Polo kinase and separase regulate the mitotic licensing of centriole duplication in human cells

Cdk5rap2 exposes the centrosomal root of microcephaly syndromes

Autosomal recessive primary microcephaly (MCPH) is characterized by small brain size as a result of deficient neuron production in the developing cerebral cortex. Although MCPH is a rare disease, the questions surrounding its etiology strike at the core of stem cell biology. The seven genes implicated in MCPH all encode centrosomal proteins and disruption of the MCPH gene Cdk5rap2 in mice revealed its role in neural progenitor proliferation and in maintaining normal centriole replication control. We discuss here the impact that centrosome regulation has upon neural progenitors in the developing brain. We integrate the impact of centriole replication defects with the functions of Cdk5rap2 and other MCPH proteins, propose mechanisms for progenitor loss in MCPH, and discuss links to two other microcephaly syndromes.

The HPV-16 E7 oncoprotein induces centriole multiplication through deregulation of Polo-like kinase 4 expression

Background

Infection with high-risk human papillomaviruses (HPVs) such as HPV-16 is intimately associated with squamous cell carcinomas (SCCs) of the anogenital tract and a subset of oropharyngeal carcinomas. Such lesions, including pre-invasive precursors, frequently show multipolar mitoses and aneuploidy. The high-risk HPV-16-encoded E7 oncoprotein has been shown to rapidly induce centrosome abnormalities thereby causing the formation of supernumerary mitotic spindle poles and increasing the risk for chromosome missegregation. HPV-16 E7 has been found to rapidly induce centriole overduplication, in part, through the simultaneous formation of more than one daughter centriole at single maternal centrioles (centriole multiplication). The precise molecular mechanism that underlies HPV-16 E7-induced centriole multiplication, however, remains poorly understood.

Findings

Here, we show that human keratinocytes engineered to stably express the HPV-16 E7 oncoprotein exhibit aberrant Polo-like kinase 4 (PLK4) protein expression at maternal centrioles. Real-time quantitative reverse transcriptase (qRT-PCR) analysis of these cells revealed an increase of PLK4 mRNA levels compared to control cells. Importantly, the ability of the HPV-16 E7 oncoprotein to induce centriole multiplication was found to correlate with its ability to activate the PLK4 promoter and to up-regulate PLK4 mRNA.

Conclusions

These results highlight the critical role of PLK4 transcriptional deregulation in centriole multiplication in HPV-16 E7-expressing cells. Our findings encourage further experiments to test transcriptional inhibitors or small molecules targeting PLK4 to prevent centriole abnormalities, mitotic infidelity and malignant progression in HPV-associated neoplasms and other tumors in which PLK4 regulation is disrupted.

The conversion of centrioles to centrosomes: essential coupling of duplication with segregation

Centrioles are self-reproducing organelles that form the core structure of centrosomes or microtubule-organizing centers (MTOCs). However, whether duplication and MTOC organization reflect innate activities of centrioles or activities acquired conditionally is unclear. In this paper, we show that newly formed full-length centrioles had no inherent capacity to duplicate or to organize pericentriolar material (PCM) but acquired both after mitosis through a Plk1-dependent modification that occurred in early mitosis. Modified centrioles initiated PCM recruitment in G1 and segregated equally in mitosis through association with spindle poles. Conversely, unmodified centrioles segregated randomly unless passively tethered to modified centrioles. Strikingly, duplication occurred only in centrioles that were both modified and disengaged, whereas unmodified centrioles, engaged or not, were "infertile," indicating that engagement specifically blocks modified centrioles from reduplication. These two requirements, centriole modification and disengagement, fully exclude unlimited duplication in one cell cycle. We thus uncovered a Plk1-dependent mechanism whereby duplication and segregation are coupled to maintain centriole homeostasis.

The life cycle of centrioles

Centrioles organize the centrosome and nucleate the ciliary axoneme, and the centriole life cycle has many parallels to the chromosome cycle. The centriole cycle in animals begins at fertilization with the contribution of two centrioles by the male gamete. In the ensuing cell cycles, the duplication of centrioles is controlled temporally, spatially, and numerically. As a consequence of the duplication mechanism, the two centrioles in a typical interphase cell are of different ages and have different functions. Here, we discuss how new centrioles are assembled, what mechanisms limit centriole number, and the consequences of the inherent asymmetry of centriole duplication and segregation.

