polo encodes a protein kinase homolog required for mitosis in Drosophila

Polo-like kinase 1 regulates cell proliferation and is targeted by miR-593* in esophageal cancer

Polo-like kinase 1 (PLK1) is overexpressed in various human cancers. However, the biological functions and the post-transcriptional regulations of PLK1 in esophageal cancer (EC) are still unknown. The purposes of our study are to determine whether PLK1 can be a molecular target of EC therapy and to identify a microRNA (miRNA) targeting PLK1. We performed loss-of-function and gain-of-function experiments regarding cell proliferation, cell cycle, apoptosis, in vivo tumor formation and luciferase reporter assays, using siRNAs against PLK1 and miRNA. PLK1 protein was expressed in all 11 EC cell lines, but not in normal esophageal epithelial cells (HEEpiC). Knockdown of PLK1 in EC cells induced G2/M arrest (p < 0.001) in cell cycle assay and reduced cell proliferation (p = 0.019) and tumor formation ability in vivo (p < 0.0001). MiR-593*, identified as a miRNA targeting PLK1 by a database search, was less expressed especially in six EC cell lines than HEEpiC cells. Moreover, miR-593* expression level was inversely correlated with PLK1 mRNA level in 48 clinical tissue specimens of EC (p = 0.006). Introduction of synthetic miR-593* suppressed PLK1 expression by 69-73%, reduced cell proliferation (p = 0.008) and increased cell proportion of G2/M phase (p = 0.01) in HSA/c (an EC cells), whereas a miR-593* inhibitor upregulated PLK1 expression by 11-55%. Additionally, luciferase assay demonstrated that miR-593* interacted two binding sites in the PLK1 3'-UTR and reduced 56.8-71.5% of luciferase activity by degrading luciferase mRNA in HSA/c cells. In conclusion, PLK1 is post-transcriptionally regulated by miR-593* and could be a promising molecular target for EC treatment.

Tumor suppressors and cell-cycle proteins in lung cancer

The cell cycle is the cascade of events that allows a growing cell to duplicate all its components and split into two daughter cells. Cell cycle progression is mediated by the activation of a highly conserved family of protein kinases, the cyclin-dependent kinases (CDKs). CDKs are also regulated by related proteins called cdk inhibitors grouped into two families: the INK4 inhibitors (p16, p15, p19, and p18) and the Cip/Kip inhibitors (p21, p27, and p53). Several studies report the importance of cell-cycle proteins in the pathogenesis and the prognosis of lung cancer. This paper will review the most recent data from the literature about the regulation of cell cycle. Finally, based essentially on the data generated in our laboratory, the expression, the diagnostic, and prognostic significance of cell-cycle molecules in lung cancer will be examined.

Hsp90 in non-mammalian metazoan model systems

The molecular chaperone Hsp90 has been discovered in the heat-shock response of the fruit fly more than 30years ago. Today, it is becoming clear that Hsp90 is in the middle of a regulatory system, participating in the modulation of many essential client proteins and signaling pathways. Exerting these activities, Hsp90 works together with about a dozen of cochaperones. Due to their organismal simplicity and the possibility to influence their genetics on a large scale, many studies have addressed the function of Hsp90 in several multicellular model systems. Defined pathways involving Hsp90 client proteins have been identified in the metazoan model systems of Caenorhabditis elegans, Drosophila melanogaster and the zebrafish Danio rerio. Here, we summarize the functions of Hsp90 during muscle maintenance, development of phenotypic traits and the involvement of Hsp90 in stress responses, all of which were largely uncovered using the model organisms covered in this review. These findings highlight the many specific and general actions of the Hsp90 chaperone machinery. This article is part of a Special Issue entitled: Heat Shock Protein 90 (HSP90).

Physiological relevance of cell cycle kinases

The basic biology of the cell division cycle and its control by protein kinases was originally studied through genetic and biochemical studies in yeast and other model organisms. The major regulatory mechanisms identified in this pioneer work are conserved in mammals. However, recent studies in different cell types or genetic models are now providing a new perspective on the function of these major cell cycle regulators in different tissues. Here, we review the physiological relevance of mammalian cell cycle kinases such as cyclin-dependent kinases (Cdks), Aurora and Polo-like kinases, and mitotic checkpoint regulators (Bub1, BubR1, and Mps1) as well as other less-studied enzymes such as Cdc7, Nek proteins, or Mastl and their implications in development, tissue homeostasis, and human disease. Among these functions, the control of self-renewal or asymmetric cell division in stem/progenitor cells and the ability to regenerate injured tissues is a central issue in current research. In addition, many of these proteins play previously unexpected roles in metabolism, cardiovascular function, or neuron biology. The modulation of their enzymatic activity may therefore have multiple therapeutic benefits in human disease.

From Plk1 to Plk5: functional evolution of polo-like kinases

Mammalian polo-like kinases (Plks) are characterized by the presence of an N-terminal protein kinase domain and a C-terminal polo-box domain (PBD) involved in substrate binding and regulation of kinase activity. Plk1-4 have traditionally been linked to cell cycle progression, genotoxic stress and, more recently, neuron biology. Recently, a fifth mammalian Plk family member, Plk5, has been characterized in murine and human cells. Plk5 is expressed mainly in differentiated tissues such as the cerebellum. Despite apparent loss of catalytic activity and a stop codon in the middle of the human gene, Plk5 proteins retain important functions in neuron biology. Notably, its expression is silenced by epigenetic alterations in brain tumors, such as glioblastomas, and its re-expression prevents cell proliferation of these tumor cells. In this review, we will focus on the non-cell cycle roles of Plks, the biology of the new member of the family and the possible kinase- and PBD-independent functions of polo-like kinases.

Nek9 is a Plk1-activated kinase that controls early centrosome separation through Nek6/7 and Eg5

The NIMA-family kinases Nek9/Nercc1, Nek6 and Nek7 form a signalling module required for mitotic spindle assembly. Nek9, the upstream kinase, is activated during prophase at centrosomes although the details of this have remained elusive. We now identify Plk1 as Nek9 direct activator and propose a two-step activation mechanism that involves Nek9 sequential phosphorylation by CDK1 and Plk1. Furthermore, we show that Plk1 controls prophase centrosome separation through the activation of Nek9 and ultimately the phosphorylation of the mitotic kinesin Eg5 at Ser1033, a Nek6/7 site that together with the CDK1 site Thr926 we establish contributes to the accumulation of Eg5 at centrosomes and is necessary for subsequent centrosome separation and timely mitosis. Our results provide a basis to understand signalling downstream of Plk1 and shed light on the role of Eg5, Plk1 and the NIMA-family kinases in the control of centrosome separation and normal mitotic progression.

