long time underappreciated and is just emerging. Misconceptions regarding the specificity and activity of protein phosphatases led to the belief that protein kinases are the primary determinants of mitotic regulation, leaving PPPs out of the limelight. Recent studies have shown that protein phosphatases are specific and selective enzymes, and that their activity is tightly regulated. In this review, we discuss the emerging roles of PPPs in mitosis and their regulation of and by mitotic kinases, as well as mechanisms that determine PPP substrate recognition and specificity.

The rationale for druggability of CCDC6-tyrosine kinase fusions in lung cancer

Gene fusions occur in up to 17% of solid tumours. Oncogenic kinases are often involved in such fusions. In lung cancer, almost 30% of patients carrying an activated oncogene show the fusion of a tyrosine kinase to an heterologous gene. Several genes are partner in the fusion with the three kinases ALK, ROS1 and RET in lung. The impaired function of the partner gene, in combination with the activation of the kinase, may alter the cell signaling and promote the cancer cell addiction to the oncogene. Moreover, the gene that is partner in the fusion to the kinase may affect the response to therapeutics and/or promote resistance in the cancer cells. Few genes are recurrent partners in tyrosine kinase fusions in lung cancer, including CCDC6, a recurrent partner in ROS1 and RET fusions, that can be selected as possible target for new strategies of combined therapy including TKi.

Evidence that PP2A activity is dispensable for spindle assembly checkpoint-dependent control of Cdk1

Progression through mitosis, the cell cycle phase deputed to segregate replicated chromosomes, is granted by a protein phosphorylation wave that follows an activation-inactivation cycle of cyclin B-dependent kinase (Cdk) 1, the major mitosis-promoting enzyme. To ensure correct chromosome segregation, the safeguard mechanism spindle assembly checkpoint (SAC) delays Cdk1 inactivation by preventing cyclin B degradation until mitotic spindle assembly. At the end of mitosis, reversal of bulk mitotic protein phosphorylation, downstream Cdk1 inactivation, is required to complete mitosis and crucially relies on the activity of major protein phosphatases like PP2A. A role for PP2A, however, has also been suggested in spindle assembly and SAC-dependent control of Cdk1. Indeed, PP2A was found in complex with SAC proteins while small interfering RNAs (siRNAs)-mediated downregulation of PP2A holoenzyme components affected mitosis completion in mammalian cells. However, whether the SAC-dependent control of Cdk1 required the catalytic activity of PP2A has never been directly assessed. Here, using two PP2A inhibitors, okadaic acid and LB-100, we provide evidence that PP2A activity is dispensable for SAC control of Cdk1 in human cells.

Phosphatase-regulated recruitment of the spindle- and kinetochore-associated (Ska) complex to kinetochores

Kinetochores move chromosomes on dynamic spindle microtubules and regulate signaling of the spindle checkpoint. The spindle- and kinetochore-associated (Ska) complex, a hexamer composed of two copies of Ska1, Ska2 and Ska3, has been implicated in both roles. Phosphorylation of kinetochore components by the well-studied mitotic kinases Cdk1, Aurora B, Plk1, Mps1, and Bub1 regulate chromosome movement and checkpoint signaling. Roles for the opposing phosphatases are more poorly defined. Recently, we showed that the C terminus of Ska1 recruits protein phosphatase 1 (PP1) to kinetochores. Here we show that PP1 and protein phosphatase 2A (PP2A) both promote accumulation of Ska at kinetochores. Depletion of PP1 or PP2A by siRNA reduces Ska binding at kinetochores, impairs alignment of chromosomes to the spindle midplane, and causes metaphase delay or arrest, phenotypes that are also seen after depletion of Ska. Artificial tethering of PP1 to the outer kinetochore protein Nuf2 promotes Ska recruitment to kinetochores, and it reduces but does not fully rescue chromosome alignment and metaphase arrest defects seen after Ska depletion. We propose that Ska has multiple functions in promoting mitotic progression and that kinetochore-associated phosphatases function in a positive feedback cycle to reinforce Ska complex accumulation at kinetochores.

Summary: Feedback between protein phosphatases and the spindle- and kinetochore-associated (Ska) complex regulates chromosome movement and the metaphase-to-anaphase cell cycle transition. This article has an associated First Person interview with the first author of the paper as part of the supplementary information.