Separase phosphosite mutation leads to genome instability and primordial germ cell depletion during oogenesis

To ensure equal chromosome segregation and the stability of the genome during cell division, Separase is strictly regulated primarily by Securin binding and inhibitory phosphorylation. By generating a mouse model that contained a mutation to the inhibitory phosphosite of Separase, we demonstrated that mice of both sexes are infertile. We showed that Separase deregulation leads to chromosome mis-segregation, genome instability, and eventually apoptosis of primordial germ cells (PGCs) during embryonic oogenesis. Although the PGCs of mutant male mice were completely depleted, a population of PGCs from mutant females survived Separase deregulation. The surviving PGCs completed oogenesis but produced deficient initial follicles. These results indicate a sexual dimorphism effect on PGCs from Separase deregulation, which may be correlated with a gender-specific discrepancy of Securin. Our results reveal that Separase phospho-regulation is critical for genome stability in oogenesis. Furthermore, we provided the first evidence of a pre-zygotic mitotic chromosome segregation error resulting from Separase deregulation, whose sex-specific differences may be a reason for the sexual dimorphism of aneuploidy in gametogenesis.

Securin and separase modulate membrane traffic by affecting endosomal acidification

Securin and separase play a key role in sister chromatid separation during anaphase. However, a growing body of evidence suggests that in addition to regulating chromosome segregation, securin and separase display functions implicated in membrane traffic in Caenorhabditis elegans and Drosophila. Here we show that in mammalian cells both securin and separase associate with membranes and that depletion of either protein causes robust swelling of the trans-Golgi network (TGN) along with the appearance of large endocytic vesicles in the perinuclear region. These changes are accompanied by diminished constitutive protein secretion as well as impaired receptor recycling and degradation. Unexpectedly, cells depleted of securin or separase display defective acidification of early endosomes and increased membrane recruitment of vacuolar (V-) ATPase complexes, mimicking the effect of the specific V-ATPase inhibitor Bafilomycin A1. Taken together, our findings identify a new functional role of securin and separase in the modulation of membrane traffic and protein secretion that implicates regulation of V-ATPase assembly and function.

Centriole duplication: analogue control in a digital age

In preparation for mitosis, the centrosome doubles once and only once to provide the two poles of the mitotic spindle. The presence of more than two centrosomes increases the chances that mitosis will be multipolar, and chromosomes will be distributed unequally. Since the number of mother-daughter centriole pairs determines the number of centrosomes, it is important that only one daughter centriole is assembled at, but slightly separated from, the proximal end of each mother centriole. This numerical and spatial specificity has led to the belief that a 'template' on the mother centriole provides a unique site for procentriole assembly. We review observations that are leading to the demise of this intuitively attractive idea. In its place, we are left with the notion that pericentriolar material at the wall of the mother centriole provides a local environment that promotes the assembly of a macromolecular complex that seeds the daughter centriole. Even though the system normally behaves in a digital fashion to go from zero to just one daughter centriole per mother, this behaviour appears to be based in the precise analogue control of multiple proteins, their activities, and the structure provided by the mother centriole.

Cdc14: a highly conserved family of phosphatases with non-conserved functions?

CDC14 was originally identified by L. Hartwell in his famous screen for genes that regulate the budding yeast cell cycle. Subsequent work showed that Cdc14 belongs to a family of highly conserved dual-specificity phosphatases that are present in a wide range of organisms from yeast to human. Human CDC14B is even able to fulfill the essential functions of budding yeast Cdc14. In budding yeast, Cdc14 counteracts the activity of cyclin dependent kinase (Cdk1) at the end of mitosis and thus has important roles in the regulation of anaphase, mitotic exit and cytokinesis. On the basis of the functional conservation of other cell-cycle genes it seemed obvious to assume that Cdc14 phosphatases also have roles in late mitosis in mammalian cells and regulate similar targets to those found in yeast. However, analysis of the human Cdc14 proteins (CDC14A, CDC14B and CDC14C) by overexpression or by depletion using small interfering RNA (siRNA) has suggested functions that are quite different from those of ScCdc14. Recent studies in avian and human somatic cell lines in which the gene encoding either Cdc14A or Cdc14B had been deleted, have shown - surprisingly - that neither of the two phosphatases on its own is essential for viability, cell-cycle progression and checkpoint control. In this Commentary, we critically review the available data on the functions of yeast and vertebrate Cdc14 phosphatases, and discuss whether they indeed share common functions as generally assumed.

