The polo-like kinase Plx1 is required for M phase exit and destruction of mitotic regulators in Xenopus egg extracts

Virtual screening of natural compounds as selective inhibitors of polo-like kinase-1 at C-terminal polo box and N-terminal catalytic domain

The over-expression of Polo-like kinase-1 (PLK1) is associated with cancer prognosis due to its pivotal role in cell proliferation. The N-terminal catalytic domain (NCD) and C-terminal polo box domain (PBD) of PLK1 are critical for the activity of the protein. Drugs that inhibit PLK1 by targeting these domains are on clinical trials, but so far, none has been approved by FDA. Thus, this study targets the two domains of PLK1 to identify compounds with inhibitory potential. Four validated e-pharmacophore models from NCD (PDB ID: 2OU7 and 4J52) and PBD (PDB ID: 5NEI and 5NN2) were used to screen over 26,000 natural compounds from NPASS database. Hits were identified after the well-fitted compounds were subjected to molecular docking study and ADME prediction. The pIC50 and electronic behaviour of the identified hits selectively targeting NCD and PBD of PLK1 were predicted via an externally validated QSAR model and quantum mechanics. The results showed that CAA180504, CAA197326, CAA74619, CAA328856 modulating PLK1 at NCD, and CBB130581, CBB230713, CBB206123, CBB12656 and CBB267117 modulating PLK1 at PBD had better molecular docking scores, pharmacokinetics and drug-like properties than NCD (volasertib) and PBD (purpurogallin) reference inhibitors. The compounds all had satisfactory inhibitory (pIC50) values which range from 6.187 to 7.157. The electronic behaviours of understudied compounds using HOMO/LUMO and global descriptive parameters revealed the atomic portion of the compounds prone to donating and accepting electrons. In conclusion, the hit compounds identified from the library of natural compounds are worthy of further experimental validation.Communicated by Ramaswamy H. Sarma.

Polo-like kinase 1 (Plk1) is a positive regulator of DNA replication in the Xenopus in vitro system

Polo-like kinase 1 (Plk1) is a cell cycle kinase essential for mitosis progression, but also important for checkpoint recovery and adaptation in response to DNA damage and replication stress. However, although Plk1 is expressed in S phase, little is known about its function during unperturbed DNA replication. Using Xenopus laevis egg extracts, mimicking early embryonic replication, we demonstrate that Plk1 is simultaneously recruited to chromatin with pre-replication proteins where it accumulates throughout S phase. Further, we found that chromatin-bound Plk1 is phosphorylated on its activating site T201, which appears to be sensitive to dephosphorylation by protein phosphatase 2A. Extracts immunodepleted of Plk1 showed a decrease in DNA replication, rescued by wild type recombinant Plk1. Inversely, modest Plk1 overexpression accelerated DNA replication. Plk1 depletion led to an increase in Chk1 phosphorylation and to a decrease in Cdk2 activity, which strongly suggests that Plk1 could inhibit the ATR/Chk1-dependent intra-S phase checkpoint during normal S phase. In addition, we observed that phosphorylated Plk1 levels are high during the rapid, early cell cycles of Xenopus development but decrease after the mid-blastula transition when the cell cycle and the replication program slow down along with more active checkpoints. These data shed new light on the role of Plk1 as a positive regulating factor for DNA replication in early, rapidly dividing embryos.

MCC1019, a selective inhibitor of the Polo-box domain of Polo-like kinase 1 as novel, potent anticancer candidate

Polo-like kinase (PLK1) has been identified as a potential target for cancer treatment. Although a number of small molecules have been investigated as PLK1 inhibitors, many of which showed limited selectivity. PLK1 harbors a regulatory domain, the Polo box domain (PBD), which has a key regulatory function for kinase activity and substrate recognition. We report on 3-bromomethyl-benzofuran-2-carboxylic acid ethyl ester (designated: MCC1019) as selective PLK1 inhibitor targeting PLK1 PBD. Cytotoxicity and fluorescence polarization-based screening were applied to a library of 1162 drug-like compounds to identify potential inhibitors of PLK1 PBD. The activity of compound MC1019 against the PLK1 PBD was confirmed using fluorescence polarization and microscale thermophoresis. This compound exerted specificity towards PLK1 over PLK2 and PLK3. MCC1019 showed cytotoxic activity in a panel of different cancer cell lines. Mechanistic investigations in A549 lung adenocarcinoma cells revealed that MCC1019 induced cell growth inhibition through inactivation of AKT signaling pathway, it also induced prolonged mitotic arrest—a phenomenon known as mitotic catastrophe, which is followed by immediate cell death via apoptosis and necroptosis. MCC1019 significantly inhibited tumor growth in vivo in a murine lung cancer model without affecting body weight or vital organ size, and reduced the growth of metastatic lesions in the lung. We propose MCC1019 as promising anti-cancer drug candidate.

A selective inhibitor of the Polo-box domain of Polo-like kinase 1 identified by virtual screening

Polo-like kinase 1 (PLK1), a member of the Polo-like kinase family, plays an important regulatory role in mitosis and cell cycle progression. PLK1 overexpression is correlated with tumourigenesis and poor prognosis in cancer patients. Therefore, the identification of novel compounds that inhibit PLK1 would provide attractive therapeutic approaches. Although some PLK1 kinase inhibitors have been developed, their application has been limited by off-target effects. PLK1 contains a regulatory domain named the Polo-box domain (PBD), which is characteristic only for the Polo-like kinase family. This domain represents an alternative therapeutic target with higher selectivity for PLK1. In this study, we applied in silico virtual drug screening, fluorescence polarization and microscale thermophoresis to identify new scaffolds targeting the PBD of PLK1. One compound, 3-{[(1R,9S)-3-(naphthalen-2-yl)-6-oxo-7,11-diazatricyclo[7.3.1.02,7]trideca-2,4-dien-11-yl]methyl}benzonitrile (designated compound (1)), out of a total of 30,793 natural product derivatives, inhibited the PLK1 PBD with high selectivity (IC50: 17.9 ± 0.5 µM). This compound inhibited the growth of cultured leukaemia cells (CCRF-CEM and CEM/ADR5000) and arrested the cell cycle in the G2/M phase, which is characteristic for PLK1 inhibitors. Immunofluorescence analyses showed that treatment with compound (1) disrupted spindle formation due to the aberrant localization of PLK1 during the mitotic process, leading to G2/M arrest and ultimately cell death. In conclusion, compound (1) is a selective PLK1 inhibitor that inhibits cancer cell growth. It represents a chemical scaffold for the future synthesis of new selective PLK1 inhibitors for cancer therapy.

Regulating a key mitotic regulator, polo-like kinase 1 (PLK1)

During cell division, duplicated genetic material is separated into two distinct daughter cells. This process is essential for initial tissue formation during development and to maintain tissue integrity throughout an organism's lifetime. To ensure the efficacy and efficiency of this process, the cell employs a variety of regulatory and signaling proteins that function as mitotic regulators and checkpoint proteins. One vital mitotic regulator is polo-like kinase 1 (PLK1), a highly conserved member of the polo-like kinase family. Unique from its paralogues, it functions specifically during mitosis as a regulator of cell division. PLK1 is spatially and temporally enriched at three distinct subcellular locales; the mitotic centrosomes, kinetochores, and the cytokinetic midbody. These localization patterns allow PLK1 to phosphorylate specific downstream targets to regulate mitosis. In this review, we will explore how polo-like kinases were originally discovered and diverged into the five paralogues (PLK1-5) in mammals. We will then focus specifically on the most conserved, PLK1, where we will discuss what is known about how its activity is modulated, its role during the cell cycle, and new, innovative tools that have been developed to examine its function and interactions in cells.

