The Polo-like kinase Plx1 interacts with and inhibits Myt1 after fertilization of Xenopus eggs

Chk2 sustains PLK1 activity in mitosis to ensure proper chromosome segregation

Polo-like kinase 1 (PLK1) protects against genome instability by ensuring timely and accurate mitotic cell division. PLK1 activity is tightly regulated throughout the cell cycle. Although the pathways that initially activate PLK1 in G2 are well-characterized, the factors that directly regulate PLK1 in mitosis remain poorly understood. Here, we identify that human PLK1 activity is sustained by the DNA damage response kinase Checkpoint kinase 2 (Chk2) in mitosis. Chk2 directly phosphorylates PLK1 T210, a residue on its T-loop whose phosphorylation is essential for full PLK1 kinase activity. Loss of Chk2-dependent PLK1 activity causes increased mitotic errors, including chromosome misalignment, chromosome missegregation, and cytokinetic defects. Moreover, Chk2 deficiency increases sensitivity to PLK1 inhibitors, suggesting that Chk2 status may be an informative biomarker for PLK1 inhibitor efficacy. This work demonstrates that Chk2 sustains mitotic PLK1 activity and protects genome stability through discrete functions in interphase DNA damage repair and mitotic chromosome segregation.

Computational analysis of synergism in small networks with different logic

Cell fate decision processes are regulated by networks which contain different molecules and interactions. Different network topologies may exhibit synergistic or antagonistic effects on cellular functions. Here, we analyze six most common small networks with regulatory logic AND or OR, trying to clarify the relationship between network topologies and synergism (or antagonism) related to cell fate decisions. We systematically examine the contribution of both network topologies and regulatory logic to the cell fate synergism by bifurcation and combinatorial perturbation analysis. Initially, under a single set of parameters, the synergism of three types of networks with AND and OR logic is compared. Furthermore, to consider whether these results depend on the choices of parameter values, statistics on the synergism of five hundred parameter sets is performed. It is shown that the results are not sensitive to parameter variations, indicating that the synergy or antagonism mainly depends on the network topologies rather than the choices of parameter values. The results indicate that the topology with "Dual Inhibition" shows good synergism, while the topology with "Dual Promotion" or "Hybrid" shows antagonism. The results presented here may help us to design synergistic networks based on network structure and regulation combinations, which has promising implications for cell fate decisions and drug combinations.

A Review of the Regulatory Mechanisms of N-Myc on Cell Cycle

Neuroblastoma has obvious heterogeneity. It is one of the few undifferentiated malignant tumors that can spontaneously degenerate into completely benign tumors. However, for its high-risk type, even with various intensive treatment options, the prognosis is still unsatisfactory. At the same time, a large number of research data show that the abnormal amplification and high-level expression of the MYCN gene are positively correlated with the malignant progression, poor prognosis, and mortality of neuroblastoma. In this context, this article explores the role of the N-Myc, MYCN gene expression product on its target genes related to the cell cycle and reveals its regulatory network in promoting tumor proliferation and malignant progression. We hope it can provide ideas and direction for the research and development of drugs targeting N-Myc and its downstream target genes.

Multiple Roles of PLK1 in Mitosis and Meiosis

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

Revisiting the multisite phosphorylation that produces the M-phase supershift of key mitotic regulators

The term M-phase supershift denotes the phosphorylation-dependent substantial increase in the apparent molecular weight of numerous proteins of varied biological functions during M-phase induction. Although the M-phase supershift of multiple key mitotic regulators has been attributed to the multisite phosphorylation catalyzed by the Cdk1/cyclin B/Cks complex, this view is challenged by multiple lines of paradoxical observations. To solve this problem, we reconstituted the M-phase supershift of Xenopus Cdc25C, Myt1, Wee1A, APC3 and Greatwall in Xenopus egg extracts and characterized the supershift-producing phosphorylations. Our results demonstrate that their M-phase supershifts are each due to simultaneous phosphorylation of a considerable portion of S/T/Y residues in a long intrinsically disordered region that is enriched in both S/T residues and S/TP motifs. Although the major mitotic kinases in Xenopus egg extracts, Cdk1, MAPK, Plx1 and RSK2, are able to phosphorylate the five mitotic regulators, they are neither sufficient nor required to produce the M-phase supershift. Accordingly, inhibition of the four major mitotic kinase activities in Xenopus oocytes did not inhibit the M-phase supershift in okadaic acid-induced oocyte maturation. These findings indicate that the M-phase supershift is produced by a previously unrecognized category of mitotic phosphorylation that likely plays important roles in M-phase induction.

PPP4C facilitates homologous recombination DNA repair by dephosphorylating PLK1 during early embryo development

Mammalian early embryo cells have complex DNA repair mechanisms to maintain genomic integrity, and homologous recombination (HR) plays the main role in response to double-strand DNA breaks (DSBs) in these cells. Polo-like kinase 1 (PLK1) participates in the HR process and its overexpression has been shown to occur in a variety of human cancers. Nevertheless, the regulatory mechanism of PLK1 remains poorly understood, especially during the S and G2 phase. Here, we show that protein phosphatase 4 catalytic subunit (PPP4C) deletion causes severe female subfertility due to accumulation of DNA damage in oocytes and early embryos. PPP4C dephosphorylated PLK1 at the S137 site, negatively regulating its activity in the DSB response in early embryonic cells. Depletion of PPP4C induced sustained activity of PLK1 when cells exhibited DNA lesions that inhibited CHK2 and upregulated the activation of CDK1, resulting in inefficient loading of the essential HR factor RAD51. On the other hand, when inhibiting PLK1 in the S phase, DNA end resection was restricted. These results demonstrate that PPP4C orchestrates the switch between high-PLK1 and low-PLK1 periods, which couple the checkpoint to HR.

Oocyte maturation abnormalities - A systematic review of the evidence and mechanisms in a rare but difficult to manage fertility pheneomina

A small proportion of infertile women experience repeated oocyte maturation abnormalities (OMAS). OMAS include degenerated and dysmorphic oocytes, empty follicle syndrome, oocyte maturation arrest (OMA), resistant ovary syndrome and maturation defects due to primary ovarian insufficiency. Genetic factors play an important role in OMAS but still need specifications. This review documents the spectrum of OMAS and to evaluate the multiple subtypes classified as OMAS. In this review, readers will be able to understand the oocyte maturation mechanism, gene expression and their regulation that lead to different subtypes of OMAs, and it will discuss the animal and human studies related to OMAS and lastly the treatment options for OMAs. Literature searches using PubMed, MEDLINE, Embase, National Institute for Health and Care Excellence were performed to identify articles written in English focusing on Oocyte Maturation Abnormalities by looking for the following relevant keywords. A search was made with the specified keywords and included books and documents, clinical trials, animal studies, human studies, meta-analysis, randomized controlled trials, reviews, systematic reviews and options written in english. The search detected 3,953 sources published from 1961 to 2021. After title and abstract screening for study type, duplicates and relevancy, 2,914 studies were excluded. The remaining 1,039 records were assessed for eligibility by full-text reading and 886 records were then excluded. Two hundred and twenty seven full-text articles and 0 book chapters from the database were selected for inclusion. Overall, 227 articles, one unpublished and one abstract paper were included in this final review. In this review study, OMAS were classified and extensively evaluatedand possible treatment options under the light of current information, present literature and ongoing studies. Either genetic studies or in vitro maturation studies that will be handled in the future will lead more informations to be reached and may make it possible to obtain pregnancies.