CDK11(p58) is required for centriole duplication and Plk4 recruitment to mitotic centrosomes

Background

CDK11^p58 is a mitotic protein kinase, which has been shown to be required for different mitotic events such as centrosome maturation, chromatid cohesion and cytokinesis.

Methodology/Principal Findings

In addition to these previously described roles, our study shows that CDK11^p58 inhibition induces a failure in the centriole duplication process in different human cell lines. We propose that this effect is mediated by the defective centrosomal recruitment of proteins at the onset of mitosis. Indeed, Plk4 protein kinase and the centrosomal protein Cep192, which are key components of the centriole duplication machinery, showed reduced levels at centrosomes of mitotic CDK11-depleted cells. CDK11^p58, which accumulates only in the vicinity of mitotic centrosomes, directly interacts with the centriole-associated protein kinase Plk4 that regulates centriole number in cells. In addition, we show that centriole from CDK11 defective cells are not able to be over duplicated following Plk4 overexpression.

Conclusion/Significance

We thus propose that CDK11 is required for centriole duplication by two non-mutually-exclusive mechanisms. On one hand, the observed duplication defect could be caused indirectly by a failure of the centrosome to fully maturate during mitosis. On the other hand, CDK11^p58 could also directly regulate key centriole components such as Plk4 during mitosis to trigger essential mitotic centriole modifications, required for centriole duplication during subsequent interphase.

Building the centriole

Centrioles are conserved microtubule-based organelles that lie at the core of the animal centrosome and play a crucial role in nucleating the formation of cilia and flagella in most eukaryotes. Centrioles have a complex ultrastructure with ninefold symmetry and a well-defined length. This structure is assembled from a host of proteins, including a variety of disease gene products. Over a century after the discovery of centrioles, the mechanisms underlying the assembly of these fascinating organelles, in particular the establishment of ninefold symmetry and the control of centriole length, are now starting to be uncovered.

Urine from current smokers induces centrosome aberrations and spindle defects in vitro in nonmalignant human cell lines

Tobacco smoke containing numerous derived chemical carcinogens is the main risk factor for urothelial carcinoma. These carcinogens can induce DNA damage leading to chromosomal instability, which plays a fundamental role in urothelial carcinogenesis. Possible mechanisms could be centrosomal aberrations, which cause defective spindles and may be responsible for genetic instability. We evaluated the effect of urine from never smokers (NS) and current smokers (CS) in concentrations of 0 to 50% on cell proliferation, chromosomes, centrosomes, and the spindle status of normal human dermal fibroblasts and normal human urothelial cells (UROtsa). After 2 weeks of urine treatment, cell cultures were analyzed by centrosome and spindle immunostaining and conventional cytogenetics. Effects were compared to results of untreated controls. Analysis of normal human dermal fibroblasts and UROtsa cells revealed that urine from CS induced higher values of centrosome aberrations in a dose-dependent and cell line-independent manner when compared to cultures treated with urine from NS and untreated controls. Centrosomal alterations correlated with spindle defects and an increase of sporadic chromosomal aberrations. The observations suggest a causative role of chemical carcinogens in urine from CS in the origin of centrosome and spindle defects in vitro leading to chromosomal instability and may be involved in urothelial carcinogenesis.

The first cell cycle of the Caenorhabditis elegans embryo: spatial and temporal control of an asymmetric cell division

Throughout the development of an organism, it is essential that the cell cycle machinery is fine-tuned to generate cells of different fate. A series of asymmetric cell divisions leads to lineage specification. The Caenorhabditis elegans embryo is an excellent system to study various aspects of the early embryonic cell cycle. The invariant nature of the rapid cell divisions is the key feature for studying the effects of small perturbations to a complex process such as the cell cycle. The thorough characterization of the asymmetric first cell division of the C. elegans embryo has given great insight on how the oscillations of the cell cycle coordinate with the cytoplasmic rearrangements that ultimately lead to two developmentally distinct daughter cells.

Centrioles regulate centrosome size by controlling the rate of Cnn incorporation into the PCM

BACKGROUND

centrosomes are major microtubule organizing centers in animal cells, and they comprise a pair of centrioles surrounded by an amorphous pericentriolar material (PCM). Centrosome size is tightly regulated during the cell cycle, and it has recently been shown that the two centrosomes in certain stem cells are often asymmetric in size. There is compelling evidence that centrioles influence centrosome size, but how centrosome size is set remains mysterious.