Regulation of PLK1 through competition between hnRNPK, miR-149-3p and miR-193b-5p

Polo-like kinase 1 (PLK1) is a critical regulator of cell cycle progression and apoptosis. However, its regulation remains poorly understood. In the present study, we investigated the molecular mechanism underlying the post-transcriptional regulation of PLK1. We observed that heterogeneous nuclear ribonucleoprotein K (hnRNPK) and PLK1 were positively associated in several different cancers and high expression levels of them correlated with poor prognosis in patients with cancer. Knockdown of hnRNPK resulted in reduced expression of PLK1, whereas conversely, PLK1 expression was increased in hnRNPK-overexpressing cells. We found that hnRNPK regulated PLK1 expression through KH1- and KH2-dependent interactions with the 3'UTR of PLK1 mRNA. In addition, microRNA-149-3p (miR-149-3p) and miR-193b-5p suppressed PLK1 expression by targeting the 3'UTR of PLK1 mRNA. MicroRNA-elicited enrichment of PLK1 mRNA in Ago2 immunoprecipitation was altered by the presence or absence of hnRNPK. Furthermore, the deletion of the cytosine (C)-rich sequences of the 3'UTR of PLK1 mRNA abolished the decreased PLK1 expression observed via hnRNPK silencing and administration of miRNAs, a finding that suggests that hnRNPK shares this C-rich motif with miR-149-3p and miR-193b-5p. We also found that downregulation of PLK1 by either silencing hnRNPK or overexpression of miR-149-3p and miR-193b-5p decreased clonogenicity and induced apoptosis. Our findings from this study demonstrate that hnRNPK regulates PLK1 expression by competing with the PLK1-targeting miRNAs, miR-149-3p and miR-193b-5p.

Inhibition of the mitochondrial fission protein dynamin-related protein 1 (Drp1) impairs mitochondrial fission and mitotic catastrophe after x-irradiation

The role of mitochondrial dynamics in cellular responses to ionizing radiation (IR) is still largely unknown. This study demonstrates that IR triggers Drp1-dependent mitochondrial fission and that Drp1 inhibition attenuates radiation-induced mitotic catastrophe, suggesting that Drp1 is involved in determining the fate of cells after irradiation.

Accumulating evidence suggests that mitochondrial dynamics is crucial for the maintenance of cellular quality control and function in response to various stresses. However, the role of mitochondrial dynamics in cellular responses to ionizing radiation (IR) is still largely unknown. In this study, we provide evidence that IR triggers mitochondrial fission mediated by the mitochondrial fission protein dynamin-related protein 1 (Drp1). We also show IR-induced mitotic catastrophe (MC), which is a type of cell death associated with defective mitosis, and aberrant centrosome amplification in mouse embryonic fibroblasts (MEFs). These are attenuated by genetic or pharmacological inhibition of Drp1. Whereas radiation-induced aberrant centrosome amplification and MC are suppressed by the inhibition of Plk1 and CDK2 in wild-type MEFs, the inhibition of these kinases is ineffective in Drp1-deficient MEFs. Furthermore, the cyclin B1 level after irradiation is significantly higher throughout the time course in Drp1-deficient MEFs than in wild-type MEFs, implying that Drp1 is involved in the regulation of cyclin B1 level. These findings strongly suggest that Drp1 plays an important role in determining the fate of cells after irradiation via the regulation of mitochondrial dynamics.

Polo-like kinases (plks), a key regulator of cell cycle and new potential target for cancer therapy

Cell cycle process is regulated by a number of protein kinases and among them, serine/threonine kinases carry phosphate group from ATP to substrates. The most important three kinase families are Cyclin-dependent kinase (Cdk), Polo-like kinase (Plk), and Aurora kinase. Polo-like kinase family consists of 5 members (Plk1-Plk5) and they are involved in multiple functions in eukaryotic cell division. It regulates a variety of aspects such as, centrosome maturation, checkpoint recovery, spindle assembly, cytokinesis, apoptosis and many other features. Recently, it has been reported that Plks are related to tumor development and over-expressed in many kinds of tumor cells. When injected the anti-Plk antibody into human cells, the cells show aneuploidy, and if inhibit Plks, most of the mitotic cell division does not proceed properly. For that reasons, many inhibitors of Plk have been recently emerged as new target for remedy of the cancer therapeutic research. In this paper, we reviewed briefly the characteristics of Plk families and how Plks work in regulating cell cycles and cancer formation, and the possibilities of Plks as target for cancer therapy.

GSK3 and Polo-like kinase regulate ADAM13 function during cranial neural crest cell migration

ADAMs are cell surface metalloproteases that control multiple biological processes by cleaving signaling and adhesion molecules. ADAM13 controls cranial neural crest (CNC) cell migration both by cleaving cadherin-11 to release a promigratory extracellular fragment and by controlling expression of multiple genes via its cytoplasmic domain. The latter activity is regulated by γ-secretase cleavage and the translocation of the cytoplasmic domain into the nucleus. One of the genes regulated by ADAM13, the protease calpain8, is essential for CNC migration. Although the nuclear function of ADAM13 is evolutionarily conserved, it is unclear whether the transcriptional regulation is also performed by other ADAMs and how this process may be regulated. We show that ADAM13 function to promote CNC migration is regulated by two phosphorylation events involving GSK3 and Polo-like kinase (Plk). We further show that inhibition of either kinase blocks CNC migration and that the respective phosphomimetic forms of ADAM13 can rescue these inhibitions. However, these phosphorylations are not required for ADAM13 proteolysis of its substrates, γ-secretase cleavage, or nuclear translocation of its cytoplasmic domain. Of significance, migration of the CNC can be restored in the absence of Plk phosphorylation by expression of calpain-8a, pointing to impaired nuclear activity of ADAM13.

Identification of a high affinity selective inhibitor of Polo-like kinase 1 for cancer chemotherapy by computational approach

Polo-like kinase (Plk)1 is a key regulator of the cell cycle during mitotic phase and is an attractive anti-mitotic drug target for cancer. Plk1 is a member of Ser/Thr kinase family which also includes Plk2-4 in human. Plk1 promotes the cell division whereas Plk2 and Plk3 are reported to act as tumour suppressors. The available inhibitors of Plk1 also suppress Plk2 and Plk3 activity significantly resulting in the cell death of normal cells in addition to the cancer cells. Hence, it is imperative to explore Plk1 specific inhibitors as anti-cancer drugs. In this work, a selective potential inhibitor of Plk1 has been identified by molecular docking based high throughput virtual screening. The identified compound exploits the subtle differences between the binding sites of Plk1 and other Ser/Thr kinases including Plk2-4. The predicted binding affinity of identified inhibitor is higher than available inhibitors with a 100-fold selectivity towards Plk1 over Plk2-4 and several cell cycle kinases. It also satisfies the Lipinski's criteria of drug-like molecules and passes the other ADMET filters. This triazole compound with aryl substituent belongs to a novel class of potential inhibitor for Plk1. The suggested potential lead molecule can thus be tested and developed further as a potent and selective anti-cancer drug.

Phosphorylation-mediated stabilization of Bora in mitosis coordinates Plx1/Plk1 and Cdk1 oscillations

Cdk1 and Plk1/Plx1 activation leads to their inactivation through negative feedback loops. Cdk1 deactivates itself by activating the APC/C, consequently generating embryonic cell cycle oscillations. APC/C inhibition by the mitotic checkpoint in somatic cells and the cytostatic factor (CSF) in oocytes sustain the mitotic state. Plk1/Plx1 targets its co-activator Bora for degradation, but it remains unclear how embryonic oscillations in Plx1 activity are generated, and how Plk1/Plx1 activity is sustained during mitosis. We show that Plx1-mediated degradation of Bora in interphase generates oscillations in Plx1 activity and is essential for development. In CSF extracts, phosphorylation of Bora on the Cdk consensus site T52 blocks Bora degradation. Upon fertilization, Calcineurin dephosphorylates T52, triggering Plx1 oscillations. Similarly, we find that GFP-Bora is degraded when Plk1 activity spreads to somatic cell cytoplasm before mitosis. Interestingly, GFP-Bora degradation stops upon mitotic entry when Cdk1 activity is high. We hypothesize that Cdk1 controls Bora through an incoherent feedforward loop synchronizing the activities of mitotic kinases.