Polo-like kinase 1 (PLK1) signaling in cancer and beyond

PLK1 is an evolutionary conserved Ser/Thr kinase that is best known for its role in cell cycle regulation and is expressed predominantly during the G2/S and M phase of the cell cycle. PLK1-mediated phosphorylation of specific substrates controls cell entry into mitosis, centrosome maturation, spindle assembly, sister chromatid cohesion and cytokinesis. In addition, a growing body of evidence describes additional roles of PLK1 beyond the cell cycle, more specifically in the DNA damage response, autophagy, apoptosis and cytokine signaling. PLK1 has an indisputable role in cancer as it controls several key transcription factors and promotes cell proliferation, transformation and epithelial-to-mesenchymal transition. Furthermore, deregulation of PLK1 results in chromosome instability and aneuploidy. PLK1 is overexpressed in many cancers, which is associated with poor prognosis, making PLK1 an attractive target for cancer treatment. Additionally, PLK1 is involved in immune and neurological disorders including Graft versus Host Disease, Huntington's disease and Alzheimer's disease. Unfortunately, newly developed small compound PLK1 inhibitors have only had limited success so far, due to low therapeutic response rates and toxicity. In this review we will highlight the current knowledge about the established roles of PLK1 in mitosis regulation and beyond. In addition, we will discuss its tumor promoting but also tumor suppressing capacities, as well as the available PLK1 inhibitors, elaborating on their efficacy and limitations.

Regulation of Myt1 kinase activity via its N-terminal region in Xenopus meiosis and mitosis

Immature animal oocytes are naturally arrested at the first meiotic prophase (Pro-I), which corresponds to the G2 phase of the cell cycle. In Xenopus oocytes, Myt1 kinase phosphorylates and inactivates cyclin-dependent kinase 1 (Cdk1) at Pro-I, thereby preventing oocytes from entering meiosis I (MI) prematurely. Previous studies have shown that, upon resuming MI, Cdk1 and p90rsk, which is a downstream kinase of the Mos-MAPK pathway, in turn phosphorylate the C-terminal region of Myt1, to suppress its activity, thereby ensuring high Cdk1 activity during M phase. However, the roles of the N-terminal region of Myt1 during meiosis and mitosis remain to be elucidated. In the present study, we show that the N-terminal region of Myt1 participates in the regulation of Myt1 activity in the Xenopus cell cycle. In particular, we found that a short, conserved sequence in the N-terminal region, termed here as the PAYF motif, is required for the normal activity of Myt1 in oocytes. Furthermore, multiple phosphorylations by Cdk1 at the Myt1 N-terminal region were found to be involved in the negative regulation of Myt1. In particular, phosphorylations at Thr11 and Thr16 of Myt1, which are adjacent to the PAYF motif, were found to be important for the inactivation of Myt1 in the M phase of the cell cycle. These results suggest that in addition to the regulation of Myt1 activity via the C-terminal region, the N-terminal region of Myt1 also plays an important role in the regulation of Myt1 activity.

Clinical Candidates Targeting the ATR-CHK1-WEE1 Axis in Cancer

Selective killing of cancer cells while sparing healthy ones is the principle of the perfect cancer treatment and the primary aim of many oncologists, molecular biologists, and medicinal chemists. To achieve this goal, it is crucial to understand the molecular mechanisms that distinguish cancer cells from healthy ones. Accordingly, several clinical candidates that use particular mutations in cell-cycle progressions have been developed to kill cancer cells. As the majority of cancer cells have defects in G1 control, targeting the subsequent intra‑S or G2/M checkpoints has also been extensively pursued. This review focuses on clinical candidates that target the kinases involved in intra‑S and G2/M checkpoints, namely, ATR, CHK1, and WEE1 inhibitors. It provides insight into their current status and future perspectives for anticancer treatment. Overall, even though CHK1 inhibitors are still far from clinical establishment, promising accomplishments with ATR and WEE1 inhibitors in phase II trials present a positive outlook for patient survival.

The G2-to-M transition from a phosphatase perspective: a new vision of the meiotic division

Cell division is orchestrated by the phosphorylation and dephosphorylation of thousands of proteins. These post-translational modifications underlie the molecular cascades converging to the activation of the universal mitotic kinase, Cdk1, and entry into cell division. They also govern the structural events that sustain the mechanics of cell division. While the role of protein kinases in mitosis has been well documented by decades of investigations, little was known regarding the control of protein phosphatases until the recent years. However, the regulation of phosphatase activities is as essential as kinases in controlling the activation of Cdk1 to enter M-phase. The regulation and the function of phosphatases result from post-translational modifications but also from the combinatorial association between conserved catalytic subunits and regulatory subunits that drive their substrate specificity, their cellular localization and their activity. It now appears that sequential dephosphorylations orchestrated by a network of phosphatase activities trigger Cdk1 activation and then order the structural events necessary for the timely execution of cell division. This review discusses a series of recent works describing the important roles played by protein phosphatases for the proper regulation of meiotic division. Many breakthroughs in the field of cell cycle research came from studies on oocyte meiotic divisions. Indeed, the meiotic division shares most of the molecular regulators with mitosis. The natural arrests of oocytes in G2 and in M-phase, the giant size of these cells, the variety of model species allowing either biochemical or imaging as well as genetics approaches explain why the process of meiosis has served as an historical model to decipher signalling pathways involved in the G2-to-M transition. The review especially highlights how the phosphatase PP2A-B55δ critically orchestrates the timing of meiosis resumption in amphibian oocytes. By opposing the kinase PKA, PP2A-B55δ controls the release of the G2 arrest through the dephosphorylation of their substrate, Arpp19. Few hours later, the inhibition of PP2A-B55δ by Arpp19 releases its opposing kinase, Cdk1, and triggers M-phase. In coordination with a variety of phosphatases and kinases, the PP2A-B55δ/Arpp19 duo therefore emerges as the key effector of the G2-to-M transition.