RESULTS

we show that the conserved Drosophila PCM protein Cnn exhibits an unusual dynamic behavior, because Cnn molecules only incorporate into the PCM closest to the centrioles and then spread outward through the rest of the PCM. Cnn incorporation into the PCM is driven by an interaction with the conserved centriolar proteins Asl (Cep152 in humans) and DSpd-2 (Cep192 in humans). The rate of Cnn incorporation into the PCM is tightly regulated during the cell cycle, and this rate influences the amount of Cnn in the PCM, which in turn is an important determinant of overall centrosome size. Intriguingly, daughter centrioles in syncytial embryos only start to incorporate Cnn as they disengage from their mothers; this generates a centrosome size asymmetry, with mother centrioles always initially organizing more Cnn than their daughters.

CONCLUSIONS

centrioles can control the amount of PCM they organize by regulating the rate of Cnn incorporation into the PCM. This mechanism can explain how centrosome size is regulated during the cell cycle and also allows mother and daughter centrioles to set centrosome size independently of one another.

BRCA2 and nucleophosmin coregulate centrosome amplification and form a complex with the Rho effector kinase ROCK2

BRCA2 germline mutations account for the majority of heredity breast and ovarian cancer. Besides its role in DNA damage repair, BRCA2 also plays an important role in cytokinesis, transcription regulation, and cancer cell proliferation. Recently, we reported that BRCA2 localizes to centrosomes as well as nuclei and the dysfunction of BRCA2 in a centrosome causes abnormalities in cell division. Here, we identified a nucleolar phosphoprotein, nucleophosmin (NPM), as a novel BRCA2-associated protein. We also detected the binding of BRCA2 to ROCK2, an effector of Rho small GTPase. Because it is known that ROCK2 binds to NPM at centrosomes, these 3 proteins may form a complex. NPM-binding region was within amino acids 639-1,000 of BRCA2. Exogenous expression of this BRCA2 region resulted in aberrant centrosome amplification and a high frequency of multinucleated cells. Our results suggested that a complex consisting of BRCA2, NPM, and ROCK2 maintains the numerical integrity of centrosomes and accurate cell division and that dysfunction of this regulation might be involved in the tumorigenesis of breast cancer.

Genomic instability and cancer: lessons learned from human papillomaviruses

High-risk HPV E6 and E7 oncoproteins cooperate to subvert critical host cell cycle checkpoint control mechanisms in order to promote viral genome replication. This results not only in aberrant proliferation but also in host cellular changes that can promote genomic instability. The HPV-16 E7 oncoprotein was found to induce centrosome abnormalities thereby disrupting mitotic fidelity and increasing the risk for chromosome missegregation and aneuploidy. In addition, expression of the high-risk HPV E7 oncoprotein stimulates DNA replication stress as a potential source of DNA breakage and structural chromosomal instability. Proliferation of genomically unstable cells is sustained by several mechanisms including the accelerated degradation of claspin by HPV-16 E7 and the degradation of p53 by the high-risk HPV E6 oncoprotein. These results highlight the oncogenic potential of aberrant proliferation and opens new avenues for prevention of malignant progression, not only in HPV-associated cervical cancer but also in non-virally associated malignancies with disrupted cell cycle checkpoint control mechanisms.

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.

Polo-like kinase 4: the odd one out of the family

Polo-like kinase 4 (PLK4) is a unique member of the Polo-like family of kinases that shares little homology with its siblings and has an essential role in centriole duplication. The turn-over of this kinase must be strictly controlled to prevent centriole amplification. This is achieved, in part, by an autoregulatory mechanism, whereby PLK4 autophosphorylates residues in a PEST sequence located carboxy-terminal to its catalytic domain. Phosphorylated PLK4 is subsequently recognized by the SCF complex, ubiquitinylated and targeted to the proteasome for degradation. Recent data have also shown that active PLK4 is restricted to the centrosome, a mechanism that could serve to prevent aberrant centriole assembly elsewhere in the cell. While significant advances have been made in understanding how PLK4 is regulated it is certain that additional regulatory mechanisms exist to safeguard the fidelity of centriole duplication. Here, we overview past and present data discussing the regulation and functions of PLK4.