In-silico screening of cancer associated mutation on PLK1 protein and its structural consequences

The Polo-like kinases (Plks) are a conserved subfamily of serine-threonine protein kinases that have significant roles in cell proliferation. The serine/threonine protein kinases or polo-like kinase 1 (PLK1) exist in centrosome during interphase and is an important regulatory enzyme in cell cycle progression during M phase. Mutations in mammalian PLK1 were found to be over expressed in various human cancers and it is disrupting the binding ability of polo box domain with target peptide. In this analysis we implemented a computational approach to filter the most deleterious and cancer associated mutation on PLK1 protein. We found W414F as the most deleterious and cancer associated by Polyphen 2.0, SIFT, I-mutant 3.0, PANTHER, PhD-SNP, SNP&GO, Mutpred and Dr Cancer tools. Molecular docking and molecular dynamics simulation (MDS) approach was used to investigate the structural and functional behavior of PLK1 protein upon mutation. MDS and docking results showed stability loss in mutant PLK1 protein. Due to mutation, PLK1 protein became more flexible and alters the dynamic property of protein which might affect the interaction with target peptide and leads to cell proliferation. Our study provided a well designed computational methodology to examine the cancer associated nsSNPs and their molecular mechanism. It further helps scientists to develop a drug therapy against PLK1 cancer-associated diseases.

APC/C(Cdh1)-dependent degradation of Cdc20 requires a phosphorylation on CRY-box by Polo-like kinase-1 during somatic cell cycle

Cdc20 is an activator of the anaphase-promoting complex (APC/C), and APC/C(Cdc20) is essential for metaphase-anaphase transition. To allow progression beyond mitosis, Cdc20 is degraded through KEN-box-dependent APC/C(Cdh1) activity. Mammalian Cdc20 contains the CRY box, a second APC/C(Cdh1)-dependent degron, but the molecular mechanism in degradation process remains undefined. Polo-like kinase-1 (Plk1) is an essential mitotic kinase regulating various targets in kinetochore, centrosome, and midbody for proper mitotic progression. Plk1 directly bound to Cdc20 and phosphorylates it on serine-170 located in CRY-box. Whereas wild-type Cdc20 was degraded according to progress cell cycle beyond mitosis, the phosphorylation-defective mutant, which serine-170 was changed into alanine, was not destroyed in early G1 phase. The phosphorylation on serine-170 by Plk1 was important for ubiquitination and Cdh1-dependent proteolysis. However, this modification by Plk1 on CRY box had no effect on the subcellular localization of Cdc20 and the formation of APC/C-inhibitory checkpoint complexes under spindle assembly checkpoint. This mechanism will be the first finding of inhibitory phosphorylation related to Cdc20 instability.

Drosophila Polo regulates the spindle assembly checkpoint through Mps1-dependent BubR1 phosphorylation

Maintenance of genomic stability during eukaryotic cell division relies on the spindle assembly checkpoint (SAC) that prevents mitotic exit until all chromosomes are properly attached to the spindle. Polo is a mitotic kinase proposed to be involved in SAC function, but its role has remained elusive. We demonstrate that Polo and Aurora B functional interdependency comprises a positive feedback loop that promotes Mps1 kinetochore localization and activity. Expression of constitutively active Polo restores normal Mps1 kinetochore levels even after Aurora B inhibition, highlighting a role for Polo in Mps1 recruitment to unattached kinetochores downstream of Aurora B. We also show that Mps1 kinetochore localization is required for BubR1 hyperphosphorylation and formation of the 3F3/2 phosphoepitope. This is essential to allow recruitment of Cdc20 to unattached kinetochores and the assembly of anaphase-promoting complex/cyclosome-inhibitory complexes to levels that ensure long-term SAC activity. We propose a model in which Polo controls Mps1-dependent BubR1 phosphorylation to promote Cdc20 kinetochore recruitment and sustained SAC function.

Mitotic Kinases and p53 Signaling

Mitosis is tightly regulated and any errors in this process often lead to aneuploidy, genomic instability, and tumorigenesis. Deregulation of mitotic kinases is significantly associated with improper cell division and aneuploidy. Because of their importance during mitosis and the relevance to cancer, mitotic kinase signaling has been extensively studied over the past few decades and, as a result, several mitotic kinase inhibitors have been developed. Despite promising preclinical results, targeting mitotic kinases for cancer therapy faces numerous challenges, including safety and patient selection issues. Therefore, there is an urgent need to better understand the molecular mechanisms underlying mitotic kinase signaling and its interactive network. Increasing evidence suggests that tumor suppressor p53 functions at the center of the mitotic kinase signaling network. In response to mitotic spindle damage, multiple mitotic kinases phosphorylate p53 to either activate or deactivate p53-mediated signaling. p53 can also regulate the expression and function of mitotic kinases, suggesting the existence of a network of mutual regulation, which can be positive or negative, between mitotic kinases and p53 signaling. Therefore, deciphering this regulatory network will provide knowledge to overcome current limitations of targeting mitotic kinases and further improve the results of targeted therapy.

A critical balance between Cyclin B synthesis and Myt1 activity controls meiosis entry in Xenopus oocytes

In fully grown oocytes, meiosis is arrested at first prophase until species-specific initiation signals trigger maturation. Meiotic resumption universally involves early activation of M phase-promoting factor (Cdc2 kinase-Cyclin B complex, MPF) by dephosphorylation of the inhibitory Thr14/Tyr15 sites of Cdc2. However, underlying mechanisms vary. In Xenopus oocytes, deciphering the intervening chain of events has been hampered by a sensitive amplification loop involving Cdc2-Cyclin B, the inhibitory kinase Myt1 and the activating phosphatase Cdc25. In this study we provide evidence that the critical event in meiotic resumption is a change in the balance between inhibitory Myt1 activity and Cyclin B neosynthesis. First, we show that in fully grown oocytes Myt1 is essential for maintaining prophase I arrest. Second, we demonstrate that, upon upregulation of Cyclin B synthesis in response to progesterone, rapid inactivating phosphorylation of Myt1 occurs, mediated by Cdc2 and without any significant contribution of Mos/MAPK or Plx1. We propose a model in which the appearance of active MPF complexes following increased Cyclin B synthesis causes Myt1 inhibition, upstream of the MPF/Cdc25 amplification loop.

Ca2+ signaling during mammalian fertilization: requirements, players, and adaptations

Changes in the intracellular concentration of calcium ([Ca(2+)](i)) represent a vital signaling mechanism enabling communication among cells and between cells and the environment. The initiation of embryo development depends on a [Ca(2+)](i) increase(s) in the egg, which is generally induced during fertilization. The [Ca(2+)](i) increase signals egg activation, which is the first stage in embryo development, and that consist of biochemical and structural changes that transform eggs into zygotes. The spatiotemporal patterns of [Ca(2+)](i) at fertilization show variability, most likely reflecting adaptations to fertilizing conditions and to the duration of embryonic cell cycles. In mammals, the focus of this review, the fertilization [Ca(2+)](i) signal displays unique properties in that it is initiated after gamete fusion by release of a sperm-derived factor and by periodic and extended [Ca(2+)](i) responses. Here, we will discuss the events of egg activation regulated by increases in [Ca(2+)](i), the possible downstream targets that effect these egg activation events, and the property and identity of molecules both in sperm and eggs that underpin the initiation and persistence of the [Ca(2+)](i) responses in these species.