RNA polymerase II kinetics in polo polyadenylation signal selection

Regulated alternative polyadenylation is an important feature of gene expression, but how gene transcription rate affects this process remains to be investigated. polo is a cell-cycle gene that uses two poly(A) signals in the 3' untranslated region (UTR) to produce alternative messenger RNAs that differ in their 3'UTR length. Using a mutant Drosophila strain that has a lower transcriptional elongation rate, we show that transcription kinetics can determine alternative poly(A) site selection. The physiological consequences of incorrect polo poly(A) site choice are of vital importance; transgenic flies lacking the distal poly(A) signal cannot produce the longer transcript and die at the pupa stage due to a failure in the proliferation of the precursor cells of the abdomen, the histoblasts. This is due to the low translation efficiency of the shorter transcript produced by proximal poly(A) site usage. Our results show that correct polo poly(A) site selection functions to provide the correct levels of protein expression necessary for histoblast proliferation, and that the kinetics of RNA polymerase II have an important role in the mechanism of alternative polyadenylation.

Tuberous sclerosis complex and Myc coordinate the growth and division of Drosophila intestinal stem cells

Excessive cell growth in Drosophila intestinal stem cells lacking TSC blocks further cell division.

Intestinal stem cells (ISCs) in the adult Drosophila melanogaster midgut can respond to damage and support repair. We demonstrate in this paper that the tuberous sclerosis complex (TSC) plays a critical role in balancing ISC growth and division. Previous studies have shown that imaginal disc cells that are mutant for TSC have increased rates of growth and division. However, we report in this paper that loss of TSC in the adult Drosophila midgut results in the formation of much larger ISCs that have halted cell division. These mutant ISCs expressed proper stem cell markers, did not differentiate, and had defects in multiple steps of the cell cycle. Slowing the growth by feeding rapamycin or reducing Myc was sufficient to rescue the division defect. The TSC mutant guts had a thinner epithelial structure than wild-type tissues, and the mutant flies were more susceptible to tissue damage. Therefore, we have uncovered a context-dependent phenotype of TSC mutants in adult ISCs, such that the excessive growth leads to inhibition of division.

Polo-like kinases and DNA damage checkpoint: beyond the traditional mitotic functions

Polo-like kinases (Plks) are a family of serine-threonine kinases that play a pivotal role in cell cycle progression and in cellular response to DNA damage. The Plks are highly conserved from yeast to mammals. There are five Plk family members (Plk1-5) in humans, of which Plk1, is the best characterized. The Plk1 isoform is being aggressively pursued as a target for cancer therapy, following observations that this protein is overexpressed in human tumors and is actively involved in malignant transformation. The roles of Plks in mitotic entry, spindle pole functions and cytokinesis are well established and have been the subject of several recent reviews. In this review, we discuss functions of Plks other than their classical roles in mitotic progression. When cells incur DNA damage, they activate checkpoint mechanisms that result in cell cycle arrest and allow time for repair. If the damage is extensive and cannot be repaired, cells will undergo cell death by apoptosis. If the damage is repaired, cells can resume cycling, as part of the process known as checkpoint recovery. If the damage is not repaired or incompletely repaired, cells can override the checkpoint and resume cycling with damaged DNA, a process called checkpoint adaptation. The Plks play a role in all three outcomes and their involvement in these processes will be the subject of this review.

Targeting the cell cycle in esophageal adenocarcinoma: An adjunct to anticancer treatment

Esophageal adenocarcinoma is a major cause of cancer death in men in the developed world. Continuing poor outcomes with conventional therapies that predominantly target apoptosis pathways have lead to increasing interest in treatments that target the cell cycle. A large international effort has led to the development of a large number of inhibitors, which target cell cycle kinases, including cyclin-dependent kinases, Aurora kinases and polo-like kinase. Initial phase I/II trials in solid tumors have often demonstrated only modest clinical benefits of monotherapy. This may relate in part to a failure to identify the patient populations that will gain the most clinical benefit. Newer compounds lacking the side effect profile of first-generation compounds may show utility as adjunctive treatments targeted to an individual’s predicted response to treatment.

Absence of polo-like kinase 3 in mice stabilizes Cdc25A after DNA damage but is not sufficient to produce tumors

The polo-like kinases (Plks1-5) are emerging as an important class of proteins involved in many facets of cell cycle regulation and response to DNA damage and stress. Here we show that Plk3 phosphorylates the key cell cycle protein phosphatase Cdc25A on two serine residues in its cyclinB/cdk1 docking domain and regulates its stability in response to DNA damage. We generated a Plk3 knock-out mouse and show that Cdc25A protein from Plk3-deficient cells is less susceptible to DNA damage-mediated degradation than cells with functional Plk3. We also show that absence of Plk3 correlates with loss of the G1/S cell cycle checkpoint. However, neither this compromised DNA damage checkpoint nor reduced susceptibility to proteasome-mediated degradation after DNA damage translated into a significant increase in tumor incidence in the Plk3-deficient mice.

Plk5, a polo box domain-only protein with specific roles in neuron differentiation and glioblastoma suppression

Polo-like kinases (Plks) are characterized by the presence of a specific domain, known as the polo box (PBD), involved in protein-protein interactions. Plk1 to Plk4 are involved in centrosome biology as well as the regulation of mitosis, cytokinesis, and cell cycle checkpoints in response to genotoxic stress. We have analyzed here the new member of the vertebrate family, Plk5, a protein that lacks the kinase domain in humans. Plk5 does not seem to have a role in cell cycle progression; in fact, it is downregulated in proliferating cells and accumulates in quiescent cells. This protein is mostly expressed in the brain of both mice and humans, and it modulates the formation of neuritic processes upon stimulation of the brain-derived neurotrophic factor (BDNF)/nerve growth factor (NGF)-Ras pathway in neurons. The human PLK5 gene is significantly silenced in astrocytoma and glioblastoma multiforme by promoter hypermethylation, suggesting a tumor suppressor function for this gene. Indeed, overexpression of Plk5 has potent apoptotic effects in these tumor cells. Thus, Plk5 seems to have evolved as a kinase-deficient PBD-containing protein with nervous system-specific functions and tumor suppressor activity in brain cancer.

