POLO ensures chromosome bi-orientation by preventing and correcting erroneous chromosome-spindle attachments

The Role of Mitotic Kinases and the RZZ Complex in Kinetochore-Microtubule Attachments: Doing the Right Link

During mitosis, the interaction of kinetochores (KTs) with microtubules (MTs) drives chromosome congression to the spindle equator and supports the segregation of sister chromatids. Faithful genome partition critically relies on the ability of chromosomes to establish and maintain proper amphitelic end-on attachments, a configuration in which sister KTs are connected to robust MT fibers emanating from opposite spindle poles. Because the capture of spindle MTs by KTs is error prone, cells use mechanisms that sense and correct inaccurate KT-MT interactions before committing to segregate sister chromatids in anaphase. If left unresolved, these errors can result in the unequal distribution of chromosomes and lead to aneuploidy, a hallmark of cancer. In this review, we provide an overview of the molecular strategies that monitor the formation and fine-tuning of KT-MT attachments. We describe the complex network of proteins that operates at the KT-MT interface and discuss how AURORA B and PLK1 coordinate several concurrent events so that the stability of KT-MT attachments is precisely modulated throughout mitotic progression. We also outline updated knowledge on how the RZZ complex is regulated to ensure the formation of end-on attachments and the fidelity of mitosis.

BUB1 and CENP-U, Primed by CDK1, Are the Main PLK1 Kinetochore Receptors in Mitosis

Reflecting its pleiotropic functions, Polo-like kinase 1 (PLK1) localizes to various sub-cellular structures during mitosis. At kinetochores, PLK1 contributes to microtubule attachments and mitotic checkpoint signaling. Previous studies identified a wealth of potential PLK1 receptors at kinetochores, as well as requirements for various mitotic kinases, including BUB1, Aurora B, and PLK1 itself. Here, we combine ectopic localization, in vitro reconstitution, and kinetochore localization studies to demonstrate that most and likely all of the PLK1 is recruited through BUB1 in the outer kinetochore and centromeric protein U (CENP-U) in the inner kinetochore. BUB1 and CENP-U share a constellation of sequence motifs consisting of a putative PP2A-docking motif and two neighboring PLK1-docking sites, which, contingent on priming phosphorylation by cyclin-dependent kinase 1 and PLK1 itself, bind PLK1 and promote its dimerization. Our results rationalize previous observations and describe a unifying mechanism for recruitment of PLK1 to human kinetochores.

RZZ-SPINDLY-DYNEIN: you got to keep 'em separated

To maintain genome stability, chromosomes must be equally distributed among daughter cells at the end of mitosis. The accuracy of chromosome segregation requires sister-kinetochores to stably attach to microtubules emanating from opposite spindle poles. However, initial kinetochore-microtubule interactions are able to turnover so that defective attachment configurations that typically arise during early mitosis may be corrected. Growing evidence supports a role for the RZZ complex in preventing the stabilization of erroneous kinetochore-microtubule attachments. This inhibitory function of RZZ toward end-on attachments is relieved by DYNEIN-mediated transport of the complex as chromosomes congress and appropriate interactions with microtubules are established. However, it remains unclear how DYNEIN is antagonized to prevent premature RZZ removal. We recently described a new mechanism that sheds new light on this matter. We found that POLO kinase phosphorylates the DYNEIN adaptor SPINDLY to promote the uncoupling between RZZ and DYNEIN. Elevated POLO activity during prometaphase ensures that RZZ is retained at kinetochores to allow the dynamic turnover of kinetochore-microtubule interactions and prevent the stabilization of erroneous attachments. Here, we discuss additional interpretations to explain a model for POLO-dependent regulation of the RZZ-SPINDLY-DYNEIN module during mitosis.

Regulation of Polo Kinase by Matrimony Is Required for Cohesin Maintenance during Drosophila melanogaster Female Meiosis

Polo-like kinases (PLKs) have numerous roles in both mitosis and meiosis, including functions related to chromosome segregation, cohesin removal, and kinetochore orientation [1-7]. PLKs require specific regulation during meiosis to control those processes. Genetic studies demonstrate that the Drosophila PLK Polo kinase (Polo) is inhibited by the female meiosis-specific protein Matrimony (Mtrm) in a stoichiometric manner [8]. Drosophila Polo localizes strongly to kinetochores and to central spindle microtubules during prometaphase and metaphase I of female meiosis [9, 10]. Mtrm protein levels increase dramatically after nuclear envelope breakdown [11]. We show that Mtrm is enriched along the meiotic spindle and that loss of mtrm results in mislocalization of the catalytically active form of Polo. The mtrm gene is haploinsufficient, and heterozygosity for mtrm (mtrm/+) results in high levels of achiasmate chromosome missegregation [8, 12]. In mtrm/+ heterozygotes, there is a low level of sister centromere separation, as well as precocious loss of cohesion along the arms of achiasmate chromosomes. However, mtrm-null females are sterile [13], and sister chromatid cohesion is abolished on all chromosomes, leading to a failure to properly congress or orient chromosomes in metaphase I. These data demonstrate a requirement for the inhibition of Polo, perhaps by sequestering Polo to the microtubules during Drosophila melanogaster female meiosis and suggest that catalytically active Polo is a distinct subset of the total Polo population within the oocyte that requires its own regulation.