Transcriptional analysis and differentially expressed gene screening of spontaneous liver tumors in CBA/CaJ mice

Hepatocellular carcinoma (HCC) is the third most common cause of cancer-related deaths worldwide due to its frequent metastasis, tumor recurrence, and lack of curative treatment. However, the underlying molecular mechanisms involved in HCC progression remain unclear. Here, we analyzed the global gene expression of spontaneous liver tumor tissue from CBA/CaJ mice by RNA-Seq and identified 10,706 and 10,374 genes in the normal and liver tumor groups, respectively. Only 9793 genes were expressed in both, 913 genes were identified in only the liver tumor group, and 581 genes were found in normal liver tissues. There were 2054 differentially expressed genes (DEGs), with 975 down-regulated genes and 1079 up-regulated genes. Gene ontology (GO) term enrichment analysis showed that 43 up-regulated genes were significantly associated with cell cycle regulation and hundreds of up-regulated genes were related to cell migration, adhesion, or metabolic processes. KEGG pathway enrichment also demonstrated that some DEGs were tightly associated with the cell cycle, extracellular matrix (ECM)-receptor interactions, as well as protein digestion and absorption pathways, indicating that the activation of these oncogenic cascades was closely related to tumor liver progression in CBA/CaJ mice. Ninety-three genes with elevated expression levels preferentially localized in microtubules, kinetochores, and spindles play an important role during mitosis and meiosis and are associated with the reorganization of the cytoskeleton in cancer cells during migration and invasion. Some ECM-related genes were significantly different in the tumor group, including collagen types I, III, IV, V, and VI, non-collagenous glycoproteins, laminin, and fibronectin. We further validated the functions of upregulated genes, such as cyclin-dependent kinase 1 (CDK1) and polo-like kinase 1 (PLK1), with regards to cell cycle regulation, apoptosis, and proliferation in normal human liver or liver tumor-derived cell lines. Our results indicated that the cell cycle dysregulation, ECM-receptor interaction, and cytoskeleton-associated genes in mouse livers may promote HCC progression and deciphering the function of the genes will help investigators understand the underlying molecular mechanism of HCC.

Involvement of Myt1 kinase in the G2 phase of the first cell cycle in Xenopus laevis

During cleavage of Xenopus laevis, the first mitotic cell cycle immediately following fertilization is approximately 90 min and consists of S, G2, and M phases. In contrast, the subsequent eleven cell cycles are approximately 30 min and consist mostly of S and M phases. The balance between Cdc25 and Wee1A/Myt1 is thought to be crucial for Xenopus first cell cycle progression; however, the role of Myt1 in this period has not been fully investigated. In this study, we examined the roles of Myt1, Wee1A, and Cdc25A in the first cell cycle of Xenopus laevis. Inhibition of Cdc25A with antisense morpholino oligonucleotides lengthened the duration of the first cell cycle to some extent, whereas it was slightly shortened by ectopic Cdc25A expression, suggesting that the low concentration of Cdc25A during the first cell cycle does not fully account for the long duration of this cycle. Using the Wee1A antisense morpholino oligonucleotide and neutralizing antibody against Myt1, we found that Myt1 phosphorylates and inhibits Cdk1 much more effectively than Wee1A during the first cell cycle in Xenopus. Taken together, these results suggest that the activity of Myt1 is predominantly responsible for the duration of the long G2 phase in the first mitotic cell cycle in Xenopus.

Coupling of Polo kinase activation to nuclear localization by a bifunctional NLS is required during mitotic entry

The Polo kinase is a master regulator of mitosis and cytokinesis conserved from yeasts to humans. Polo is composed of an N-term kinase domain (KD) and a C-term polo-box domain (PBD), which regulates its subcellular localizations. The PBD and KD can interact and inhibit each other, and this reciprocal inhibition is relieved when Polo is phosphorylated at its activation loop. How Polo activation and localization are coupled during mitotic entry is unknown. Here we report that PBD binding to the KD masks a nuclear localization signal (NLS). Activating phosphorylation of the KD leads to exposure of the NLS and entry of Polo into the nucleus before nuclear envelope breakdown. Failures of this mechanism result in misregulation of the Cdk1-activating Cdc25 phosphatase and lead to mitotic and developmental defects in Drosophila. These results uncover spatiotemporal mechanisms linking master regulatory enzymes during mitotic entry.

Drosophila Polo kinase is the founding member of the Polo-Like Kinase (PLK) family and a master regulator of mitosis and cytokinesis. Here the authors uncover a molecular mechanism for the spatiotemporal regulation of Polo kinase during mitotic entry through a phosphorylation event that triggers nuclear import.

Regulatory functional territory of PLK-1 and their substrates beyond mitosis

Polo-like kinase 1 (PLK-1) is a well-known (Ser/Thr) mitotic protein kinase and is considered as a proto-oncogene. As hyper-activation of PLK-1 is broadly associated with poor prognosis and cancer progression, it is one of the most extensively studied mitotic kinases. During mitosis, PLK-1 regulates various cell cycle events, such as spindle pole maturation, chromosome segregation and cytokinesis. However, studies have demonstrated that the role of PLK-1 is not only restricted to mitosis, but PLK-1 can also regulate other vital events beyond mitosis, including transcription, translation, ciliogenesis, checkpoint adaptation and recovery, apoptosis, chromosomes dynamics etc. Recent reviews have tried to define the regulatory role of PLK-1 during mitosis progression and tumorigenesis, but its' functional role beyond mitosis is still largely unexplored. PLK-1 can regulate the activity of many proteins that work outside of its conventional territory. The dysregulation of these proteins can cause diseases such as Alzheimer's disease, tumorigenesis etc. and may also lead to drug resistance. Thus, in this review, we discussed the versatile role of PLK-1 and tried to collect data to validate its' functional role in cell cycle regulation apart from mitosis.

Regulation of Cell Division

The challenging task of mitotic cell divisions is to generate two genetically identical daughter cells from a single precursor cell. To accomplish this task, a complex regulatory network evolved, which ensures that all events critical for the duplication of cellular contents and their subsequent segregation occur in the correct order, at specific intervals and with the highest possible fidelity. Transitions between cell cycle stages are triggered by changes in the phosphorylation state and levels of components of the cell cycle machinery. Entry into S-phase and M-phase are mediated by cyclin-dependent kinases (Cdks), serine-threonine kinases that require a regulatory cyclin subunit for their activity. Resetting the system to the interphase state is mediated by protein phosphatases (PPs) that counteract Cdks by dephosphorylating their substrates. To avoid futile cycles of phosphorylation and dephosphorylation, Cdks and PPs must be regulated in a manner such that their activities are mutually exclusive.

Mitotic entry: The interplay between Cdk1, Plk1 and Bora

Polo-like kinase 1 (Plk1) is an important mitotic kinase that is crucial for entry into mitosis after recovery from DNA damage-induced cell cycle arrest. Plk1 activation is promoted by the conserved protein Bora (SPAT-1 in C. elegans), which stimulates the phosphorylation of a conserved residue in the activation loop by the Aurora A kinase. In a recent article published in Cell Reports, we show that the master mitotic kinase Cdk1 contributes to Plk1 activation through SPAT-1/Bora phosphorylation. We identified 3 conserved Sp/Tp residues that are located in the N-terminal, most conserved part, of SPAT-1/Bora. Phosphorylation of these sites by Cdk1 is essential for Plk1 function in mitotic entry in C. elegans embryos and during DNA damage checkpoint recovery in mammalian cells. Here, using an untargeted Förster Resonance Energy Transfer (FRET) biosensor to monitor Plk1 activation, we provide additional experimental evidence supporting the importance of these phosphorylation sites for Plk1 activation and subsequent mitotic entry after DNA damage. We also briefly discuss the mechanism of Plk1 activation and the potential role of Bora phosphorylation by Cdk1 in this process. As Plk1 is overexpressed in cancer cells and this correlates with poor prognosis, understanding how Bora contributes to Plk1 activation is paramount for the development of innovative therapeutical approaches.