Polo-like kinase 1 as predictive marker and therapeutic target for radiotherapy in rectal cancer

The ability to predict tumor sensitivity toward radiotherapy may significantly impact the selection of patients for preoperative combined-modality therapy. The aim of the present study was to test the predictive value of Polo-like kinase 1 (PLK1) in rectal cancer patients and to investigate whether PLK1 plays a direct role in mediating radiation sensitivity. PLK1 expression was evaluated by immunohistochemistry (n = 76) or Affymetrix HG133 microarray (n = 20) on pretreatment biopsies of patients with advanced rectal cancer. Expression was correlated with both tumor regression in the resected specimen and long-term clinical outcome. Furthermore, we used small interfering RNAs (siRNAs) to down-regulate PLK1 expression in colorectal cancer cells and analyzed the effects of PLK1-specific siRNAs by Western blot and quantitative real-time PCR analysis, FACScan analysis, caspase 3/7 assays, and colony-forming assays. We observed that increased PLK1 protein expression was significantly related to a poorer tumor regression and a higher risk of local recurrence in uni- and multivariate analysis. A significant decrease of PLK1 expression by siRNAs in combination with ionizing radiation induced an increased percentage of apoptotic cells and increased caspase 3/7 activity. Furthermore, enhanced G(2)-M levels, decreased cellular viability, and reduced clonogenic survival were demonstrated, indicating a radiosensitizing effect of PLK1 depletion. Therefore, PLK1 may be a novel predictive marker for radiation response as well as a promising therapeutic target in rectal cancer patients.

Targeted depletion of Polo-like kinase (Plk) 1 through lentiviral shRNA or a small-molecule inhibitor causes mitotic catastrophe and induction of apoptosis in human melanoma cells

Melanoma, one of the most lethal forms of skin cancer, remains resistant to currently available treatments. Therefore, additional target-based approaches are needed for the management of this neoplasm. Polo-like kinase 1 (Plk1) has been shown to be a crucial regulator of mitotic entry, progression, and exit. Elevated Plk1 level has been associated with aggressiveness of several cancer types and with poor disease prognosis. However, the role of Plk1 in melanoma is not well established. Here, we show that Plk1 is overexpressed in both clinical tissue specimens and cultured human melanoma cells (WM115, A375, and HS294T) when compared with normal skin tissues and cultured normal melanocytes, respectively. Furthermore, Plk1 gene knockdown through Plk1-specific shRNA or its activity inhibition by a small-molecule inhibitor resulted in a significant decrease in the viability and growth of melanoma cells without affecting normal human melanocytes. In addition, Plk1 inhibition resulted in a significant (i) decrease in clonogenic survival, (ii) multiple mitotic errors, (iii) G(2)/M cell-cycle arrest, and (iv) apoptosis of melanoma cells. This study suggests that Plk1 may have a functional relevance toward melanoma development and/or progression. We suggest that the targeting of Plk1 may be a viable approach for the treatment of melanoma.

Polo-like kinase-1 controls Aurora A destruction by activating APC/C-Cdh1

Polo-like kinase-1 (Plk1) is activated before mitosis by Aurora A and its cofactor Bora. In mitosis, Bora is degraded in a manner dependent on Plk1 kinase activity and the E3 ubiquitin ligase SCF-betaTrCP. Here, we show that Plk1 is also required for the timely destruction of its activator Aurora A in late anaphase. It has been shown that Aurora A destruction is controlled by the auxiliary subunit Cdh1 of the Anaphase-Promoting Complex/Cyclosome (APC/C). Remarkably, we found that Plk1-depletion prevented the efficient dephosphorylation of Cdh1 during mitotic exit. Plk1 mediated its effect on Cdh1, at least in part, through direct phosphorylation of the human phosphatase Cdc14A, controlling the phosphorylation state of Cdh1. We conclude that Plk1 facilitates efficient Aurora A degradation through APC/C-Cdh1 activation after mitosis, with a potential role for hCdc14A.

Inositol 1,4,5-trisphosphate receptor 1, a widespread Ca2+ channel, is a novel substrate of polo-like kinase 1 in eggs

To initiate embryo development, the sperm induces in the egg release of intracellular calcium ([Ca2+](i)). During oocyte maturation, the inositol 1,4,5-trisphosphate receptor (IP(3)R1), the channel implicated, undergoes modifications that enhance its function. We found that IP(3)R1 becomes phosphorylated during maturation at an MPM-2 epitope and that this persists until the fertilization-associated [Ca2+](i) responses cease. We also reported that maturation without ERK activity diminishes IP(3)R1 MPM-2 reactivity and [Ca2+](i) responses. Here, we show that IP(3)R1 is a novel target for Polo-like kinase1 (Plk1), a conserved M-phase kinase, which phosphorylates it at an MPM-2 epitope. Plk1 and IP(3)R1 interact in an M-phase preferential manner, and they exhibit close co-localization in the spindle/spindle poles area. This co-localization is reduced in the absence of ERK activity, as the ERK pathway regulates spindle organization and IP(3)R1 cortical re-distribution. We propose that IP(3)R1 phosphorylation by Plk1, and possibly by other M-phase kinases, underlies the delivery of spatially and temporally regulated [Ca2+](i) signals during meiosis/mitosis and cytokinesis.

Second meiotic arrest and exit in frogs and mice

Mature vertebrate oocytes typically undergo programmed arrest at the second meiotic cell cycle until they are signalled to initiate embryonic development at fertilization. Here, we describe the underlying molecular mechanisms of this second meiotic arrest and release in Xenopus, and compare and contrast them with their counterparts in mice.

Polo on the Rise-from Mitotic Entry to Cytokinesis with Plk1

Polo-like kinase 1 (Plk1) is a key regulator of cell division in eukaryotic cells. New techniques, including the application of small-molecule inhibitors, have greatly expanded our knowledge of the functions, targets, and regulation of this key mitotic enzyme. In this review, we focus on how Plk1 is recruited to centrosomes, kinetochores, and the spindle midzone and what the specific tasks of Plk1 at these distinct subcellular structures might be. In particular, we highlight new work on the role of Plk1 in cytokinesis in human cells. Finally, we describe how better understanding of Plk1 functions allows critical evaluation of Plk1 as a potential drug target for cancer therapy.

14-3-3 theta binding to cell cycle regulatory factors is enhanced by HIV-1 Vpr

Background

Despite continuing advances in our understanding of AIDS pathogenesis, the mechanism of CD4+ T cell depletion in HIV-1-infected individuals remains unclear. The HIV-1 Vpr accessory protein causes cell death, likely through a mechanism related to its ability to arrest cells in the G₂,M phase. Recent evidence implicated the scaffold protein, 14-3-3, in Vpr cell cycle blockade.

Results

We found that in human T cells, 14-3-3 plays an active role in mediating Vpr-induced cell cycle arrest and reveal a dramatic increase in the amount of Cdk1, Cdc25C, and CyclinB1 bound to 14-3-3 θ during Vpr_v-induced G₂,M arrest. By contrast, a cell-cycle-arrest-dead Vpr mutant failed to augment 14-3-3 θ association with Cdk1 and CyclinB1. Moreover, G₂,M arrest caused by HIV-1 infection strongly correlated with a disruption in 14-3-3 θ binding to centrosomal proteins, Plk1 and centrin. Finally, Vpr caused elevated levels of CyclinB1, Plk1, and Cdk1 in a complex with the nuclear transport and spindle assembly protein, importin β.

Conclusion

Thus, our data reveal a new facet of Vpr-induced cell cycle arrest involving previously unrecognized abnormal rearrangements of multiprotein assemblies containing key cell cycle regulatory proteins.

Reviewers

This article was reviewed by David Kaplan, Nathaniel R. Landau and Yan Zhou.