Polo regulates Spindly to prevent premature stabilization of kinetochore-microtubule attachments

Accurate chromosome segregation in mitosis requires sister kinetochores to bind to microtubules from opposite spindle poles. The stability of kinetochore-microtubule attachments is fine-tuned to prevent or correct erroneous attachments while preserving amphitelic interactions. Polo kinase has been implicated in both stabilizing and destabilizing kinetochore-microtubule attachments. However, the mechanism underlying Polo-destabilizing activity remains elusive. Here, resorting to an RNAi screen in Drosophila for suppressors of a constitutively active Polo mutant, we identified a strong genetic interaction between Polo and the Rod-ZW10-Zwilch (RZZ) complex, whose kinetochore accumulation has been shown to antagonize microtubule stability. We find that Polo phosphorylates Spindly and impairs its ability to bind to Zwilch. This precludes dynein-mediated removal of the RZZ from kinetochores and consequently delays the formation of stable end-on attachments. We propose that high Polo-kinase activity following mitotic entry directs the RZZ complex to minimize premature stabilization of erroneous attachments, whereas a decrease in active Polo in later mitotic stages allows the formation of stable amphitelic spindle attachments. Our findings demonstrate that Polo tightly regulates the RZZ-Spindly-dynein module during mitosis to ensure the fidelity of chromosome segregation.

Another string to the polo bow: a new mitotic role of PLK1 in centromere protection

Polo-like kinase 1 (PLK1) plays a fundamental role in the spatiotemporal control of mitosis. Cells lacking PLK1 activity exhibit characteristic chromosome misalignment due to defects in microtubule-kinetochore organization and attachment. In our recently published paper, we uncover a new role for PLK1 in the preservation and maintenance of centromere integrity.

Phosphatases in Mitosis: Roles and Regulation

Mitosis requires extensive rearrangement of cellular architecture and of subcellular structures so that replicated chromosomes can bind correctly to spindle microtubules and segregate towards opposite poles. This process originates two new daughter nuclei with equal genetic content and relies on highly-dynamic and tightly regulated phosphorylation of numerous cell cycle proteins. A burst in protein phosphorylation orchestrated by several conserved kinases occurs as cells go into and progress through mitosis. The opposing dephosphorylation events are catalyzed by a small set of protein phosphatases, whose importance for the accuracy of mitosis is becoming increasingly appreciated. This review will focus on the established and emerging roles of mitotic phosphatases, describe their structural and biochemical properties, and discuss recent advances in understanding the regulation of phosphatase activity and function.

Cyclin A/Cdk1 modulates Plk1 activity in prometaphase to regulate kinetochore-microtubule attachment stability

The fidelity of chromosome segregation in mitosis is safeguarded by the precise regulation of kinetochore microtubule (k-MT) attachment stability. Previously, we demonstrated that Cyclin A/Cdk1 destabilizes k-MT attachments to promote faithful chromosome segregation. Here, we use quantitative phosphoproteomics to identify 156 Cyclin A/Cdk1 substrates in prometaphase. One Cyclin A/Cdk1 substrate is myosin phosphatase targeting subunit 1 (MYPT1), and we show that MYPT1 localization to kinetochores depends on Cyclin A/Cdk1 activity and that MYPT1 destabilizes k-MT attachments by negatively regulating Plk1 at kinetochores. Thus, Cyclin A/Cdk1 phosphorylation primes MYPT1 for Plk1 binding. Interestingly, priming of PBIP1 by Plk1 itself (self-priming) increased in MYPT1-depleted cells showing that MYPT1 provides a molecular link between the processes of Cdk1-dependent priming and self-priming of Plk1 substrates. These data demonstrate cross-regulation between Cyclin A/Cdk1-dependent and Plk1-dependent phosphorylation of substrates during mitosis to ensure efficient correction of k-MT attachment errors necessary for high mitotic fidelity.

Playing polo during mitosis: PLK1 takes the lead

Polo-like kinase 1 (PLK1), the prototypical member of the polo-like family of serine/threonine kinases, is a pivotal regulator of mitosis and cytokinesis in eukaryotes. Many layers of regulation have evolved to target PLK1 to different subcellular structures and to its various mitotic substrates in line with its numerous functions during mitosis. Collective work is starting to illuminate an important set of substrates for PLK1: the mitotic kinases that together ensure the fidelity of the cell division process. Amongst these, recent developments argue that PLK1 regulates the activity of the histone kinases Aurora B and Haspin to define centromere identity, of MPS1 to initiate spindle checkpoint signaling, and of BUB1 and its pseudokinase paralog BUBR1 to coordinate spindle checkpoint activation and inactivation. Here, we review the recent work describing the regulation of these kinases by PLK1. We highlight common themes throughout and argue that a major mitotic function of PLK1 is as a master regulator of these key kinases.