Effect of guaianolides in the meiosis reinitiation of amphibian oocytes

Sesquiterpene lactones (STLs) are a large and structurally diverse group of plant metabolites generally found in the Asteraceae family. STLs exhibit a wide spectrum of biological activities and it is generally accepted that their major mechanism of action is the alkylation of the thiol groups of biological molecules. The guaianolides is one of various groups of STLs. Anti-tumour and anti-migraine effects, an allergenic agent, an inhibitor of smooth muscle cells and of meristematic cell proliferation are only a few of the most commonly reported activities of STLs. In amphibians, fully grown ovarian oocytes are arrested at the beginning of meiosis I. Under stimulus with progesterone, this meiotic arrest is released and meiosis progresses to metaphase II, a process known as oocyte maturation. There are previous records of the inhibitory effect of dehydroleucodin (DhL), a guaianolide lactone, on the progression of meiosis. It has been also shown that DhL and its 11,13-dihydroderivative (2H-DhL; a mixture of epimers at C-11) act as blockers of the resumption of meiosis in fully grown ovarian oocytes from the amphibian Rhinella arenarum (formerly classified as Bufo arenarum). The aim of this study was to analyze the effect of four closely related guaianolides, i.e., DhL, achillin, desacetoxymatricarin and estafietin as possible inhibitors of meiosis in oocytes of amphibians in vitro and discuss some structure-activity relationships. It was found that the inhibitory effect on meiosis resumption is greater when the lactone has two potentially reactive centres, either a α,β-α',β'-diunsaturated cyclopentanone moiety or an epoxide group plus an exo-methylene-γ-lactone function.

Myt1 inhibition of Cyclin A/Cdk1 is essential for fusome integrity and premeiotic centriole engagement in Drosophila spermatocytes

Drosophila Myt1 is essential for male fertility. Loss of Myt1 activity causes defective fusomes and premature centriole disengagement during premeiotic G2 phase due to lack of Myt1 inhibition of Cyclin A/Cdk1. These functions are distinct from known roles for Myt1 inhibition of Cyclin B/Cdk1 used to regulate G2/MI timing.

Regulation of cell cycle arrest in premeiotic G2 phase coordinates germ cell maturation and meiotic cell division with hormonal and developmental signals by mechanisms that control Cyclin B synthesis and inhibitory phosphorylation of the M-phase kinase, Cdk1. In this study, we investigated how inhibitory phosphorylation of Cdk1 by Myt1 kinase regulates premeiotic G2 phase of Drosophila male meiosis. Immature spermatocytes lacking Myt1 activity exhibit two distinct defects: disrupted intercellular bridges (fusomes) and premature centriole disengagement. As a result, the myt1 mutant spermatocytes enter meiosis with multipolar spindles. These myt1 defects can be suppressed by depletion of Cyclin A activity or ectopic expression of Wee1 (a partially redundant Cdk1 inhibitory kinase) and phenocopied by expression of a Cdk1F mutant defective for inhibitory phosphorylation. We therefore conclude that Myt1 inhibition of Cyclin A/Cdk1 is essential for normal fusome behavior and centriole engagement during premeiotic G2 arrest of Drosophila male meiosis. The novel meiotic functions we discovered for Myt1 kinase are spatially and temporally distinct from previously described functions of Myt1 as an inhibitor of Cyclin B/Cdk1 to regulate G2/MI timing.

PLK-1: Angel or devil for cell cycle progression

PLK-1 is a key player in the eukaryotic cell cycle. Cell cycle progression is precisely controlled by cell cycle regulatory kinases. PLK-1 is a mitotic kinase that actively regulates the G2/M transition, mitosis, mitotic exit, and cytokinesis. During cell cycle progression, PLK-1 controls various events related to the cell cycle maturation, directly and/or indirectly. On the contrary, aberrant expression of PLK-1 is strongly associated with tumorigenesis and its poor prognosis. The misexpression of PLK-1 causes the abnormalities including aneuploidy, mitotic defects, leading to tumorigenesis through inhibiting the p53 and pRB genes. Therefore, we reviewed the role of PLK-1 in the cell cycle progression and in the tumorigenesis either as a cell cycle regulator or on an attractive anti-cancer drug target.

Phosphorylation of ARPP19 by protein kinase A prevents meiosis resumption in Xenopus oocytes

During oogenesis, oocytes are arrested in prophase and resume meiosis by activating the kinase Cdk1 upon hormonal stimulation. In all vertebrates, release from prophase arrest relies on protein kinase A (PKA) downregulation and on the dephosphorylation of a long-sought but still unidentified substrate. Here we show that ARPP19 is the PKA substrate whose phosphorylation at serine 109 is necessary and sufficient for maintaining Xenopus oocytes arrested in prophase. By downregulating PKA, progesterone, the meiotic inducer in Xenopus, promotes partial dephosphorylation of ARPP19 that is required for the formation of a threshold level of active Cdk1. Active Cdk1 then initiates MPF autoamplification loop that occurs independently of both PKA and ARPP19 phosphorylation at serine 109 but requires the Greatwall-dependent phosphorylation of ARPP19 at serine 67. Therefore, ARPP19 stands at a crossroads in the meiotic M-phase control network by integrating differential effects of PKA and Greatwall, two essential kinases for meiosis resumption.

Polo-like kinase and its inhibitors: Ready for the match to start?

Polo-like kinases (Plks) plays a central role in the normal cell cycle and their upregulation has been shown to play a role in the pathogenesis of multiple human cancers. Preclinical work demonstrates that targeting Plk has a significant impact on the treatment of both solid and hematologic malignancies in vitro and in vivo. We review here the basic science and clinical work to date with the Plks as well as future directions with this novel class of mitotic inhibitors.

PLK-1 Targeted Inhibitors and Their Potential against Tumorigenesis

Mitotic kinases are the key components of the cell cycle machinery and play vital roles in cell cycle progression. PLK-1 (Polo-like kinase-1) is a crucial mitotic protein kinase that plays an essential role in both the onset of G2/M transition and cytokinesis. The overexpression of PLK-1 is strongly correlated with a wide spectrum of human cancers and poor prognosis. The (si)RNA-mediated depletion of PLK-1 arrests tumor growth and triggers apoptosis in cancer cells without affecting normal cells. Therefore, PLK-1 has been selected as an attractive anticancer therapeutic drug target. Some small molecules have been discovered to target the catalytic and noncatalytic domains of PLK-1. These domains regulate the catalytic activation and subcellular localization of PLK-1. However, while PLK-1 inhibitors block tumor growth, they have been shown to cause severe adverse complications, such as toxicity, neutropenia, and bone marrow suppression during clinical trials, due to a lack of selectivity and specificity within the human kinome. To minimize these toxicities, inhibitors should be tested against all protein kinases in vivo and in vitro to enhance selectivity and specificity against targets. Here, we discuss the potency and selectivity of PLK-1-targeted inhibitors and their molecular interactions with PLK-1 domains.