A centrosome-autonomous signal that involves centriole disengagement permits centrosome duplication in G2 phase after DNA damage

DNA damage can induce centrosome overduplication in a manner that requires G2-to-M checkpoint function, suggesting that genotoxic stress can decouple the centrosome and chromosome cycles. How this happens is unclear. Using live-cell imaging of cells that express fluorescently tagged NEDD1/GCP-WD and proliferating cell nuclear antigen, we found that ionizing radiation (IR)-induced centrosome amplification can occur outside S phase. Analysis of synchronized populations showed that significantly more centrosome amplification occurred after irradiation of G2-enriched populations compared with G1-enriched or asynchronous cells, consistent with G2 phase centrosome amplification. Irradiated and control populations of G2 cells were then fused to test whether centrosome overduplication is allowed through a diffusible stimulatory signal, or the loss of a duplication-inhibiting signal. Irradiated G2/irradiated G2 cell fusions showed significantly higher centrosome amplification levels than irradiated G2/unirradiated G2 fusions. Chicken-human cell fusions demonstrated that centrosome amplification was limited to the irradiated partner. Our finding that only the irradiated centrosome can duplicate supports a model where a centrosome-autonomous inhibitory signal is lost upon irradiation of G2 cells. We observed centriole disengagement after irradiation. Although overexpression of dominant-negative securin did not affect IR-induced centrosome amplification, Plk1 inhibition reduced radiation-induced amplification. Together, our data support centriole disengagement as a licensing signal for DNA damage-induced centrosome amplification.

SPICE--a previously uncharacterized protein required for centriole duplication and mitotic chromosome congression

Proper assembly and function of a bipolar mitotic spindle is crucial for faithful bidirectional chromosome segregation during cell division. In animal cells, the two poles of the mitotic spindle are organized by centrosomes, microtubule-organizing structures composed of a pair of centrioles surrounded by the so-called pericentriolar material. Proteomic studies have revealed a large number of centrosome proteins, but many remain uncharacterized. Here, we characterize SPICE, a protein that localizes to spindle microtubules in mitosis and to centrioles throughout the cell cycle. RNAi-mediated depletion of SPICE in human cells impairs centriole duplication and causes severe mitotic defects. SPICE depletion compromises spindle architecture, spindle pole integrity and chromosome congression, even in cells in which centriole duplication has occurred. Our data suggest that SPICE is an important dual-function regulator required for centriole duplication and for proper bipolar spindle formation and chromosome congression in mitosis.

Centriole reduplication during prolonged interphase requires procentriole maturation governed by Plk1

Supernumerary centrioles lead to abnormal mitosis, which in turn promotes tumorigenesis. Thus, centriole duplication must be coordinated with the cell cycle to ensure that the number of centrioles in the cell doubles precisely during each cell cycle. However, in some transformed cells, centrioles undergo multiple rounds of duplication (reduplication) during prolonged interphase. Mechanisms responsible for centriole reduplication are poorly understood. Here, we report that centrioles reduplicate consistently in cancerous and nontransformed human cells during G2 arrests and that this reduplication requires the activity of Polo-like kinase 1 (Plk1). We also find that a cell's ability to reduplicate centrioles during S arrests depends on the presence of activated (Thr210-phosphorylated) Plk1 at the centrosome. In the absence of activated Plk1, nascent procentrioles remain associated with mother centrioles, which prevents centriole reduplication. In contrast, if Plk1(pT210) appears at the centrosome, procentrioles mature, disengage from mother centrioles, and ultimately duplicate. Plk1 activity is not required for the assembly of procentrioles, however. Thus, the role of Plk1 is to coordinate the centriole duplication cycle with the cell cycle. Activation of Plk1 during late S/G2 induces procentriole maturation, and after this point, the centriole cycle can be completed autonomously, even in the absence of cell-cycle progression.

Centrioles: active players or passengers during mitosis?

Centrioles are cylinders made of nine microtubule (MT) triplets present in many eukaryotes. Early studies, where centrosomes were seen at the poles of the mitotic spindle led to their coining as "the organ for cell division". However, a variety of subsequent observational and functional studies showed that centrosomes might not always be essential for mitosis. Here we review the arguments in this debate. We describe the centriole structure and its distribution in the eukaryotic tree of life and clarify its role in the organization of the centrosome and cilia, with an historical perspective. An important aspect of the debate addressed in this review is how centrioles are inherited and the role of the spindle in this process. In particular, germline inheritance of centrosomes, such as their de novo formation in parthenogenetic species, poses many interesting questions. We finish by discussing the most likely functions of centrioles and laying out new research avenues.