The Plk1-dependent phosphoproteome of the early mitotic spindle

Polo-like kinases regulate many aspects of mitotic and meiotic progression from yeast to man. In early mitosis, mammalian Polo-like kinase 1 (Plk1) controls centrosome maturation, spindle assembly, and microtubule attachment to kinetochores. However, despite the essential and diverse functions of Plk1, the full range of Plk1 substrates remains to be explored. To investigate the Plk1-dependent phosphoproteome of the human mitotic spindle, we combined stable isotope labeling by amino acids in cell culture with Plk1 inactivation or depletion followed by spindle isolation and mass spectrometry. Our study identified 358 unique Plk1-dependent phosphorylation sites on spindle proteins, including novel substrates, illustrating the complexity of the Plk1-dependent signaling network. Over 100 sites were validated by in vitro phosphorylation of peptide arrays, resulting in a broadening of the Plk1 consensus motif. Collectively, our data provide a rich source of information on Plk1-dependent phosphorylation, Plk1 docking to substrates, the influence of phosphorylation on protein localization, and the functional interaction between Plk1 and Aurora A on the early mitotic spindle.

Protein phosphatase 6 regulates mitotic spindle formation by controlling the T-loop phosphorylation state of Aurora A bound to its activator TPX2

Many protein kinases are activated by a conserved regulatory step involving T-loop phosphorylation. Although there is considerable focus on kinase activator proteins, the importance of specific T-loop phosphatases reversing kinase activation has been underappreciated. We find that the protein phosphatase 6 (PP6) holoenzyme is the major T-loop phosphatase for Aurora A, an essential mitotic kinase. Loss of PP6 function by depletion of catalytic or regulatory subunits interferes with spindle formation and chromosome alignment because of increased Aurora A activity. Aurora A T-loop phosphorylation and the stability of the Aurora A-TPX2 complex are increased in cells depleted of PP6 but not other phosphatases. Furthermore, purified PP6 acts as a T-loop phosphatase for Aurora A-TPX2 complexes in vitro, whereas catalytically inactive mutants cannot dephosphorylate Aurora A or rescue the PPP6C depletion phenotype. These results demonstrate a hitherto unappreciated role for PP6 as the T-loop phosphatase regulating Aurora A activity during spindle formation and suggest the general importance of this form of regulation.

The substrates of Plk1, beyond the functions in mitosis

Polo-like kinase 1 (Plk1) is a key regulator of cell division in eukaryotic cells. In this short review, we briefly summarized the well-established functions modulated by Plk1 during mitosis. Beyond mitosis, we focused mainly on the unexpected processes in which Plk1 emerges as a critical player, including microtubule dynamics, DNA replication, chromosome dynamics, p53 regulation, and recovery from the G2 DNA-damage checkpoint. Our discussion is mainly based on the critical substrates targeted by Plk1 during these cellular events and the functional significance associated with each phosphorylation event.

A conserved arginine-rich motif within the hypervariable N-domain of Drosophila centromeric histone H3 (CenH3) mediates BubR1 recruitment

Background

Centromere identity is determined epigenetically by deposition of CenH3, a centromere-specific histone H3 variant that dictates kinetochore assembly. The molecular basis of the contribution of CenH3 to centromere/kinetochore functions is, however, incompletely understood, as its interactions with the rest of centromere/kinetochore components remain largely uncharacterised at the molecular/structural level.

Principal Findings

Here, we report on the contribution of Drosophila CenH3^CID to recruitment of BubR1, a conserved kinetochore protein that is a core component of the spindle attachment checkpoint (SAC). This interaction is mediated by the N-terminal domain of CenH3^CID (NCenH3^CID), as tethering NCenH3^CID to an ectopic reporter construct results in BubR1 recruitment and BubR1-dependent silencing of the reporter gene. Here, we also show that this interaction depends on a short arginine (R)-rich motif and that, most remarkably, it appears to be evolutionarily conserved, as tethering constructs carrying the highly divergent NCenH3 of budding yeast and human also induce silencing of the reporter. Interestingly, though NCenH3 shows an exceedingly low degree of conservation, the presence of R-rich motives is a common feature of NCenH3 from distant species. Finally, our results also indicate that two other conserved sequence motives within NCenH3^CID might also be involved in interactions with kinetochore components.

Conclusions

These results unveil an unexpected contribution of the hypervariable N-domain of CenH3 to recruitment of kinetochore components, identifying simple R-rich motives within it as evolutionary conserved structural determinants involved in BubR1 recruitment.

Alternative mRNA polyadenylation in eukaryotes: an effective regulator of gene expression

Alternative RNA processing mechanisms, including alternative splicing and alternative polyadenylation, are increasingly recognized as important regulators of gene expression. This article will focus on what has recently been described about alternative polyadenylation in development, differentiation, and disease in higher eukaryotes. We will also describe how the evolving global methodologies for examining the cellular transcriptome, both experimental and bioinformatic, are revealing new details about the complex nature of alternative 3(') end formation as well as interactions with other RNA-mediated and RNA processing mechanisms.

Inhibition of Plk1 induces mitotic infidelity and embryonic growth defects in developing zebrafish embryos

Polo-like kinase 1 (Plk1) is central to cell division. Here, we report that Plk1 is critical for mitosis in the embryonic development of zebrafish. Using a combination of several cell biology tools, including single-cell live imaging applied to whole embryos, we show that Plk1 is essential for progression into mitosis during embryonic development. Plk1 morphant cells displayed mitotic infidelity, such as abnormal centrosomes, irregular spindle assembly, hypercondensed chromosomes, and a failure of chromosome arm separation. Consequently, depletion of Plk1 resulted in mitotic arrest and finally death by 6days post-fertilization. In comparison, Plk2 or Plk3 morphant embryos did not display any significant abnormalities. Treatment of embryos with the Plk1 inhibitor, BI 2536, caused a block in mitosis, which was more severe when used to treat plk1 morphants. Finally, using an assay to rescue the Plk1 morphant phenotype, we found that the kinase domain and PBD domains are both necessary for Plk1 function in zebrafish development. Our studies demonstrate that Plk1 is required for embryonic proliferation because its activity is crucial for mitotic integrity. Furthermore, our study suggests that zebrafish will be an efficient and economical in vivo system for the validation of anti-mitotic drugs.