Cell cycle timing regulation during asynchronous divisions of the early C. elegans embryo

A fundamental question in developmental biology is how different cell lineages acquire different cell cycle durations. With its highly stereotypical asymmetric and asynchronous cell divisions, the early Caenorhabditis elegans embryo provides an ideal system to study lineage-specific cell cycle timing regulation during development, with high spatio-temporal resolution. The first embryonic division is asymmetric and generates two blastomeres of different sizes (AB>P1) and developmental potentials that divide asynchronously, with the anterior somatic blastomere AB dividing reproducibly two minutes before the posterior germline blastomere P1. The evolutionarily conserved PAR proteins (abnormal embryonic PARtitioning of cytoplasm) regulate all of the asymmetries in the early embryo including cell cycle asynchrony between AB and P1 blastomeres. Here we discuss our current understanding and open questions on the mechanism by which the PAR proteins regulate asynchronous cell divisions in the early C. elegans embryo.

Cdk1 plays matchmaker for the Polo-like kinase and its activator SPAT-1/Bora

Mitosis is orchestrated by several protein kinases including Cdks, Plks and Aurora kinases. Despite considerable progress toward understanding the individual function of these protein kinases, how their activity is coordinated in space and time during mitosis is less well understood. In a recent article published in the Journal of Cell Biology, we show that CDK-1 regulates PLK-1 activity during mitosis in C. elegans embryos through multisite phosphorylation of the PLK-1 activator SPAT-1 (Aurora Borealis, Bora in human). SPAT-1 variants mutated on CDK-1 phosphorylation sites results in severe delays in mitotic entry, mimicking embryos lacking spat-1 or plk-1 function. We further show that SPAT-1 phosphorylation by CDK-1 promotes its binding to PLK-1 and stimulates PLK-1 phosphorylation on its activator T-loop by Aurora A kinase in vitro. Likewise, we find that phosphorylation of Bora by Cdk1 promotes phosphorylation of human Plk1 by Aurora A suggesting that this mechanism is conserved in humans. These results indicate that Cdk1 regulates Plk1 by boosting its kinase activity. Here we discuss these recent findings and open questions regarding the regulation of Plk1/PLK-1 by Cdk1/CDK-1 and Bora/SPAT-1.

Coordinating cell polarity and cell cycle progression: what can we learn from flies and worms?

Spatio-temporal coordination of events during cell division is crucial for animal development. In recent years, emerging data have strengthened the notion that tight coupling of cell cycle progression and cell polarity in dividing cells is crucial for asymmetric cell division and ultimately for metazoan development. Although it is acknowledged that such coupling exists, the molecular mechanisms linking the cell cycle and cell polarity machineries are still under investigation. Key cell cycle regulators control cell polarity, and thus influence cell fate determination and/or differentiation, whereas some factors involved in cell polarity regulate cell cycle timing and proliferation potential. The scope of this review is to discuss the data linking cell polarity and cell cycle progression, and the importance of such coupling for asymmetric cell division. Because studies in model organisms such as Caenorhabditis elegans and Drosophila melanogaster have started to reveal the molecular mechanisms of this coordination, we will concentrate on these two systems. We review examples of molecular mechanisms suggesting a coupling between cell polarity and cell cycle progression.

Modulation of cell cycle control during oocyte-to-embryo transitions

Ex ovo omnia--all animals come from eggs--this statement made in 1651 by the English physician William Harvey marks a seminal break with the doctrine that all essential characteristics of offspring are contributed by their fathers, while mothers contribute only a material substrate. More than 360 years later, we now have a comprehensive understanding of how haploid gametes are generated during meiosis to allow the formation of diploid offspring when sperm and egg cells fuse. In most species, immature oocytes are arrested in prophase I and this arrest is maintained for few days (fruit flies) or for decades (humans). After completion of the first meiotic division, most vertebrate eggs arrest again at metaphase of meiosis II. Upon fertilization, this second meiotic arrest point is released and embryos enter highly specialized early embryonic divisions. In this review, we discuss how the standard somatic cell cycle is modulated to meet the specific requirements of different developmental stages. Specifically, we focus on cell cycle regulation in mature vertebrate eggs arrested at metaphase II (MII-arrest), the first mitotic cell cycle, and early embryonic divisions.

The DNA damage response during mitosis

Cells are equipped with a cell-intrinsic signaling network called the DNA damage response (DDR). This signaling network recognizes DNA lesions and initiates various downstream pathways to coordinate a cell cycle arrest with the repair of the damaged DNA. Alternatively, the DDR can mediate clearance of affected cells that are beyond repair through apoptosis or senescence. The DDR can be activated in response to DNA damage throughout the cell cycle, although the extent of DDR signaling is different in each cell cycle phase. Especially in response to DNA double strand breaks, only a very marginal response was observed during mitosis. Early on it was recognized that cells which are irradiated during mitosis continued division without repairing broken chromosomes. Although these initial observations indicated diminished DNA repair and lack of an acute DNA damage-induced cell cycle arrest, insight into the mechanistic re-wiring of DDR signaling during mitosis was only recently provided. Different mechanisms appear to be at play to inactivate specific signaling axes of the DDR network in mitosis. Importantly, mitotic cells not simply inactivate the entire DDR, but appear to mark their DNA damage for repair after mitotic exit. Since the treatment of cancer frequently involves agents that induce DNA damage as well as agents that block mitotic progression, it is clinically relevant to obtain a better understanding of how cancer cells deal with DNA damage during interphase versus mitosis. In this review, the molecular details concerning DDR signaling during mitosis as well as the consequences of encountering DNA damage during mitosis for cellular fate are discussed.

The phosphorylation of ARPP19 by Greatwall renders the auto-amplification of MPF independently of PKA in Xenopus oocytes

Entry into mitosis or meiosis relies on the coordinated action of kinases and phosphatases that ultimately leads to the activation of Cyclin-B-Cdk1, also known as MPF for M-phase promoting factor. Vertebrate oocytes are blocked in prophase of the first meiotic division, an arrest that is tightly controlled by high PKA activity. Re-entry into meiosis depends on activation of Cdk1, which obeys a two-step mechanism: a catalytic amount of Cdk1 is generated in a PKA and protein-synthesis-dependent manner; then a regulatory network known as the MPF auto-amplification loop is initiated. This second step is independent of PKA and protein synthesis. However, none of the molecular components of the auto-amplification loop identified so far act independently of PKA. Therefore, the protein rendering this process independent of PKA in oocytes remains unknown. Using a physiologically intact cell system, the Xenopus oocyte, we show that the phosphorylation of ARPP19 at S67 by the Greatwall kinase promotes its binding to the PP2A-B55δ phosphatase, thus inhibiting its activity. This process is controlled by Cdk1 and has an essential role within the Cdk1 auto-amplification loop for entry into the first meiotic division. Moreover, once phosphorylated by Greatwall, ARPP19 escapes the negative regulation exerted by PKA. It also promotes activation of MPF independently of protein synthesis, provided that a small amount of Mos is present. Taken together, these findings reveal that PP2A-B55δ, Greatwall and ARPP19 are not only required for entry into meiotic divisions, but are also pivotal effectors within the Cdk1 auto-regulatory loop responsible for its independence with respect to the PKA-negative control.