PLK2 phosphorylation is critical for CPAP function in procentriole formation during the centrosome cycle

Control of centrosome duplication is tightly linked with the progression of the cell cycle. Recent studies suggest that the fundamental process of centriole duplication is evolutionally conserved. Here, we identified centrosomal P4.1-associated protein (CPAP), a human homologue of SAS-4, as a substrate of PLK2 whose activity oscillates during the cell cycle. PLK2 phosphorylates the S589 and S595 residues of CPAP in vitro and in vivo. This phosphorylation is critical for procentriole formation during the centrosome cycle. PLK4 also phosphorylates S595 of CPAP, but PLK4 phosphorylation is not a critical step in the PLK4 function in procentriole assembly. CPAP is phosphorylated in a cell cycle stage-specific manner, so that its phosphorylation increases at the G1/S transition phase and decreases during the exit of mitosis. Phosphorylated CPAP is preferentially located at the procentriole. Furthermore, overexpression of a phospho-resistant CPAP mutant resulted in the failure to form elongated centrioles. On the basis of these results, we propose that phosphorylated CPAP is involved in procentriole assembly, possibly for centriole elongation. This work demonstrates an example of how procentriole formation is linked to the progression of the cell cycle.

Cell cycle: the art of multi-tasking

Separase is the protease that cleaves the cohesive link between sister chromatids to trigger chromosome segregation in mitosis and meiosis. This enzyme is known to orchestrate additional mitotic events and we now gain new insight into how it promotes cytokinesis in the nematode Caenorhabditis elegans.

Stepwise evolution of the centriole-assembly pathway

The centriole and basal body (CBB) structure nucleates cilia and flagella, and is an essential component of the centrosome, underlying eukaryotic microtubule-based motility, cell division and polarity. In recent years, components of the CBB-assembly machinery have been identified, but little is known about their regulation and evolution. Given the diversity of cellular contexts encountered in eukaryotes, but the remarkable conservation of CBB morphology, we asked whether general mechanistic principles could explain CBB assembly. We analysed the distribution of each component of the human CBB-assembly machinery across eukaryotes as a strategy to generate testable hypotheses. We found an evolutionarily cohesive and ancestral module, which we term UNIMOD and is defined by three components (SAS6, SAS4/CPAP and BLD10/CEP135), that correlates with the occurrence of CBBs. Unexpectedly, other players (SAK/PLK4, SPD2/CEP192 and CP110) emerged in a taxon-specific manner. We report that gene duplication plays an important role in the evolution of CBB components and show that, in the case of BLD10/CEP135, this is a source of tissue specificity in CBB and flagella biogenesis. Moreover, we observe extreme protein divergence amongst CBB components and show experimentally that there is loss of cross-species complementation among SAK/PLK4 family members, suggesting species-specific adaptations in CBB assembly. We propose that the UNIMOD theory explains the conservation of CBB architecture and that taxon- and tissue-specific molecular innovations, gained through emergence, duplication and divergence, play important roles in coordinating CBB biogenesis and function in different cellular contexts.

Polo-like kinase 2-dependent phosphorylation of NPM/B23 on serine 4 triggers centriole duplication

Duplication of the centrosome is well controlled during faithful cell division while deregulation of this process leads to supernumary centrosomes, chromosome missegregation and aneuploidy, a hallmark of many cancer cells. We previously reported that Polo-like kinase 2 (Plk2) is activated near the G1/S phase transition, and regulates the reproduction of centrosomes. In search for Plk2 interacting proteins we have identified NPM/B23 (Nucleophosmin) as a novel Plk2 binding partner. We find that Plk2 and NPM/B23 interact in vitro in a Polo-box dependent manner. An association between both proteins was also observed in vivo. Moreover, we show that Plk2 phosphorylates NPM/B23 on serine 4 in vivo in S-phase. Notably, expression of a non-phosphorylatable NPM/B23 S4A mutant interferes with centriole reduplication in S-phase arrested cells and leads to a dilution of centriole numbers in unperturbed U2OS cells. The corresponding phospho-mimicking mutants have the opposite effect and their expression leads to the accumulation of centrioles. These findings suggest that NPM/B23 is a direct target of Plk2 in the regulation of centriole duplication and that phosphorylation on serine 4 can trigger this process.