The roles of Ca2+, downstream protein kinases, and oscillatory signaling in regulating fertilization and the activation of development

Reviews in Developmental Biology have covered the pathways that generate the all-important intracellular calcium (Ca(2+)) signal at fertilization [Miyazaki, S., Shirakawa, H., Nakada, K., Honda, Y., 1993a. Essential role of the inositol 1,4,5-trisphosphate receptor/Ca(2+) release channel in Ca(2+) waves and Ca(2+) oscillations at fertilization of mammalian eggs. Dev. Biol. 158, 62-78; Runft, L., Jaffe, L., Mehlmann, L., 2002. Egg activation at fertilization: where it all begins. Dev. Biol. 245, 237-254] and the different temporal responses of Ca(2+) in many organisms [Stricker, S., 1999. Comparative biology of calcium signaling during fertilization and egg activation in animals. Dev. Biol. 211, 157-176]. Those reviews raise the importance of identifying how Ca(2+) causes the events of egg activation (EEA) and to what extent these temporal Ca(2+) responses encode developmental information. This review covers recent studies that have analyzed how these Ca(2+) signals are interpreted by specific proteins, and how these proteins regulate various EEA responsible for the onset of development. Many of these proteins are protein kinases (CaMKII, PKC, MPF, MAPK, MLCK) whose activity is directly or indirectly regulated by Ca(2+), and whose amount increases during late oocyte maturation. We cover biochemical progress in defining the signaling pathways between Ca(2+) and the EEA, as well as discuss how oscillatory or multiple Ca(2+) signals are likely to have specific advantages biochemically and/or developmentally. These emerging concepts are put into historical context, emphasizing that key contributions have come from many organisms. The intricate interdependence of Ca(2+), Ca(2+)-dependent proteins, and the EEA raise many new questions for future investigations that will provide insight into the extent to which fertilization-associated signaling has long-range implications for development. In addition, answers to these questions should be beneficial to establishing parameters of egg quality for human and animal IVF, as well as improving egg activation protocols for somatic cell nuclear transfer to generate stem cells and save endangered species.

Roles of Greatwall kinase in the regulation of cdc25 phosphatase

We previously reported that immunodepletion of Greatwall kinase prevents Xenopus egg extracts from entering or maintaining M phase due to the accumulation of inhibitory phosphorylations on Thr14 and Tyr15 of Cdc2. M phase-promoting factor (MPF) in turn activates Greatwall, implying that Greatwall participates in an MPF autoregulatory loop. We show here that activated Greatwall both accelerates the mitotic G2/M transition in cycling egg extracts and induces meiotic maturation in G2-arrested Xenopus oocytes in the absence of progesterone. Activated Greatwall can induce phosphorylations of Cdc25 in the absence of the activity of Cdc2, Plx1 (Xenopus Polo-like kinase) or mitogen-activated protein kinase, or in the presence of an activator of protein kinase A that normally blocks mitotic entry. The effects of active Greatwall mimic in many respects those associated with addition of the phosphatase inhibitor okadaic acid (OA); moreover, OA allows cycling extracts to enter M phase in the absence of Greatwall. Taken together, these findings support a model in which Greatwall negatively regulates a crucial phosphatase that inhibits Cdc25 activation and M phase induction.

Hsp90-inhibitor geldanamycin abrogates G2 arrest in p53-negative leukemia cell lines through the depletion of Chk1

Checkpoint protein Chk1 has been identified as an Hsp90 client. Treatment with 100 nM geldanamycin (GM) for 24 h markedly reduced the Chk1 amount in Jurkat and ML-1 leukemia cell lines. Because Chk1 plays a central role in G2 checkpoint, we added GM to G2-arrested Jurkat and HL-60 cells pretreated with 50 nM doxorubicin for 24 h. GM slowly released both cell lines from doxorubicin-induced G2 arrest into G1 phase. GM also abrogated ICRF-193-induced decatenation G2 checkpoint in Jurkat and HL-60 cells. Western blot analysis showed that addition of GM attenuates doxorubicin- and ICRF-193-induced Chk1 phosphorylation at Ser345. GM, however, failed to abrogate G2 arrest in p53-positive ML-1 cells maybe due to the p21 induction. GM released HeLa cells from doxorubicin-induced G2 arrest but trapped them at M phase. Flow cytometric analysis showed that addition of GM converted doxorubicin-induced necrosis into apoptosis in Jurkat cells. Colony assay indicated that although GM has a weak cytotoxic effect as a single agent, it dramatically intensifies the cytotoxicity of doxorubicin and ICRF-193 in Jurkat and HL-60 cells. These results suggest that abrogation of G2 checkpoint by GM may play a central role in sensitizing p53-negative tumor cells to DNA-damaging and decatenation-inhibiting agents.

Cdk1 phosphorylation of BubR1 controls spindle checkpoint arrest and Plk1-mediated formation of the 3F3/2 epitope

Accurate chromosome segregation is controlled by the spindle checkpoint, which senses kinetochore- microtubule attachments and tension across sister kinetochores. An important step in the tension-signaling pathway involves the phosphorylation of an unknown protein by polo-like kinase 1/Xenopus laevis polo-like kinase 1 (Plx1) on kinetochores lacking tension to generate the 3F3/2 phosphoepitope. We report here that the checkpoint protein BubR1 interacts with Plx1 and that phosphorylation of BubR1 by Plx1 generates the 3F3/2 epitope. Formation of the BubR1 3F3/2 epitope by Plx1 requires a prior phosphorylation of BubR1 on Thr 605 by cyclin-dependant kinase 1 (Cdk1). This priming phosphorylation of BubR1 by Cdk1 is required for checkpoint-mediated mitotic arrest and for recruitment of Plx1 and the checkpoint protein Mad2 to unattached kinetochores. Biochemically, formation of the 3F3/2 phosphoepitope by Cdk1 and Plx1 greatly enhances the kinase activity of BubR1. Thus, Cdk1-mediated phosphorylation of BubR1 controls checkpoint arrest and promotes the formation of the kinetochore 3F3/2 epitope.

Polo-like kinase 1 may regulate G2/M transition of mouse fertilized eggs by means of inhibiting the phosphorylation of Tyr 15 of Cdc2

Polo-like kinase 1(Plk1) has been reported to be a multifunctional kinase that plays pivotal regulatory roles in microtubule assembly during mammalian early embryonic mitosis. In the present study, we examined the expression of Plk1 at protein and mRNA level in mouse fertilized eggs by Western blot and RT-PCR. We also examined the kinase activity of Plk1. At various developmental phases of mouse one-cell stage embryos, both the protein and the mRNA of Plk1 were uniformly distributed; but the kinase activity of Plk1 increased at G2/M phase and decreased at the end of M phase. At the meantime, the phosphorylation of Tyr 15 of Cdc2 was inhibited at M phase. To investigate its function in mammalian fertilized eggs further, we used specific short hairpin RNAs (shRNA) and scytonemin, the putative inhibitor of Plk1 to suppress the activity of Plk1 in mouse fertilized eggs. Upon blockage of the activation of with Plk1 shRNA and scytonemin in mouse one-cell stage embryos, the cleavage rate decreased and the phosphorylation level of Tyr 15 of Cdc2 increased. These results imply that the Plk1 may regulate cell cycle progression of mouse fertilized eggs by means of inhibiting the phosphorylation of Tyr 15 of Cdc2.

Regulation of mitosis via mitotic kinases: new opportunities for cancer management

Mitosis, a critical and highly orchestrated event in the cell cycle, decides how cells divide and transmit genetic information from one cell generation to the next. Errors in the choreography of these events may lead to uncontrolled proliferation, aneuploidy, and genetic instability culminating in cancer development. Considering the central role of phosphorylation in mitotic checkpoints, spindle function, and chromosome segregation, it is not surprising that several mitotic kinases have been implicated in tumorigenesis. These kinases play pivotal roles throughout cellular division. From DNA damage and spindle assembly checkpoints before entering mitosis, to kinetochore and centrosome maturation and separation, to regulating the timing of entrance and exit of mitosis, mitotic kinases are essential for cellular integrity. Therefore, targeting the mitotic kinases that control the fidelity of chromosome transmission seems to be a promising avenue in the management of cancer. This review provides an insight into the mechanism of mitotic signaling, especially the role of critical mitotic kinases. We have also discussed the possibilities of the use of mitotic kinases in crafting novel strategies in cancer management.