Plk1 phosphorylation of orc2 and hbo1 contributes to gemcitabine resistance in pancreatic cancer

Although gemcitabine is the standard chemotherapeutic drug for treatment of pancreatic cancer, almost all patients eventually develop resistance to this agent. Previous studies identified Polo-like kinase 1 (Plk1) as the mediator of gemcitabine resistance, but the molecular mechanism remains unknown. In this study, we show that Plk1 phosphorylation of Orc2 and Hbo1 mediates the resistance to gemcitabine. We show that the level of Plk1 expression positively correlates with gemcitabine resistance, both in pancreatic cancer cells and xenograft tumors. Overexpression of Plk1 increases gemcitabine resistance, while inhibition of Plk1 sensitizes pancreatic cancer cells to gemcitabine treatment. To validate our findings, we show that inhibition of Plk1 sensitizes tumors to gemcitabine treatment in a mouse xenograft study. Mechanistically, we find that Plk1 phosphorylation of Orc2 maintains DNA replication on gemcitabine treatment. Furthermore, Plk1 phosphorylation of Hbo1 transcriptionally increases cFos expression and consequently elevates its target multidrug resistance 1 (MDR1), which was previously reported to confer chemotherapeutic drug resistance. Knockdown of cFos or MDR1 sensitizes gemcitabine-resistant cells to gemcitabine treatment. Finally, pancreatic cancer cells expressing Plk1-unphosphorylatable mutants of Orc2 or Hbo1 are more sensitive to gemcitabine than cells expressing wild-type Orc2 or Hbo1. In short, our study provides a mechanism for Plk1-mediated gemcitabine resistance, suggesting that Plk1 is a promising target for treatment of gemcitabine-resistant pancreatic cancer.

MEK1 inactivates Myt1 to regulate Golgi membrane fragmentation and mitotic entry in mammalian cells

The pericentriolar stacks of Golgi cisternae are separated from each other in G2 and fragmented extensively during mitosis. MEK1 is required for Golgi fragmentation in G2 and for the entry of cells into mitosis. We now report that Myt1 mediates MEK1's effects on the Golgi complex. Knockdown of Myt1 by siRNA increased the efficiency of Golgi complex fragmentation by mitotic cytosol in permeabilized and intact HeLa cells. Myt1 knockdown eliminated the requirement of MEK1 in Golgi fragmentation and alleviated the delay in mitotic entry due to MEK1 inhibition. The phosphorylation of Myt1 by MEK1 requires another kinase but is independent of RSK, Plk, and CDK1. Altogether our findings reveal that Myt1 is inactivated by MEK1 mediated phosphorylation to fragment the Golgi complex in G2 and for the entry of cells into mitosis. It is known that Myt1 inactivation is required for CDK1 activation. Myt1 therefore is an important link by which MEK1 dependent fragmentation of the Golgi complex in G2 is connected to the CDK1 mediated breakdown of Golgi into tubules and vesicles in mitosis.

Involvement of polo-like kinase 1 (Plk1) in mitotic arrest by inhibition of mitogen-activated protein kinase-extracellular signal-regulated kinase-ribosomal S6 kinase 1 (MEK-ERK-RSK1) cascade

Cell division is controlled through cooperation of different kinases. Of these, polo-like kinase 1 (Plk1) and p90 ribosomal S6 kinase 1 (RSK1) play key roles. Plk1 acts as a G(2)/M trigger, and RSK1 promotes G(1) progression. Although previous reports show that Plk1 is suppressed by RSK1 during meiosis in Xenopus oocytes, it is still not clear whether this is the case during mitosis or whether Plk1 counteracts the effects of RSK1. Few animal models are available for the study of controlled and transient cell cycle arrest. Here we show that encysted embryos (cysts) of the primitive crustacean Artemia are ideal for such research because they undergo complete cell cycle arrest when they enter diapause (a state of obligate dormancy). We found that Plk1 suppressed the activity of RSK1 during embryonic mitosis and that Plk1 was inhibited during embryonic diapause and mitotic arrest. In addition, studies on HeLa cells using Plk1 siRNA interference and overexpression showed that phosphorylation of RSK1 increased upon interference and decreased after overexpression, suggesting that Plk1 inhibits RSK1. Taken together, these findings provide insights into the regulation of Plk1 during cell division and Artemia diapause cyst formation and the correlation between the activity of Plk1 and RSK1.

Polo-like kinase-activating kinases: Aurora A, Aurora B and what else?

The events of cell division are regulated by a complex interplay between kinases and phosphatases. Cyclin-dependent kinases (Cdks), polo-like kinases (Plks) and Aurora kinases play central roles in this process. Polo kinase (Plk1 in humans) regulates a wide range of events in mitosis and cytokinesis. To ensure the accuracy of these processes, polo activity itself is subject to complex regulation. Phosphorylation of polo in its T loop (or activation loop) increases its kinase activity several-fold. It has been shown that Aurora A kinase, with its co-factor Bora, activates Plk1 in G(2), and that this is essential for recovery from cell cycle arrest induced by DNA damage. In a recent article published in PLoS Biology, we report that Drosophila polo is activated by Aurora B kinase at centromeres, and that this is crucial for polo function in regulating chromosome dynamics in prometaphase. Our results suggest that this regulatory pathway is conserved in humans. Here, we propose a model for the collaboration between Aurora B and polo in the regulation of kinetochore attachment to microtubules in early mitosis. Moreover, we suggest that Aurora B could also function to activate Polo/Plk1 in cytokinesis. Finally, we discuss recent findings and open questions regarding the activation of polo and polo-like kinases by different kinases in mitosis, cytokinesis and other processes.

Polo-like kinase 1, on the rise from cell cycle regulation to prostate cancer development

Polo-like kinase 1 (Plk1), a well-characterized member of serine/threonine kinases Plk family, has been shown to play pivotal roles in mitosis and cytokinesis in eukaryotic cells. Recent studies suggest that Plk1 not only controls the process of mitosis and cytokinesis, but also, going beyond those previously described functions, plays critical roles in DNA replication and Pten null prostate cancer initiation. In this review, we briefly summarize the functions of Plk1 in mitosis and cytokinesis, and then mainly focus on newly discovered functions of Plk1 in DNA replication and in Pten-null prostate cancer initiation. Furthermore, we briefly introduce the architectures of human and mouse prostate glands and the possible roles of Plk1 in human prostate cancer development. And finally, the newly chemotherapeutic development of small-molecule Plk1 inhibitors to target Plk1 in cancer treatment and their translational studies are also briefly reviewed.

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.