Overexpression of the E2 ubiquitin-conjugating enzyme UbcH10 causes chromosome missegregation and tumor formation

An overabundance of UbcH10 disrupts mitotic checkpoint signaling as a result of a degradation of cyclin B, increasing spontaneous and carcinogen-induced tumor formation in transgenic mice.

The anaphase-promoting complex/cyclosome (APC/C) E3 ubiquitin ligase functions with the E2 ubiquitin–conjugating enzyme UbcH10 in the orderly progression through mitosis by marking key mitotic regulators for destruction by the 26-S proteasome. UbcH10 is overexpressed in many human cancer types and is associated with tumor progression. However, whether UbcH10 overexpression causes tumor formation is unknown. To address this central question and to define the molecular and cellular consequences of UbcH10 overexpression, we generated a series of transgenic mice in which UbcH10 was overexpressed in graded fashion. In this study, we show that UbcH10 overexpression leads to precocious degradation of cyclin B by the APC/C, supernumerary centrioles, lagging chromosomes, and aneuploidy. Importantly, we find that UbcH10 transgenic mice are prone to carcinogen-induced lung tumors and a broad spectrum of spontaneous tumors. Our results identify UbcH10 as a prominent protooncogene that causes whole chromosome instability and tumor formation over a wide gradient of overexpression levels.

Cohesin cleavage and Cdk inhibition trigger formation of daughter nuclei

The metaphase-anaphase transition is orchestrated through proteolysis of numerous proteins by a ubiquitin protein ligase called the anaphase-promoting complex or cyclosome (APC/C). A crucial aspect of this process is sister chromatid separation, which is thought to be mediated by separase, a thiol protease activated by the APC/C. Separase cleaves cohesin, a ring-shaped complex that entraps sister DNAs. It is a matter of debate whether cohesin-independent forces also contribute to sister chromatid cohesion. Using 4D live-cell imaging of Drosophila melanogaster syncytial embryos blocked in metaphase (via APC/C inhibition), we show that artificial cohesin cleavage is sufficient to trigger chromosome disjunction. This is nevertheless insufficient for correct chromosome segregation. Kinetochore-microtubule attachments are rapidly destabilized by the loss of tension caused by cohesin cleavage in the presence of high Cdk1 (cyclin-dependent kinase 1) activity, as occurs when the APC/C cannot destroy mitotic cyclins. Metaphase chromosomes undergo a bona fide anaphase when cohesin cleavage is combined with Cdk1 inhibition. We conclude that only two key events, opening of cohesin rings and downregulation of Cdk1, are sufficient to drive proper segregation of chromosomes in anaphase.

A role for separase in the regulation of RAB-11-positive vesicles at the cleavage furrow and midbody

Cell division requires coordinated regulation of chromosome segregation and cytokinesis. Although much is known about the function of the protease separase in promoting sister chromosome separation, the role of separase during cytokinesis is unclear. We show that separase localizes to the ingressing furrow and midbody during cytokinesis in the C. elegans embryo. Loss of separase function during the early mitotic divisions causes cytokinesis failure that is not due to eggshell defects or chromosome nondisjunction. Moreover, depletion of separase causes the accumulation of RAB-11-positive vesicles at the cleavage furrow and midbody that is not a consequence of chromosome nondisjunction, but is mimicked by depletion of vesicle fusion machinery. Collectively, these data indicate that separase is required for cytokinesis by regulating the incorporation of RAB-11-positive vesicles into the plasma membrane at the cleavage furrow and midbody.

Centrioles, centrosomes, and cilia in health and disease

Centrioles are barrel-shaped structures that are essential for the formation of centrosomes, cilia, and flagella. Here we review recent advances in our understanding of the function and biogenesis of these organelles, and we emphasize their connection to human disease. Deregulation of centrosome numbers has long been proposed to contribute to genome instability and tumor formation, whereas mutations in centrosomal proteins have recently been genetically linked to microcephaly and dwarfism. Finally, structural or functional centriole aberrations contribute to ciliopathies, a variety of complex diseases that stem from the absence or dysfunction of cilia.