Cell cycle-dependent regulation of Greatwall kinase by protein phosphatase 1 and regulatory subunit 3B

Greatwall (Gwl) kinase plays an essential role in the regulation of mitotic entry and progression. Mitotic activation of Gwl requires both cyclin-dependent kinase 1 (CDK1)-dependent phosphorylation and its autophosphorylation at an evolutionarily conserved serine residue near the carboxyl terminus (Ser-883 in Xenopus). In this study we show that Gwl associates with protein phosphatase 1 (PP1), particularly PP1γ, which mediates the dephosphorylation of Gwl Ser-883. Consistent with the mitotic activation of Gwl, its association with PP1 is disrupted in mitotic cells and egg extracts. During mitotic exit, PP1-dependent dephosphorylation of Gwl Ser-883 occurs prior to dephosphorylation of other mitotic substrates; replacing endogenous Gwl with a phosphomimetic S883E mutant blocks mitotic exit. Moreover, we identified PP1 regulatory subunit 3B (PPP1R3B) as a targeting subunit that can direct PP1 activity toward Gwl. PPP1R3B bridges PP1 and Gwl association and promotes Gwl Ser-883 dephosphorylation. Consistent with the cell cycle-dependent association of Gwl and PP1, Gwl and PPP1R3B dissociate in M phase. Interestingly, up-regulation of PPP1R3B facilitates mitotic exit and blocks mitotic entry. Thus, our study suggests PPP1R3B as a new cell cycle regulator that functions by governing Gwl dephosphorylation.

Unfair competition governs the interaction of pCPI-17 with myosin phosphatase (PP1-MYPT1)

The small phosphoprotein pCPI-17 inhibits myosin light-chain phosphatase (MLCP). Current models postulate that during muscle relaxation, phosphatases other than MLCP dephosphorylate and inactivate pCPI-17 to restore MLCP activity. We show here that such hypotheses are insufficient to account for the observed rapidity of pCPI-17 inactivation in mammalian smooth muscles. Instead, MLCP itself is the critical enzyme for pCPI-17 dephosphorylation. We call the mutual sequestration mechanism through which pCPI-17 and MLCP interact inhibition by unfair competition: MLCP protects pCPI-17 from other phosphatases, while pCPI-17 blocks other substrates from MLCP’s active site. MLCP dephosphorylates pCPI-17 at a slow rate that is, nonetheless, both sufficient and necessary to explain the speed of pCPI-17 dephosphorylation and the consequent MLCP activation during muscle relaxation.

DOI:http://dx.doi.org/10.7554/eLife.24665.001

Cell cycle checkpoint in cancer: a therapeutically targetable double-edged sword

Major currently used anticancer therapeutics either directly damage DNA or target and upset basic cell division mechanisms like DNA replication and chromosome segregation. These insults elicit activation of cell cycle checkpoints, safeguard mechanisms that cells implement to correctly complete cell cycle phases, repair damage or eventually commit suicide in case damage is unrepairable. Although cancer cells appear to be advantageously defective in some aspects of checkpoint physiology, recent acquisitions on the biochemical mechanisms of the various checkpoints are offering new therapeutic approaches against cancer. Indeed, chemical manipulation of these mechanisms is providing new therapeutic strategies and tools to increase the killing efficacy of major cancer therapeutics as well as to directly promote cancer cell death. In this review we summarize developing concepts on how targeting cell cycle checkpoints may provide substantial improvement to cancer therapy.

Protein Phosphatases Involved in Regulating Mitosis: Facts and Hypotheses

Almost all eukaryotic proteins are subject to post-translational modifications during mitosis and cell cycle, and in particular, reversible phosphorylation being a key event. The recent use of high-throughput experimental analyses has revealed that more than 70% of all eukaryotic proteins are regulated by phosphorylation; however, the mechanism of dephosphorylation, counteracting phosphorylation, is relatively unknown. Recent discoveries have shown that many of the protein phosphatases are involved in the temporal and spatial control of mitotic events, such as mitotic entry, mitotic spindle assembly, chromosome architecture changes and cohesion, and mitotic exit. This implies that certain phosphatases are tightly regulated for timely dephosphorylation of key mitotic phosphoproteins and are essential for control of various mitotic processes. This review describes the physiological and pathological roles of mitotic phosphatases, as well as the versatile role of various protein phosphatases in several mitotic events.

Bistability of mitotic entry and exit switches during open mitosis in mammalian cells

Mitotic entry and exit are switch-like transitions that are driven by the activation and inactivation of Cdk1 and mitotic cyclins. This simple on/off reaction turns out to be a complex interplay of various reversible reactions, feedback loops, and thresholds that involve both the direct regulators of Cdk1 and its counteracting phosphatases. In this review, we summarize the interplay of the major components of the system and discuss how they work together to generate robustness, bistability, and irreversibility. We propose that it may be beneficial to regard the entry and exit reactions as two separate reversible switches that are distinguished by differences in the state of phosphatase activity, mitotic proteolysis, and a dramatic rearrangement of cellular components after nuclear envelope breakdown, and discuss how the major Cdk1 activity thresholds could be determined for these transitions.