Regulatory pathways coordinating cell cycle progression in early Xenopus development

The African clawed frog, Xenopus laevis, is used extensively as a model organism for studying both cell development and cell cycle regulation. For over 20 years now, this model organism has contributed to answering fundamental questions concerning the mechanisms that underlie cell cycle transitions--the cellular components that synthesize, modify, repair, and degrade nucleic acids and proteins, the signaling pathways that allow cells to communicate, and the regulatory pathways that lead to selective expression of subsets of genes. In addition, the remarkable simplicity of the Xenopus early cell cycle allows for tractable manipulation and dissection of the basic components driving each transition. In this organism, early cell divisions are characterized by rapid cycles alternating phases of DNA synthesis and division. The post-blastula stages incorporate gap phases, lengthening progression, and allowing more time for DNA repair. Various cyclin/Cdk complexes are differentially expressed during the early cycles with orderly progression being driven by both the combined action of cyclin synthesis and degradation and the appropriate selection of specific substrates by their Cdk components. Like other multicellular organisms, chief developmental events in early Xenopus embryogenesis coincide with profound remodeling of the cell cycle, suggesting that cell proliferation and differentiation events are linked and coordinated through crosstalk mechanisms acting on signaling pathways involving the expression of cell cycle control genes.

How protein kinases co-ordinate mitosis in animal cells

Mitosis is associated with profound changes in cell physiology and a spectacular surge in protein phosphorylation. To accomplish these, a remarkably large portion of the kinome is involved in the process. In the present review, we will focus on classic mitotic kinases, such as cyclin-dependent kinases, Polo-like kinases and Aurora kinases, as well as more recently characterized players such as NIMA (never in mitosis in Aspergillus nidulans)-related kinases, Greatwall and Haspin. Together, these kinases co-ordinate the proper timing and fidelity of processes including centrosomal functions, spindle assembly and microtubule-kinetochore attachment, as well as sister chromatid separation and cytokinesis. A recurrent theme of the mitotic kinase network is the prevalence of elaborated feedback loops that ensure bistable conditions. Sequential phosphorylation and priming phosphorylation on substrates are also frequently employed. Another important concept is the role of scaffolds, such as centrosomes for protein kinases during mitosis. Elucidating the entire repertoire of mitotic kinases, their functions, regulation and interactions is critical for our understanding of normal cell growth and in diseases such as cancers.

Illumination of mitotic orchestra during cell division: a Polo view

Protein kinase and phosphatase signaling cascade, coupled with other post-translational modifications, orchestrates temporal order of various events during cell division. Among the many mitotic kinases, Polo-like kinase 1 (PLK1) as a key regulator, participates in regulating mitosis from mitotic entry to cytokinesis. The advancement in optical reporter engineering and the recent development of specific chemical probes enable us to visualize spatiotemporal gradient of kinase activity at nano-scale. One of such tools is FRET-based optic sensor that allows us to delineate the PLK1 activity in space and time. In this review, we address the inter-relationships between PLK1 and other protein kinases/phosphatases, as well as the crosstalk between PLK1 phosphorylation and ubiquitination during cell division. In particular, we discuss the molecular mechanisms and steps underlying PLK1 kinase priming, activation and turn-off during cell division.

Cell cycle adaptations and maintenance of genomic integrity in embryonic stem cells and induced pluripotent stem cells

Pluripotent stem cells have the capability to undergo unlimited self-renewal and differentiation into all somatic cell types. They have acquired specific adjustments in the cell cycle structure that allow them to rapidly proliferate, including cell cycle independent expression of cell cycle regulators and lax G(1) to S phase transition. However, due to the developmental role of embryonic stem cells (ES) it is essential to maintain genomic integrity and prevent acquisition of mutations that would be transmitted to multiple cell lineages. Several modifications in DNA damage response of ES cells accommodate dynamic cycling and preservation of genetic information. The absence of a G(1)/S cell cycle arrest promotes apoptotic response of damaged cells before DNA changes can be fixed in the form of mutation during the S phase, while G(2)/M cell cycle arrest allows repair of damaged DNA following replication. Furthermore, ES cells express higher level of DNA repair proteins, and exhibit enhanced repair of multiple types of DNA damage. Similarly to ES cells, induced pluripotent stem (iPS) cells are poised to proliferate and exhibit lack of G(1)/S cell cycle arrest, extreme sensitivity to DNA damage, and high level of expression of DNA repair genes. The fundamental mechanisms by which the cell cycle regulates genomic integrity in ES cells and iPS cells are similar, though not identical.

Female infertility in PDE3A(-/-) mice: polo-like kinase 1 (Plk1) may be a target of protein kinase A (PKA) and involved in meiotic arrest of oocytes from PDE3A(-/-) mice

Mechanisms of cAMP/PKA-induced meiotic arrest in oocytes are not completely identified. In cultured, G2/M-arrested PDE3A(-/-) murine oocytes, elevated PKA activity was associated with inactivation of Cdc2 and Plk1, and inhibition of phosphorylation of histone H3 (S10) and of dephosphorylation of Cdc25B (S323) and Cdc2 (Thr14/Tyr15). In cultured WT oocytes, PKA activity was transiently reduced and then increased to that observed in PDE3A(-/-) oocytes; Cdc2 and Plk1 were activated, phosphorylation of histone H3 (S10) and dephosphorylation of Cdc25B (S323) and Cdc2 (Thr14/Tyr15) were observed. In WT oocytes, PKAc were rapidly translocated into nucleus, and then to the spindle apparatus, but in PDE3A(-/-) oocytes, PKAc remained in the cytosol. Plk1 was reactivated by incubation of PDE3A(-/-) oocytes with PKA inhibitor, Rp-cAMPS. PDE3A was co-localized with Plk1 in WT oocytes, and co-immunoprecipitated with Plk1 in WT ovary and Hela cells. PKAc phosphorylated rPlk1 and Hela cell Plk1 and inhibited Plk1 activity in vitro. Our results suggest that PKA-induced inhibition of Plk1 may be critical in oocyte meiotic arrest and female infertility in PDE3A(-/-) mice.

Multifaceted polo-like kinases: drug targets and antitargets for cancer therapy

The polo-like kinase 1 (PLK1) acts in concert with cyclin-dependent kinase 1-cyclin B1 and Aurora kinases to orchestrate a wide range of critical cell cycle events. Because PLK1 has been preclinically validated as a cancer target, small-molecule inhibitors of PLK1 have become attractive candidates for anticancer drug development. Although the roles of the closely related PLK2, PLK3 and PLK4 in cancer are less well understood, there is evidence showing that PLK2 and PLK3 act as tumour suppressors through their functions in the p53 signalling network, which guards the cell against various stress signals. In this article, recent insights into the biology of PLKs will be reviewed, with an emphasis on their role in malignant transformation, and progress in the development of small-molecule PLK1 inhibitors will be examined.

Coupling the cell cycle to development

The core machinery that drives the eukaryotic cell cycle has been thoroughly investigated over the course of the past three decades. It is only more recently, however, that light has been shed on the mechanisms by which elements of this core machinery are modulated to alter cell cycle progression during development. It has also become increasingly clear that, conversely, core cell cycle regulators can play a crucial role in developmental processes. Here, focusing on findings from Drosophila melanogaster and Caenorhabditis elegans, we review the importance of modulating the cell cycle during development and discuss how core cell cycle regulators participate in determining cell fates.