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.

Modulating polo-like kinase 1 as a means for cancer chemoprevention

Naturally occurring agents have always been appreciated for their medicinal value for both their chemopreventive and therapeutic effects against cancer. In fact, the majority of the drugs we use today, including the anti-cancer agents, were originally derived from natural compounds, either in their native form or modified to enhance their bioavailability or specificity. It is believed that for maximum effectiveness, it will useful to design novel target-based agents for chemoprevention as well as the treatment of cancer. Recent studies have shown that the serine/threonine kinase polo-like kinase (Plk) 1 is widely overexpressed in a variety of cancers and is being increasingly appreciated as a target for cancer management. Additionally, several chemopreventive agents have been shown to inhibit Plk1 in cancer cells. In this review, we will discuss if Plk1 could also be a target for designing novel strategies for cancer chemoprevention.

Significance of Plk1 regulation by miR-100 in human nasopharyngeal cancer

Polo-like kinase 1 (Plk1) is a critical regulator of many stages of mitosis; increasing evidence indicates that Plk1 overexpression correlates with poor clinical outcome, yet its mechanism of regulation remains unknown. Hence, a detailed evaluation was undertaken of Plk1 expression in human nasopharyngeal cancer (NPC), the cellular effects of targeting Plk1 using siRNA in combination with ionizing radiation (RT) and potential upstream microRNAs (miRs) that might regulate Plk1 expression. Using immunohistochemistry, Plk1 was observed to be overexpressed in 28 of 40 (70%) primary NPC biopsies, which in turn was associated with a higher likelihood of recurrence (p = 0.018). SiPlk1 significantly inhibited Plk1 mRNA and protein expression, and decreased Cdc25c levels in NPC cell lines. This depletion resulted in cytotoxicity of C666-1 cells, enhanced by the addition of RT, mediated by G2/M arrest, increased DNA double-strand breaks, apoptosis, and caspase activation. Immunofluorescence demonstrated that the G2/M arrest was associated with aberrant spindle formation, leading to mitotic arrest. In vivo, transfection of C666-1 cells and systemic delivery of siPlk1 decreased tumour growth. MicroRNA-100 (miR-100) was predicted to target Plk1 mRNA, which was indeed underexpressed in C666-1 cells, inversely correlating with Plk1 expression. Using luciferase constructs containing the 3'-UTR of Plk1 sequence, we document that miR-100 can directly target Plk1. Hence, our data demonstrate for the first time that underexpressed miR-100 leads to Plk1 overexpression, which in turn contributes to NPC progression. Targeting Plk1 will cause mitotic catastrophe, with significant cytotoxicity both in vitro and in vivo, underscoring the important therapeutic opportunity of Plk1 in NPC.

Targeting polo-like kinase 1 enhances radiation efficacy for head-and-neck squamous cell carcinoma

PURPOSE

To investigate the efficacy of targeting polo-like kinase 1 (Plk1) combined with ionizing radiotherapy (RT) for head-and-neck squamous cell carcinoma (HNSCC).

METHODS AND MATERIALS

Polo-like kinase 1 messenger ribonucleic acid (mRNA) was targeted by small interfering RNA (siRNA) transfection into the FaDu HNSCC cell line; reduction was confirmed using quantitative real-time polymerase chain reaction. The cellular effects were assessed using [3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium], clonogenic, flow cytometric, and caspase assays. In vivo efficacy of siPlk1 was evaluated using mouse xenograft models.

RESULTS

Small interfering Plk1 significantly decreased Plk1 mRNA expression, while also increasing cyclin B1 and p21(Waf1/CIP1) mRNA levels after 24 h. This depletion resulted in a time-dependent increase in FaDu cytotoxicity, which was enhanced by the addition of RT. Flow cytometric and caspase assays demonstrated progressive apoptosis, DNA double-strand breaks (gamma-H2AX), G2/M arrest, and activation of caspases 3 and 7. Implantation of siPlk1-treated FaDu cells in severe combined immunodeficient mice delayed tumor formation, and systemic administration of siPlk1 inhibited tumor growth enhanced by RT.

CONCLUSIONS

These data demonstrate the suitability of Plk1 as a potential therapeutic target for HNSCC, because Plk1 depletion resulted in significant cytotoxicity in vitro and abrogated tumor-forming potential in vivo. The effects of Plk1 depletion were enhanced with the addition of RT, indicating that Plk1 represents an important potential radiation sensitizer for HNSCC.

BubR1- and Polo-coated DNA tethers facilitate poleward segregation of acentric chromatids

The mechanisms that safeguard cells against chromosomal instability (CIN) are of great interest, as CIN contributes to tumorigenesis. To gain insight into these mechanisms, we studied the behavior of cells entering mitosis with damaged chromosomes. We used the endonuclease I-CreI to generate acentric chromosomes in Drosophila larvae. While I-CreI expression produces acentric chromosomes in the majority of neuronal stem cells, remarkably, it has no effect on adult survival. Our live studies reveal that acentric chromatids segregate efficiently to opposite poles. The acentric chromatid poleward movement is mediated through DNA tethers decorated with BubR1, Polo, INCENP, and Aurora-B. Reduced BubR1 or Polo function results in abnormal segregation of acentric chromatids, a decrease in acentric chromosome tethering, and a great reduction in adult survival. We propose that BubR1 and Polo facilitate the accurate segregation of acentric chromatids by maintaining the integrity of the tethers that connect acentric chromosomes to their centric partners.