Molecular and structural basis of polo-like kinase 1 substrate recognition: Implications in centrosomal localization

Polo-like kinase (Plk1) is crucial for cell cycle progression through mitosis. Here we present the molecular and structural mechanisms that regulate the substrate recognition of Plk1 and influence its centrosomal localization and activity. Our work shows that Plk1 localization is controlled not only by the polo box domain (PBD); remarkably, the kinase domain is also involved in Plk1 targeting mechanism to the centrosome. The crystal structures of the PBD in complex with Cdc25C and Cdc25C-P target peptides reveal that Trp-414 is fundamental in their recognition regardless of its phosphorylation status. Binding measurements demonstrate that W414F mutation abolishes molecular recognition and diminishes centrosomal localization. Therefore, Plk1 centrosomal localization is not controlled by His-538 and Lys-540, the residues involved in phosphorylated target binding. The different conformations of the loop, which connects the polo boxes in the apo and the PBD-Cdc25C and PBD-Cdc25C-P complex structures, together with changes in the proline adjacent to the phosphothreonine in the target peptide, suggest a regulatory mechanism to detect binding of unphosphorylated or phosphorylated target substrates. Altogether, these data propose a model for the interaction between Plk1 and Cdc25C.

How eggs arrest at metaphase II: MPF stabilisation plus APC/C inhibition equals Cytostatic Factor

Oocytes from higher chordates, including man and nearly all mammals, arrest at metaphase of the second meiotic division before fertilization. This arrest is due to an activity that has been termed 'Cytostatic Factor'. Cytostatic Factor maintains arrest through preventing loss in Maturation-Promoting Factor (MPF; CDK1/cyclin B). Physiologically, Cytostatic Factor – induced metaphase arrest is only broken by a Ca²⁺ rise initiated by the fertilizing sperm and results in degradation of cyclin B, the regulatory subunit of MPF through the Anaphase-Promoting Complex/Cyclosome (APC/C). Arrest at metaphase II may therefore be viewed as being maintained by inhibition of the APC/C, and Cytostatic Factor as being one or more pathways, one of which inhibits the APC/C, consorting in the preservation of MPF activity.

Many studies over several years have implicated the c-Mos/MEK/MAPK pathway in the metaphase arrest of the two most widely studied vertebrates, frog and mouse. Murine downstream components of this cascade are not known but in frog involve members of the spindle assembly checkpoint, which act to inhibit the APC/C. Interesting these downstream components appear not to be involved in the arrest of mouse eggs, suggesting a lack of conservation with respect to c-Mos targets. However, the recent discovery of Emi2 as an egg specific APC/C inhibitor whose degradation is Ca²⁺ dependent has greatly increased our understanding of MetII arrest. Emi2 is involved in both the establishment and maintenance of metaphase II arrest in frog and mouse suggesting a conservation of metaphase II arrest. Its identity as the physiologically relevant APC/C inhibitor involved in Cytostatic Factor arrest prompted us to re-evaluate the role of the c-Mos pathway in metaphase II arrest.

This review presents a model of Cytostatic Factor arrest, which is primarily induced by Emi2 mediated APC/C inhibition but which also requires the c-Mos pathway to set MPF levels within physiological limits, not too high to induce an arrest that cannot be broken, or too low to induce parthenogenesis.

Anti-breast cancer activity of LFM-A13, a potent inhibitor of Polo-like kinase (PLK)

Molecular modeling studies led to the identification of LFM-A13 (alpha-cyano-beta-hydroxy-beta-methyl-N-(2,5-dibromophenyl)propenamide) as a potent inhibitor of Polo-like kinase (Plk). LFM-A13 inhibited recombinant purified Plx1, the Xenopus homolog of Plk, in a concentration-dependent fashion, as measured by autophosphorylation and phosphorylation of a substrate Cdc25 peptide. LFM-A13 was a selective Plk inhibitor. While the human PLK3 kinase was also inhibited by LFM-A13 with an IC(50) value of 61 microM, none of the 7 other serine/threonine kinases, including CDK1, CDK2, CDK3, CHK1, IKK, MAPK1 or SAPK2a, none of the 10 tyrosine kinases, including ABL, BRK, BMX, c-KIT, FYN, IGF1R, PDGFR, JAK2, MET, or YES, or the lipid kinase PI3Kgamma were inhibited (IC(50) values >200-500 microM). The mode of Plk3 inhibition by LFM-A13 was competitive with respect to ATP with a K(i) value of 7.2 microM from Dixon plots. LFM-A13 blocked the cell division in a zebrafish (ZF) embryo model at the 16-cell stage of the embryonic development followed by total cell fusion and lysis. LFM-A13 prevented bipolar mitotic spindle assembly in human breast cancer cells and glioblastoma cells and when microinjected into living epithelial cells at the prometaphase stage of cell division, it caused a total mitotic arrest. Notably, LFM-A13-delayed tumor progression in the MMTV/neu transgenic mouse model of HER2 positive breast cancer at least as effectively as paclitaxel and gemcitabine. LFM-A13 showed a favorable toxicity profile in mice and rats. In particular there was no evidence of hematologic toxicity as documented by peripheral blood counts and bone marrow examinations. These results establish LFM-A13 as a small molecule inhibitor of Plk with in vitro and in vivo anti-proliferative activity against human breast cancer.

Screening of domain-specific target proteins of polo-like kinase 1: construction and application of centrosome/kinetochore-specific targeting peptide

Mammalian polo-like kinase 1 (Plk1) acts at various stages in early and late mitosis. Plk1 localizes at the centrosome and maintains this position through mitosis. Thereafter Plk1 moves to the kinetochore and midbody region, important sites during chromosome separation and cytokinesis. The catalytic domain of Plk1 is in the Nterminus region, whereas the non-catalytic region in the Cterminus of Plk1 has a conserved motif, named the Polobox. This motif is critical for Plk localization. EGFP proteins fused with the N-terminus and C-terminus of Plk1 localize in the nucleus and centrosomes, respectively. The core sequences of the polo-box (50 amino acids) also localize in Plk1 target organelles. To screen for domainspecific target proteins of Plk1, we constructed an Nterminal domain and a tandem repeat polo-box motif, and used them as templates in a yeast two-hybrid screen. The HeLa cell cDNA library indicated several proteins including the centrosome/kinetochore components or regulators, to be characterized as positive clones. Through in vitro protein binding analyses, we confirmed an interaction between these proteins and Plk1. The data reported from this study indicate that the N- and Ctermini of Plk1 may function through recruitment and/or activation of domain-specific target proteins in dividing cells. Additionally, tandem repeats of the conserved core motif of the polo-box are sufficient for targeting and may be useful as a centrosome/kinetochore-specific targeting peptide.

Polo-like kinase 1: target and regulator of anaphase-promoting complex/cyclosome-dependent proteolysis

Polo-like kinase 1 (Plk1) is a key regulator of progression through mitosis. Although Plk1 seems to be dispensable for entry into mitosis, its role in spindle formation and exit from mitosis is crucial. Recent evidence suggests that a major role of Plk1 in exit from mitosis is the regulation of inhibitors of the anaphase-promoting complex/cyclosome (APC/C), such as the early mitotic inhibitor 1 (Emi1) and spindle checkpoint proteins. Thus, Plk1 and the APC/C control mitotic regulators by both phosphorylation and targeted ubiquitylation to ensure the fidelity of chromosome separation at the metaphase to anaphase transition. The mechanisms underlying the control of genomic stability by Plk1 are discussed in this review.