Involvement of the dehydroleucodine alpha-methylene-gamma-lactone function in GVBD inhibition inBufo arenarumoocytes

Dehydroleucodine (DhL), a sesquiterpenic lactone, was isolated and purified from aerial parts ofArtemisia douglasianaBesser, a medicinal herb used in Argentina. DhL is an alpha-methylene butyro-gamma-lactone ring connected to a seven-membered ring fused to an exocyclic alpha,beta-unsaturated cyclopentenone ring

It has been previously shown that DhL selectively induces a dose-dependent transient arrest in G2of both meristematic cells and vascular smooth muscle cells. Treatment with DhL induces an inhibition of spontaneous and progesterone-induced maturation in a dose-dependent manner inBufo arenarumfully grown oocytes arrested at G2, at the beginning of meiosis I. However, the nature of the mechanisms involved in the process is still unknown.

The aim of this work was to analyse whether DhL's alpha-methylene-gamma-lactone function is responsible for the inhibition effect on meiosis reinitiation ofBufo arenarumoocytes as well as some of the transduction pathways that could be involved in this effect using a derivative of DhL inactivated for alpha-methylenelactone, the 11,13-dihydro-dehydroleucodine (2H-DhL).

The use of 2H-DhL in the maturation promoting factor (MPF) amplification experiments by injection of both cytoplasm with active MPF and of germinal vesicle content showed results similar to the ones obtained with DhL, suggesting that the hydrogenated derivative would act in a similar way to DhL.

Pretreatment with DhL or 2H-DhL did not affect the percentage of germinal vesicle breakdown (GVBD) induced by H89, a protein kinase A (PKA) inhibitor, which suggests that these lactones would act on another step of the signalling pathway that induces MPF activation. The fact that both DhL and 2H-Dhl inhibit GVBD induced by okadaic acid microinjection suggests that they could act on the activity of the Myt1 kinase. This idea is supported by the experiments of injection of GV contents in which an inhibitory effect of these lactones on GVBD was also observed.

Our results indicate that the inhibitory effect on meiosis progression of DhL does not depend only on the activity of the alpha-methylenelactone function, as its hydrogenated derivative, 2H-DhL, in which this function has been inactivated, causes similar effects on amphibian oocytes. However, 2H-DhL was less active than DhL as higher doses were required to obtain a significant inhibition. On the other hand, the analysis of the participation of certain mediators in some of the signalling pathways leading to MPF activation suggests that the Myt1 kinase could be a target of these lactones, while cdc25 phosphatase would not be affected. Besides, the PKA inhibition assays indicate that these lactones would act earlier in the signalling pathways.

Polo-like kinases: conservation and divergence in their functions and regulation

Polo-like kinases (Plks) are potent regulators of M phase that are conserved from yeasts to humans. Their roles in mitotic entry, spindle pole functions and cytokinesis are broadly conserved despite physical and molecular differences in these processes in disparate organisms. Plks are characterized by their Polo-box domain, which mediates protein interactions. They are additionally controlled by phosphorylation, proteolysis and transcription, depending on the biological context. Plks are now recognized to link cell division to developmental processes and to function in differentiated cells. A comparison of Plk function and regulation between organisms offers insight into the rich variations of cell division.

c-Jun N-terminal kinase 1 phosphorylates Myt1 to prevent UVA-induced skin cancer

The c-Jun N-terminal kinase (JNK) signaling pathway is known to mediate both survival and apoptosis of tumor cells. Although JNK1 and JNK2 have been shown to differentially regulate the development of skin cancer, the underlying mechanistic basis remains unclear. Here, we demonstrate that JNK1, but not JNK2, interacts with and phosphorylates Myt1 ex vivo and in vitro. UVA induces substantial apoptosis in JNK wild-type (JNK(+/+)) or JNK2-deficient (JNK2(-/-)) mouse embryonic fibroblasts but has no effect on JNK1-deficient (JNK1(-/-)) cells. In addition, UVA-induced caspase-3 cleavage and DNA fragmentation were suppressed by the knockdown of human Myt1 in skin cancer cells. JNK1 deficiency results in suppressed Myt1 phosphorylation and caspase-3 cleavage in skin exposed to UVA irradiation. In contrast, the absence of JNK2 induces Myt1 phosphorylation and caspase-3 cleavage in skin exposed to UVA. The overexpression of JNK1 with Myt1 promotes cellular apoptosis during the early embryonic development of Xenopus laevis, whereas the presence of JNK2 reduces the phenotype of Myt1-induced apoptotic cell death. Most importantly, JNK1(-/-) mice developed more UVA-induced papillomas than either JNK(+/+) or JNK2(-/-) mice, which was associated with suppressed Myt1 phosphorylation and decreased caspase-3 cleavage. Taken together, these data provide mechanistic insights into the distinct roles of the different JNK isoforms, specifically suggesting that the JNK1-mediated phosphorylation of Myt1 plays an important role in UVA-induced apoptosis and the prevention of skin carcinogenesis.

Kicking off the polo game

Polo-like kinase 1 (Plk1) is essential for checkpoint recovery and the activation of key mitotic enzymes; however, its own activation mechanism has remained elusive. Recent findings show that Bora, a G(2)-M expressed protein, facilitates Plk1 activation by the oncogenic kinase Aurora A in G(2). During mitosis, Plk1-dependent Bora degradation promotes Aurora A localization to the centrosome and/or spindle. Bora-dependent regulation provides important new insights into interactions between key mitotic kinases.

Drosophila myt1 is the major cdk1 inhibitory kinase for wing imaginal disc development

Mitosis is triggered by activation of Cdk1, a cyclin-dependent kinase. Conserved checkpoint mechanisms normally inhibit Cdk1 by inhibitory phosphorylation during interphase, ensuring that DNA replication and repair is completed before cells begin mitosis. In metazoans, this regulatory mechanism is also used to coordinate cell division with critical developmental processes, such as cell invagination. Two types of Cdk1 inhibitory kinases have been found in metazoans. They differ in subcellular localization and Cdk1 target-site specificity: one (Wee1) being nuclear and the other (Myt1), membrane-associated and cytoplasmic. Drosophila has one representative of each: dMyt1 and dWee1. Although dWee1 and dMyt1 are not essential for zygotic viability, loss of both resulted in synthetic lethality, indicating that they are partially functionally redundant. Bristle defects in myt1 mutant adult flies prompted a phenotypic analysis that revealed cell-cycle defects, ectopic apoptosis, and abnormal responses to ionizing radiation in the myt1 mutant imaginal wing discs that give rise to these mechanosensory organs. Cdk1 inhibitory phosphorylation was also aberrant in these myt1 mutant imaginal wing discs, indicating that dMyt1 serves Cdk1 regulatory functions that are important both for normal cell-cycle progression and for coordinating mitosis with critical developmental processes.