Inventory and phylogenomic distribution of meiotic genes in Nasonia vitripennis and among diverse arthropods

The parasitoid jewel wasp Nasonia vitripennis reproduces by haplodiploidy (arrhenotokous parthenogenesis). In diploid females, meiosis occurs during oogenesis, but in haploid males spermatogenesis is ameiotic and involves a single equational division. Here we describe the phylogenomic distribution of meiotic genes in N. vitripennis and in 10 additional arthropods. Homologues for 39 meiosis-related genes (including seven meiosis-specific genes) were identified in N. vitripennis. The meiotic genes missing from N. vitripennis are also sporadically absent in other arthropods, suggesting that certain meiotic genes are dispensable for meiosis. Among an additional set of 15 genes thought to be specific for male meiosis in Drosophila, two genes (bol and crl) were identified in N. vitripennis and Apis mellifera (both for which canonical meiosis is absent in males) and in other arthropods. The distribution of meiotic genes across arthropods and the impact of gene duplications and reproductive modes on meiotic gene evolution are discussed.

Targeting Polo-like kinase in cancer therapy

Polo-like kinases (Plk) function in mitosis and maintaining DNA integrity. There are four family members, of which Plk1 represents a target for anticancer therapy. Plk1 is only expressed in dividing cells with peak expression during G2/M. Plk1 functions in multiple steps of mitosis, and is overexpressed in many tumor types. Mitotic arrest and inhibition of proliferation, apoptosis, and tumor growth inhibition have been observed in preclinical studies using small interfering RNAs (siRNA) or small molecules that inhibit Plk1. Preclinical studies also show that Plk1 inhibitors may be active against tumors with RAS mutations and that tumor cells with mutations in TP53 are more sensitive to inhibition of Plk1. Several Plk inhibitors are in phase I or II clinical studies. As expected, hematologic toxicity is the primary dose-limiting toxicity. Some patients have achieved clinical response, although in some studies only at doses above the maximum tolerated dose defined in the study. Further evaluation is necessary to discern the clinical utility of Plk1 inhibitors.

Polo-like kinase inhibitors: an emerging opportunity for cancer therapeutics

IMPORTANCE OF THE FIELD

The Polo-like kinase (Plk) family has emerged as an important regulator in cell cycle progression. Plks belong to a family of serine/threonine kinases and exist in four isoforms Plk1- 4. However, only one of these isoforms, Plk1, is shown to be involved in the activation of Cdc2, chromosome segregation, centrosome maturation, bipolar spindle formation and execution of cytokinesis. The activity of Plk1 is elevated in tissues and cells with a high mitotic index. In patients, Plk1 is overexpressed in tumors including those derived from lung, breast, colon, pancreas, prostate and ovary. Plk1 depletion is associated with the decrease in cell viability and induction of apoptosis in various cancerous cells. Several Plk1 inhibitors are in different phases of clinical development for anticancer therapy.

AREAS COVERED IN THIS REVIEW

The focus of present review is to highlight Plk1 as a promising therapeutic approach for the treatment of cancer. The review discusses the role of Plk1 in cancer and the current status of Plk1 inhibitors, as well as highlighting the possible beneficial effect of inhibition of Plk1 as compared to other mitotic targets.

WHAT THE READER WILL GAIN

Readers will get a comprehensive overview of Plk1 as a novel anticancer drug target. This review will also update readers about the progress made in the field of Plk1 inhibitors.

TAKE HOME MESSAGE

The current literature about Plk1 inhibitors and knockout studies favor Plk1 inhibition as a potential antitumor therapy.

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.

The function of activity-regulated genes in the nervous system

The mammalian brain is plastic in the sense that it shows a remarkable capacity for change throughout life. The contribution of neuronal activity to brain plasticity was first recognized in relation to critical periods of development, when manipulating the sensory environment was found to profoundly affect neuronal morphology and receptive field properties. Since then, a growing body of evidence has established that brain plasticity extends beyond development and is an inherent feature of adult brain function, spanning multiple domains, from learning and memory to adaptability of primary sensory maps. Here we discuss evolution of the current view that plasticity of the adult brain derives from dynamic tuning of transcriptional control mechanisms at the neuronal level, in response to external and internal stimuli. We then review the identification of "plasticity genes" regulated by changes in the levels of electrical activity, and how elucidating their cellular functions has revealed the intimate role transcriptional regulation plays in fundamental aspects of synaptic transmission and circuit plasticity that occur in the brain on an every day basis.

Functional whole-genome analysis identifies Polo-like kinase 2 and poliovirus receptor as essential for neuronal differentiation upstream of the negative regulator alphaB-crystallin

This study aimed at identifying transcriptional changes associated to neuronal differentiation induced by six distinct stimuli using whole-genome microarray hybridization analysis. Bioinformatics analyses revealed the clustering of these six stimuli into two categories, suggesting separate gene/pathway dependence. Treatment with specific inhibitors demonstrated the requirement of both Janus kinase and microtubule-associated protein kinase activation to trigger differentiation with nerve growth factor (NGF) and dibutyryl cAMP. Conversely, activation of protein kinase A, phosphatidylinositol-3-kinase alpha, and mammalian target of rapamycin, although required for dibutyryl cAMP-induced differentiation, exerted a negative feedback on NGF-induced differentiation. We identified Polo-like kinase 2 (Plk2) and poliovirus receptor (PVR) as indispensable for NGF-driven neuronal differentiation and alphaB-crystallin (Cryab) as an inhibitor of this process. Silencing of Plk2 or PVR blocked NGF-triggered differentiation and Cryab down-regulation, while silencing of Cryab enhanced NGF-induced differentiation. Our results position both Plk2 and PVR upstream of the negative regulator Cryab in the pathway(s) leading to neuronal differentiation triggered by NGF.

Mitotic drivers--inhibitors of the Aurora B Kinase

In this article we review the basis for current anti-mitotic, anti-cancer, therapy and the potential for Aurora B kinase inhibitors as a new differentiated class of agents--"mitotic drivers". We review the current understanding of Aurora B inhibition from basic cell biology to inhibitors currently undergoing clinical trials.