Cytostatic factor: an activity that puts the cell cycle on hold

Fertilization is the fundamental process in which two gametes - sperm and oocyte - fuse to generate a zygote that will form a new multicellular organism. In most vertebrates, oocytes await fertilization while arrested at metaphase of meiosis II. This resting state can be stable for many hours and depends on a cytoplasmic activity termed cytostatic factor (CSF). Recently, members of the novel Emi/Erp family of proteins have been put forward as important components of CSF. These proteins inhibit the anaphase-promoting complex/cyclosome (APC/C), which acts at the very core of the cell cycle regulatory machinery. Initially, Xenopus early mitotic inhibitor 1 (Emi1) was proposed to be a component of CSF, but newer work suggests that a structural relative, Emi-related protein 1 (Erp1/Emi2), is essential for maintenance of CSF arrest in Xenopus. Most importantly, studies on Erp1/Emi2 regulation have led to a detailed molecular understanding of the Ca2+-mediated release from CSF arrest that occurs upon fertilization.

Greatwall Kinase Participates in the Cdc2 Autoregulatory Loop in Xenopus Egg Extracts

Mutations in the Drosophila gene encoding the serine-threonine protein kinase Greatwall have previously been shown to disrupt mitotic progression. To investigate Greatwall's mitotic function, we examined its behavior in Xenopus egg extracts. Greatwall is activated during mitosis by phosphorylation; in vitro evidence indicates that maturation promoting factor (MPF) is an upstream kinase. Conversely, depletion of Greatwall from mitotic extracts rapidly lowers MPF activity due to the accumulation of inhibitory phosphorylations on Cdc2 kinase. Greatwall depletion similarly prevents cycling extracts from entering M phase. The effects of Greatwall depletion can be rescued by the addition of either wild-type (wt) Greatwall or a noninhibitable form of Cdc2 kinase. These results demonstrate that Greatwall participates in an autoregulatory loop that generates and maintains sufficiently high MPF activity levels to support mitosis.

Calcium elevation at fertilization coordinates phosphorylation of XErp1/Emi2 by Plx1 and CaMK II to release metaphase arrest by cytostatic factor

BACKGROUND

Vertebrate oocytes are arrested at second meiotic metaphase by cytostatic factor (CSF) while awaiting fertilization. Accumulating evidence has suggested that inhibition of the anaphase-promoting complex/cyclosome (APC/C) is responsible for this arrest. Xenopus polo-like kinase 1 (Plx1) is required for activation of the APC/C at the metaphase-anaphase transition, and calcium elevation, upon fertilization/activation of eggs, acting through calmodulin-dependent kinase II (CaMKII) is sufficient to activate the APC/C and terminate CSF arrest. However, connections between the Plx1 pathway and the CaMKII pathway have not been identified.

RESULTS

Overexpression of Plx1 causes CSF release in the absence of calcium, and depletion of Plx1 from egg extracts blocks induction of CSF release by calcium and CaMKII. Prior phosphorylation of the APC/C inhibitor XErp1/Emi2 by CaMK II renders it a good substrate for Plx1, and phosphorylation by both kinases together promotes its degradation in egg extracts. The pathway is enhanced by the ability of Plx1 to cause calcium-independent activation of CaMKII. The results identify the targets of CaMKII and Plx1 that promote egg activation and define the first known pathway of CSF release in which an APC/C inhibitor is targeted for degradation only when both CaMKII and Plx1 are active after calcium elevation at fertilization.

CONCLUSIONS

Plx1 with an intact polo-box domain is necessary for release of CSF arrest and sufficient when overexpressed. It acts at the same level as CaMKII in the pathway of calcium-induced CSF release by cooperating with CaMKII to regulate APC/C regulator(s), such as XErp1/Emi2, rather than by directly activating the APC/C itself.

CaMKII and polo-like kinase 1 sequentially phosphorylate the cytostatic factor Emi2/XErp1 to trigger its destruction and meiotic exit

In vertebrate meiosis, unfertilized eggs are arrested in metaphase II by cytostatic factor (CSF), which is required to maintain mitotic cyclin-dependent kinase activity. Fertilization triggers a transient increase in cytosolic free Ca(2+), which leads to CSF inactivation and ubiquitin-dependent cyclin destruction through the anaphase promoting complex or cyclosome (APC/C). The Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) and the Polo-like kinase Plx1 are essential factors for Ca(2+)-induced meiotic exit, but the critical targets of these kinases were unknown. The APC/C inhibitor Emi2 or XErp1 has recently been characterized as a pivotal CSF component, required to maintain metaphase II arrest and rapidly destroyed in response to Ca(2+) signaling through phosphorylation by Plx1 and ubiquitination by the SCF(betaTrCP) complex. An important question is how the increase in free Ca(2+) targets Plx1 activity toward Emi2. Here, we demonstrate that CaMKII is required for Ca(2+)-induced Emi2 destruction, and that CaMKII functions as a "priming kinase," directly phosphorylating Emi2 at a specific motif to induce a strong interaction with the Polo Box domain of Plx1. We show that the strict requirement for CaMKII to phosphorylate Emi2 is a specific feature of CSF arrest, and we also use phosphatase inhibitors to demonstrate an additional mode of Emi2 inactivation independent of its destruction. We firmly establish the CSF component Emi2 as the first-known critical and direct target of CaMKII in CSF release, providing a detailed molecular mechanism explaining how CaMKII and Plx1 coordinately direct APC/C activation and meiotic exit upon fertilization.

Different Plk1 functions show distinct dependencies on Polo-Box domain-mediated targeting

Polo-like kinase 1 (Plk1) has multiple important functions during M-phase progression. In addition to a catalytic domain, Plk1 possesses a phosphopeptide-binding motif, the polo-box domain (PBD), which is required for proper localization. Here, we have explored the importance of correct Plk1 subcellular targeting for its mitotic functions. We either displaced endogenous Plk1 through overexpression of the PBD or introduced the catalytic domain of Plk1, lacking the PBD, into Plk1-depleted cells. Both treatments resulted in remarkably similar phenotypes, which were distinct from the Plk1 depletion phenotype. Cells depleted of Plk1 mostly arrested with monoastral spindles, because of inhibition of centrosome maturation and separation. In contrast, these functions were not impaired in cells with mislocalized Plk1. Instead, these latter cells showed a checkpoint-dependent mitotic arrest characterized by impaired chromosome congression. Thus, whereas chromosome congression requires localized Plk1 activity, other investigated Plk1 functions are less dependent on correct PBD-mediated targeting. This opens the possibility that PBD-directed drugs might be developed to selectively interfere with a subset of Plk1 functions.

Plx1 is the 3F3/2 kinase responsible for targeting spindle checkpoint proteins to kinetochores

Dynamic attachment of microtubules to kinetochores during mitosis generates pulling force, or tension, required for the high fidelity of chromosome separation. A lack of tension activates the spindle checkpoint and delays the anaphase onset. A key step in the tension–response pathway involves the phosphorylation of the 3F3/2 epitope by an unknown kinase on untensed kinetochores. Using a rephosphorylation assay in Xenopus laevis extracts, we identified the kinetochore-associated Polo-like kinase Plx1 as the kinase both necessary and sufficient for this phosphorylation. Indeed, Plx1 is the physiological 3F3/2 kinase involved in checkpoint response, as immunodepletion of Plx1 from checkpoint extracts abolished the 3F3/2 signal and blocked association of xMad2, xBubR1, xNdc80, and xNuf2 with kinetochores. Interestingly, the kinetochore localization of Plx1 is under the control of the checkpoint protein xMps1, as immunodepletion of xMps1 prevents binding of Plx1 to kinetochores. Thus, Plx1 couples the tension signal to cellular responses through phosphorylating the 3F3/2 epitope and targeting structural and checkpoint proteins to kinetochores.