Polo-like kinase (PLK) inhibitors in preclinical and early clinical development in oncology

Polo-like kinases (PLKs) are a group of highly conserved serine/threonine protein kinases that play a key role in processes such as cell division and checkpoint regulation of mitosis. About 80% of human tumors, of various origins, express high levels of PLK transcripts. However, PLK mRNA is mostly absent in surrounding healthy tissues. Overexpression of PLK is associated with a poor prognosis in several tumor types and a lower overall survival rate. The overexpression of PLKs in human tumors, but not in healthy nondividing cells, makes them an attractive, selective target for cancer drug development. PLK inhibitors interfere with different stages of mitosis, such as centrosome maturation, spindle formation, chromosome separation, and cytokinesis. They induce mitotic chaos and severely perturb cell cycle progression, eventually leading to cancer cell death. Several PLK inhibitors are in development and are undergoing evaluations as potential cancer treatments. This review includes an overview of PLK inhibitors in early clinical development (i.e., BI 2536, BI 6727, GSK461364, ON 019190.Na, and HMN-214) and in advanced preclinical development (i.e., ZK-thiazolidinone, NMS-1, CYC-800, DAP-81, and LC-445). If proof of principle is confirmed in large studies, PLK inhibitors will offer a new targeted antitumor therapy for cancer patients.

Transcriptional regulation of human polo-like kinases and early mitotic inhibitor

Human polo-like kinases (PLK1-PLK4) have been implicated in mitotic regulation and carcinogenesis. PLK1 phosphorylates early mitotic inhibitor 1 (Emi1) to ensure mitosis entry, whereas Emi2 plays a key role during the meiotic cell cycle. Transcription factor E2F is primarily considered to regulate the G(1)/S transition of the cell cycle but its involvement in the regulation of mitosis has also been recently suggested. A gap still exists between the molecular basis of E2F and mitotic regulation. The present study was designed to characterize the transcriptional regulation of human PLK and Emi genes. Adenoviral overexpression of E2F1 increased PLK1 and PLK3 mRNA levels in A549 cells. A reporter gene assay revealed that the putative promoter regions of PLK1, PLK3, and PLK4 genes were responsive to activators E2F, E2F1-E2F3. We further characterized the putative promoter regions of Emi1 and Emi2 genes, and these could be regulated by activators E2F and E2F1-E2F4, respectively. Finally, PLK1-PLK4, Emi1, and Emi2 mRNA expression levels in human adult, fetal tissues, and several cell lines indicated that each gene has a unique expression pattern but is uniquely expressed in common tissues and cells such as the testes and thymus. Collectively, these results indicate that E2F can integrate G(1)/S and G(2)/M to oscillate the cell cycle by regulating mitotic genes PLK and Emi, leading to determination of the cell fate.

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.

The centrosome protein NEDD1 as a potential pharmacological target to induce cell cycle arrest

BACKGROUND

NEDD1 is a protein that binds to the gamma-tubulin ring complex, a multiprotein complex at the centrosome and at the mitotic spindle that mediates the nucleation of microtubules.

RESULTS

We show that NEDD1 is expressed at comparable levels in a variety of tumor-derived cell lines and untransformed cells. We demonstrate that silencing of NEDD1 expression by treatment with siRNA has differential effects on cells, depending on their status of p53 expression: p53-positive cells arrest in G1, whereas p53-negative cells arrest in mitosis with predominantly aberrant monopolar spindles. However, both p53-positive and -negative cells arrest in mitosis if treated with low doses of siRNA against NEDD1 combined with low doses of the inhibitor BI2536 against the mitotic kinase Plk1. Simultaneous reduction of NEDD1 levels and inhibition of Plk1 act in a synergistic manner, by potentiating the anti-mitotic activity of each treatment.

CONCLUSION

We propose that NEDD1 may be a promising target for controlling cell proliferation, in particular if targeted in combination with Plk1 inhibitors.

Stimulation of polo-like kinase 3 mRNA decay by tristetraprolin

Polo-like protein kinase 3 (Plk3) has been proposed to regulate entry into S phase and promote apoptosis in response to oxidative stress. Its mRNA contains three AU-rich elements (AREs) in its 3' untranslated region (3'-UTR) that can contribute to the rapid degradation of labile transcripts. We investigated the possibility that tristetraprolin (TTP), a tandem CCCH zinc finger protein, could promote the decay of Plk3 transcripts. TTP is known to stimulate the deadenylation and decay of mRNAs possessing one or more copies of the consensus nonamer motif UUAUUUAUU. In stable mouse fibroblast cell lines derived from wild-type and TTP knockout littermates, the decay of Plk3 transcripts after serum stimulation was slowed in the absence of TTP. The specificity of TTP for promoting the degradation of Plk3 was demonstrated by the unaltered decay of Plk3 mRNA in cell lines deficient in the TTP family members ZFP36L1 and ZFP36L2. We also found that the AREs present in the Plk3 transcript were essential for both the binding of TTP to the 3'-UTR and promoting the destruction of target transcripts in cotransfection experiments. The regulation of Plk3 mRNA stability by TTP may influence the control of the cell cycle by this protein kinase.

Analysis of all protein phosphatase genes in Aspergillus nidulans identifies a new mitotic regulator, fcp1

Reversible protein phosphorylation is an important regulatory mechanism of cell cycle control in which protein phosphatases counteract the activities of protein kinases. In Aspergillus nidulans, 28 protein phosphatase catalytic subunit genes were identified. Systematic deletion analysis identified four essential phosphatases and four required for normal growth. Conditional alleles of these were generated using the alcA promoter. The deleted phosphatase strain collection and regulatable versions of the essential and near-essential phosphatases provide an important resource for further analysis of the role of reversible protein phosphorylation to the biology of A. nidulans. We further demonstrate that nimT and bimG have essential functions required for mitotic progression since their deletions led to classical G(2)- and M-phase arrest. Although not as obvious, cells with AnpphA and Annem1 deleted also have mitotic abnormalities. One of the essential phosphatases, the RNA polymerase II C-terminal domain phosphatase Anfcp1, was further examined for potential functions in mitosis because a temperature-sensitive Anfcp1 allele was isolated in a genetic screen showing synthetic interaction with the cdk1F mutation, a hyperactive mitotic kinase. The Anfcp1(ts) cdk1F double mutant had severe mitotic defects, including inability of nuclei to complete mitosis in a normal fashion. The severity of the Anfcp1(ts) cdk1F mitotic phenotypes were far greater than either single mutant, confirming the synthetic nature of their genetic interaction. The mitotic defects of the Anfcp1(ts) cdk1F double mutant suggests a previously unrealized function for AnFCP1 in regulating mitotic progression, perhaps counteracting Cdk1-mediated phosphorylation.

Sgt1, a co-chaperone of Hsp90 stabilizes Polo and is required for centrosome organization

Sgt1 was described previously in yeast and humans to be a Hsp90 co-chaperone and required for kinetochore assembly. We have identified a mutant allele of Sgt1 in Drosophila and characterized its function. Mutations in sgt1 do not affect overall kinetochore assembly or spindle assembly checkpoint. sgt1 mutant cells enter less frequently into mitosis and arrest in a prometaphase-like state. Mutations in sgt1 severely compromise the organization and function of the mitotic apparatus. In these cells, centrioles replicate but centrosomes fail to mature, and pericentriolar material components do not localize normally resulting in highly abnormal spindles. Interestingly, a similar phenotype was described previously in Hsp90 mutant cells and correlated with a decrease in Polo protein levels. In sgt1 mutant neuroblasts, we also observe a decrease in overall levels of Polo. Overexpression of the kinase results in a substantial rescue of the centrosome defects; most cells form normal bipolar spindles and progress through mitosis normally. Taken together, these findings suggest that Sgt1 is involved in the stabilization of Polo allowing normal centrosome maturation, entry and progression though mitosis.

Sequestration of Polo kinase to microtubules by phosphopriming-independent binding to Map205 is relieved by phosphorylation at a CDK site in mitosis

The conserved Polo kinase controls multiple events in mitosis and cytokinesis. Although Polo-like kinases are regulated by phosphorylation and proteolysis, control of subcellular localization plays a major role in coordinating their mitotic functions. This is achieved largely by the Polo-Box Domain, which binds prephosphorylated targets. However, it remains unclear whether and how Polo might interact with partner proteins when priming mitotic kinases are inactive. Here we show that Polo associates with microtubules in interphase and cytokinesis, through a strong interaction with the microtubule-associated protein Map205. Surprisingly, this interaction does not require priming phosphorylation of Map205, and the Polo-Box Domain of Polo is required but not sufficient for this interaction. Moreover, phosphorylation of Map205 at a CDK site relieves this interaction. Map205 can stabilize Polo and inhibit its cellular activity in vivo. In syncytial embryos, the centrosome defects observed in polo hypomorphs are enhanced by overexpression of Map205 and suppressed by its deletion. We propose that Map205-dependent targeting of Polo to microtubules provides a stable reservoir of Polo that can be rapidly mobilized by the activity of Cdk1 at mitotic entry.

Dual role of topoisomerase II in centromere resolution and aurora B activity

Chromosome segregation requires sister chromatid resolution. Condensins are essential for this process since they organize an axial structure where topoisomerase II can work. How sister chromatid separation is coordinated with chromosome condensation and decatenation activity remains unknown. We combined four-dimensional (4D) microscopy, RNA interference (RNAi), and biochemical analyses to show that topoisomerase II plays an essential role in this process. Either depletion of topoisomerase II or exposure to specific anti-topoisomerase II inhibitors causes centromere nondisjunction, associated with syntelic chromosome attachments. However, cells degrade cohesins and timely exit mitosis after satisfying the spindle assembly checkpoint. Moreover, in topoisomerase II-depleted cells, Aurora B and INCENP fail to transfer to the central spindle in late mitosis and remain tightly associated with centromeres of nondisjoined sister chromatids. Also, in topoisomerase II-depleted cells, Aurora B shows significantly reduced kinase activity both in S2 and HeLa cells. Codepletion of BubR1 in S2 cells restores Aurora B kinase activity, and consequently, most syntelic attachments are released. Taken together, our results support that topoisomerase II ensures proper sister chromatid separation through a direct role in centromere resolution and prevents incorrect microtubule-kinetochore attachments by allowing proper activation of Aurora B kinase.

HSF1 as a mitotic regulator: phosphorylation of HSF1 by Plk1 is essential for mitotic progression

Previously, heat shock factor 1 (HSF1) had been reported to induce genomic instability and aneuploidy by interaction with Cdc20. Here, we have further examined the functions of HSF1 in the regulation of mitosis. A null mutant or knockdown of HSF1 caused defective mitotic progression. By monitoring chromosomes in living cells, we determined that HSF1 was localized to the centrosome in mitosis and especially to the spindle poles in metaphase. HSF1 was phosphorylated by Plk1 at Ser(216) of the DSGXXS motif during the timing of mitosis and a phospho-defective mutant form of HSF1 inhibited mitotic progression. Phosphorylated HSF1 during spindle pole localization underwent ubiquitin degradation through the SCF(beta-TrCP) pathway. However, binding of HSF1 with Cdc20 stabilized the phosphorylation of HSF1. Moreover, SCF(beta-TrCP)-mediated degradation only occurred when phosphorylated HSF1 was released from Cdc20. HSF1 phosphorylation at Ser(216) occurred in the early mitotic period with simultaneous binding of Cdc20. The interaction of HSF1 with SCF(beta-TrCP) was followed and then the interaction of APC/Cdc20 was subsequently observed. From these findings, it was shown that Plk1 phosphorylates HSF1 in early mitosis and that the binding of phosphorylated HSF1 with Cdc20 and ubiquitin degradation by SCF(beta-TrCP) regulates mitotic progression.

Centrosome dysfunction in Drosophila neural stem cells causes tumors that are not due to genome instability

Genome instability (GI) and centrosomal alterations are common traits in human cancer [1, 2]. It is suspected that centrosome dysfunction may cause tumors by bringing about GI, but direct experimental proof is still lacking [3]. To explore the possible functional link between centrosome function and overgrowth, we have assayed the tumorigenic potential of a series of mutants that affect different centrosomal proteins in Drosophila. We have found that a significant number of such mutant conditions are tumorigenic in larval brain tissue, where self-renewing asymmetric division of neural stem cells is frequent, but not in symmetrically dividing epithelial cells. We have also found that mutations that increase GI without causing centrosome dysfunction are not tumorigenic in our assay. From these observations, we conclude that the tumors caused by centrosome dysfunction cannot be explained solely by the resulting genome instability. We propose that such tumors might be caused by impaired asymmetric division of neural stem cells [4]. These results show that centrosome loss, far from being innocuous, is a potentially dangerous condition in flies.