Polo-like kinase 1-mediated phosphorylation stabilizes Pin1 by inhibiting its ubiquitination in human cells

The Polo-like kinase 1 (Plk1) is a key regulator of mitosis. It is reported that the human peptidyl-prolyl cis/trans-isomerase Pin1 binds to Plk1 from mitotic cell extracts in vitro. Here we demonstrate that Ser-65 in Pin1 is the major site for Plk1-specific phosphorylation, and the polo-box domain of Plk1 is required for this phosphorylation. Interestingly, the phosphorylation of Pin1 by Plk1 does not affect its isomerase activity but rather is linked to its protein stability. Pin1 is ubiquitinated in HeLa S3 cells, and substitution of Glu for Ser-65 reduces the ubiquitination of Pin1. Furthermore, inhibition of Plk1 activity by expression of a dominant negative form of Plk1 or by transfection of small interfering RNA targeted to Plk1 enhances the ubiquitination of Pin1 and subsequently reduces the amount of Pin1 in human cancer cells. Since previous reports suggested that Plk1 is a substrate of Pin1, our work adds a new dimension to this interaction of two important mitotic regulators.

Calcium triggers exit from meiosis II by targeting the APC/C inhibitor XErp1 for degradation

Vertebrate eggs awaiting fertilization are arrested at metaphase of meiosis II by a biochemical activity termed cytostatic factor (CSF). This activity inhibits the anaphase-promoting complex/cyclosome (APC/C), a ubiquitin ligase that triggers anaphase onset and mitotic/meiotic exit by targeting securin and M-phase cyclins for destruction. On fertilization a transient rise in free intracellular calcium causes release from CSF arrest and thus APC/C activation. Although it has previously been shown that calcium induces the release of APC/C from CSF inhibition through calmodulin-dependent protein kinase II (CaMKII), the relevant substrates of this kinase have not been identified. Recently, we characterized XErp1 (Emi2), an inhibitor of the APC/C and key component of CSF activity in Xenopus egg extract. Here we show that calcium-activated CaMKII triggers exit from meiosis II by sensitizing the APC/C inhibitor XErp1 for polo-like kinase 1 (Plx1)-dependent degradation. Phosphorylation of XErp1 by CaMKII leads to the recruitment of Plx1 that in turn triggers the destruction of XErp1 by phosphorylating a site known to serve as a phosphorylation-dependent degradation signal. These results provide a molecular explanation for how the fertilization-induced calcium increase triggers exit from meiosis II.

Getting in and out of mitosis with Polo-like kinase-1

Research in different species has shown that Polo-like kinases are essential for successful cell division. In human cells, Polo-like kinase-1 (Plk1) has been implicated in the regulation of different processes, including mitotic entry, spindle formation and cytokinesis. Recently, a range of new downstream targets of Plk1 has been identified, as well as a molecular mechanism that explains recruitment of Plk1 to potential substrate proteins through its polo-box domain. On the basis of these reports, we discuss possible mechanisms by which Polo-like kinases can exert their multiple functions during mitosis. Polo-like kinases also function in DNA damage checkpoints. Plk1 has been shown to be a target of the G2 DNA damage checkpoint, while Cdc5, the Polo-like kinase in Saccharomyces cerevisiae, has long been known to be required for adaptation to persistent DNA damage. Just recently, a similar requirement for Polo-like kinases during checkpoint adaptation was demonstrated in multicellular organisms. Moreover, Plk1 was also shown to be required for checkpoint recovery following checkpoint inactivation, that is, in cells where the damage is completely repaired. Thus, Plk1 appears to play a role at multiple points during a restart of the cell cycle following DNA damage. Based on these novel observations, we discuss possible consequences of using Plk1 as a target in anticancer strategies.

Xenopus polo-like kinase Plx1 regulates XErp1, a novel inhibitor of APC/C activity

Metaphase-to-anaphase transition is a fundamental step in cell cycle progression where duplicated sister-chromatids segregate to the future daughter cells. The anaphase-promoting complex/cyclosome (APC/C) is a highly regulated ubiquitin-ligase that triggers anaphase onset and mitotic exit by targeting securin and mitotic cyclins for destruction. It was previously shown that the Xenopus polo-like kinase Plx1 is essential to activate APC/C upon release from cytostatic factor (CSF) arrest in Xenopus egg extract. Although the mechanism by which Plx1 regulates APC/C activation remained unclear, the existence of a putative APC/C inhibitor was postulated whose activity would be neutralized by Plx1 upon CSF release. Here we identify XErp1, a novel Plx1-regulated inhibitor of APC/C activity, and we demonstrate that XErp1 is required to prevent anaphase onset in CSF-arrested Xenopus egg extract. Inactivation of XErp1 leads to premature APC/C activation. Conversely, addition of excess XErp1 to Xenopus egg extract prevents APC/C activation. Plx1 phosphorylates XErp1 in vitro at a site that targets XErp1 for degradation upon CSF release. Thus, our data lead to a model of APC/C activation in Xenopus egg extract in which Plx1 targets the APC/C inhibitor XErp1 for degradation.

Closing mitosis: the functions of the Cdc14 phosphatase and its regulation

Completion of the cell cycle requires the temporal and spatial coordination of chromosome segregation with mitotic spindle disassembly and cytokinesis. In budding yeast, the protein phosphatase Cdc14 is a key regulator of these late mitotic events. Here, we review the functions of Cdc14 and how this phosphatase is regulated to accomplish the coupling of mitotic processes. We also discuss the function and regulation of Cdc14 in other eukaryotes, emphasizing conserved features.

A role for the anaphase-promoting complex inhibitor Emi2/XErp1, a homolog of early mitotic inhibitor 1, in cytostatic factor arrest of Xenopus eggs

Unfertilized vertebrate eggs are arrested in metaphase of meiosis II with high cyclin B/Cdc2 activity to prevent parthenogenesis. Until fertilization, exit from metaphase is blocked by an activity called cytostatic factor (CSF), which stabilizes cyclin B by inhibiting the anaphase-promoting complex (APC) ubiquitin ligase. The APC inhibitor early mitotic inhibitor 1 (Emi1) was recently found to be required for maintenance of CSF arrest. We show here that exogenous Emi1 is unstable in CSF-arrested Xenopus eggs and is destroyed by the SCF(betaTrCP) ubiquitin ligase, suggesting that endogenous Emi1, an apparent 44-kDa protein, requires a stabilizing factor. However, anti-Emi1 antibodies crossreact with native Emi2/Erp1/FBXO43, a homolog of Emi1 and conserved APC inhibitor. Emi2 is stable in CSF-arrested eggs, is sufficient to prevent CSF release, and is rapidly degraded in a Polo-like kinase 1-dependent manner in response to calcium-mediated egg activation. These results identify Emi2 as a candidate CSF maintenance protein.

Emi1 and Erp1: Who Can Stop These Eggs?

Vertebrate eggs prevent parthenogenetic development by producing cytostatic factor (CSF), which blocks exit from metaphase of meiosis II until fertilization. CSF was never purified but recently suspected to inhibit the anaphase-promoting complex (APC), an ubiquitin ligase required for entry into anaphase. In a recent paper in Genes & Development, Schmidt et al. describe the Xenopus APC inhibitor Erp1, which seems to be the best candidate yet for the downstream effector of CSF activity.

The forkhead-associated domain protein Cep170 interacts with Polo-like kinase 1 and serves as a marker for mature centrioles

We report the characterization of Cep170, a forkhead-associated (FHA) domain protein of previously unknown function. Cep170 was identified in a yeast two-hybrid screen for interactors of Polo-like kinase 1 (Plk1). In human cells, Cep170 is constantly expressed throughout the cell cycle but phosphorylated during mitosis. It interacts with Plk1 in vivo and can be phosphorylated by Plk1 in vitro, suggesting that it is a physiological substrate of this kinase. Both overexpression and small interfering RNA (siRNA)-mediated depletion studies suggest a role for Cep170 in microtuble organization and cell morphology. Cep170 associates with centrosomes during interphase and with spindle microtubules during mitosis. As shown by immunoelectron microscopy, Cep170 associates with subdistal appendages, typical of the mature mother centriole. Thus, anti-Cep170 antibodies stain only one centriole during G1, S, and early G2, but two centrioles during late G2 phase of the cell cycle. We show that Cep170 labeling can be used to discriminate bona fide centriole overduplication from centriole amplification that results from aborted cell division.