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.

Drosophila neuroblast asymmetric divisions: cell cycle regulators, asymmetric protein localization, and tumorigenesis

Over the past decade, many of the key components of the genetic machinery that regulate the asymmetric division of Drosophila melanogaster neural progenitors, neuroblasts, have been identified and their functions elucidated. Studies over the past two years have shown that many of these identified components act to regulate the self-renewal versus differentiation decision and appear to function as tumor suppressors during larval nervous system development. In this paper, we highlight the growing number of molecules that are normally considered to be key regulators of cell cycle events/progression that have recently been shown to impinge on the neuroblast asymmetric division machinery to control asymmetric protein localization and/or the decision to self-renew or differentiate.

Requirement for the budding yeast polo kinase Cdc5 in proper microtubule growth and dynamics

In many organisms, polo kinases appear to play multiple roles during M-phase progression. To provide new insights into the function of the budding yeast polo kinase Cdc5, we generated novel temperature-sensitive cdc5 mutants by mutagenizing the C-terminal noncatalytic polo box domain, a region that is critical for proper subcellular localization. One of these mutants, cdc5-11, exhibited a temperature-sensitive growth defect with an abnormal spindle morphology. Strikingly, provision of a moderate level of benomyl, a microtubule-depolymerizing drug, permitted cdc5-11 cells to grow significantly better than the isogenic CDC5 wild type in a FEAR (cdc Fourteen Early Anaphase Release)-independent manner. In addition, cdc5-11 required MAD2 for both cell growth and the benomyl-remedial phenotype. These results suggest that cdc5-11 is defective in proper spindle function. Consistent with this view, cdc5-11 exhibited abnormal spindle morphology, shorter spindle length, and delayed microtubule regrowth at the nonpermissive temperature. Overexpression of CDC5 moderately rescued the spc98-2 growth defect. Interestingly, both Cdc28 and Cdc5 were required for the proper modification of the spindle pole body components Nud1, Slk19, and Stu2 in vivo. They also phosphorylated these three proteins in vitro. Taken together, these observations suggest that concerted action of Cdc28 and Cdc5 on Nud1, Slk19, and Stu2 is important for proper spindle functions.

The inhibition of polo kinase by matrimony maintains G2 arrest in the meiotic cell cycle

Many meiotic systems in female animals include a lengthy arrest in G2 that separates the end of pachytene from nuclear envelope breakdown (NEB). However, the mechanisms by which a meiotic cell can arrest for long periods of time (decades in human females) have remained a mystery. The Drosophila Matrimony (Mtrm) protein is expressed from the end of pachytene until the completion of meiosis I. Loss-of-function mtrm mutants result in precocious NEB. Coimmunoprecipitation experiments reveal that Mtrm physically interacts with Polo kinase (Polo) in vivo, and multidimensional protein identification technology mass spectrometry analysis reveals that Mtrm binds to Polo with an approximate stoichiometry of 1:1. Mutation of a Polo-Box Domain (PBD) binding site in Mtrm ablates the function of Mtrm and the physical interaction of Mtrm with Polo. The meiotic defects observed in mtrm/+ heterozygotes are fully suppressed by reducing the dose of polo+, demonstrating that Mtrm acts as an inhibitor of Polo. Mtrm acts as a negative regulator of Polo during the later stages of G2 arrest. Indeed, both the repression of Polo expression until stage 11 and the inactivation of newly synthesized Polo by Mtrm until stage 13 play critical roles in maintaining and properly terminating G2 arrest. Our data suggest a model in which the eventual activation of Cdc25 by an excess of Polo at stage 13 triggers NEB and entry into prometaphase.

Mutations in Drosophila Greatwall/Scant reveal its roles in mitosis and meiosis and interdependence with Polo kinase

Polo is a conserved kinase that coordinates many events of mitosis and meiosis, but how it is regulated remains unclear. Drosophila females having only one wild-type allele of the polo kinase gene and the dominant Scant mutation produce embryos in which one of the centrosomes detaches from the nuclear envelope in late prophase. We show that Scant creates a hyperactive form of Greatwall (Gwl) with altered specificity in vitro, another protein kinase recently implicated in mitotic entry in Drosophila and Xenopus. Excess Gwl activity in embryos causes developmental failure that can be rescued by increasing maternal Polo dosage, indicating that coordination between the two mitotic kinases is crucial for mitotic progression. Revertant alleles of Scant that restore fertility to polo-Scant heterozygous females are recessive alleles or deficiencies of gwl; they show chromatin condensation defects and anaphase bridges in larval neuroblasts. One recessive mutant allele specifically disrupts a Gwl isoform strongly expressed during vitellogenesis. Females hemizygous for this allele are sterile, and their oocytes fail to arrest in metaphase I of meiosis; both homologues and sister chromatids separate on elongated meiotic spindles with little or no segregation. This allelic series of gwl mutants highlights the multiple roles of Gwl in both mitotic and meiotic progression. Our results indicate that Gwl activity antagonizes Polo and thus identify an important regulatory interaction of the cell cycle.

Asterless is a centriolar protein required for centrosome function and embryo development in Drosophila

BACKGROUND

Centrosomes, the major organizers of the microtubule network in most animal cells, are composed of centrioles embedded in a web of pericentriolar material (PCM). Recruitment and stabilization of PCM on the centrosome is a centriole-dependent function. Compared to the considerable number of PCM proteins known, the molecular characterization of centrioles is still very limited. Only a few centriolar proteins have been identified so far in Drosophila, most related to centriole duplication.

RESULTS

We have cloned asterless (asl) and found that it encodes a 120 kD highly coiled-coil protein that is a constitutive pancentriolar and basal body component. Loss of asl function impedes the stabilization/maintenance of PCM at the centrosome. In embryos deficient for Asl, development is arrested right after fertilization. Asl shares significant homology with Cep 152, a protein described as a component of the human centrosome for which no functional data is yet available.

CONCLUSIONS

The cloning of asl offers new insight into the molecular composition of Drosophila centrioles and a possible model for the role of its human homolog. In addition, the phenotype of asl-deficient flies reveals that a functional centrosome is required for Drosophila embryo development.