Loss of Polo ameliorates APP-induced Alzheimer's disease-like symptoms in Drosophila

The amyloid precursor protein (APP) has been implicated in the pathogenesis of Alzheimer’s disease (AD). Despite extensive studies, little is known about the regulation of APP’s functions in vivo. Here we report that expression of human APP in Drosophila, in the same temporal-spatial pattern as its homolog APPL, induced morphological defects in wings and larval NMJ, larva and adult locomotion dysfunctions, male choice disorder and lifespan shortening. To identify additional genes that modulate APP functions, we performed a genetic screen and found that loss of Polo, a key regulator of cell cycle, partially suppressed APP-induced morphological and behavioral defects in larval and adult stages. Finally, we showed that eye-specific expression of APP induced retina degeneration and cell cycle re-entry, both phenotypes were mildly ameliorated by loss of Polo. These results suggest Polo is an important in vivo regulator of the pathological functions of APP, and provide insight into the role of cell cycle re-entry in AD pathogenesis.

Polo kinase regulates the localization and activity of the chromosomal passenger complex in meiosis and mitosis in Drosophila melanogaster

Cell cycle progression is regulated by members of the cyclin-dependent kinase (CDK), Polo and Aurora families of protein kinases. The levels of expression and localization of the key regulatory kinases are themselves subject to very tight control. There is increasing evidence that crosstalk between the mitotic kinases provides for an additional level of regulation. We have previously shown that Aurora B activates Polo kinase at the centromere in mitosis, and that the interaction between Polo and the chromosomal passenger complex (CPC) component INCENP is essential in this activation. In this report, we show that Polo kinase is required for the correct localization and activity of the CPC in meiosis and mitosis. Study of the phenotype of different polo allele combinations compared to the effect of chemical inhibition revealed significant differences in the localization and activity of the CPC in diploid tissues. Our results shed new light on the mechanisms that control the activity of Aurora B in meiosis and mitosis.

Polo-like kinase-1 triggers histone phosphorylation by Haspin in mitosis

Histone modifications coordinate the chromatin localization of key regulatory factors in mitosis. For example, mitotic phosphorylation of Histone H3 threonine-3 (H3T3ph) by Haspin creates a binding site for the chromosomal passenger complex (CPC). However, how these histone modifications are spatiotemporally controlled during the cell cycle is unclear. Here we show that Plk1 binds to Haspin in a Cdk1-phosphorylation-dependent manner. Reducing Plk1 activity decreases the phosphorylation of Haspin and inhibits H3T3ph, particularly in prophase, suggesting that Plk1 is required for initial activation of Haspin in early mitosis. These studies demonstrate that Plk1 can positively regulate CPC recruitment in mitosis.

Linked in: formation and regulation of microtubule attachments during chromosome segregation

Accurate segregation of the replicated genome during cell division depends on dynamic attachments formed between chromosomes and the microtubule polymers of the spindle. Here we review recent advances in mechanistic analysis of microtubule attachment formation and regulation.

All together now: Polo joins the kinase network controlling the spindle assembly checkpoint in Drosophila

Maintenance of genomic stability during eukaryotic cell division relies on the Spindle Assembly Checkpoint (SAC), which has evolved as a surveillance mechanism that monitors kinetochore-microtubule attachment and prevents APC/C-mediated mitotic exit until all chromosomes are properly attached to the mitotic spindle. Reversible protein phosphorylation has long been accredited as a regulatory mechanism of the SAC. Nevertheless, knowledge of how several mitotic kinases act in concert within the signaling pathway to orchestrate SAC function is still emerging. In a recent study, we undertook a comprehensive dissection of the hierarchical framework controlling SAC function in Drosophila cells. We found that Polo lies at the top of the SAC pathway promoting the efficient recruitment of Mps1 to unattached kinetochores. This renders Mps1 fully active to control BubR1 phosphorylation that generates the 3F3/2 phosphoepitope at tensionless kinetochores. We have proposed that Polo is required for SAC function and that the molecular outcome of Mps1-dependent 3F3/2 formation is to promote the association of Cdc20 with BubR1 allowing proper kinetochore recruitment of Cdc20 and efficient assembly of the Mitotic Checkpoint Complex (MCC) required for a sustained SAC response.

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

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

High mitotic activity of Polo-like kinase 1 is required for chromosome segregation and genomic integrity in human epithelial cells

Protein kinases play key roles in regulating human cell biology, but manifold substrates and functions make it difficult to understand mechanism. We tested whether we could dissect functions of a pleiotropic mitotic kinase, Polo-like kinase 1 (Plk1), via distinct thresholds of kinase activity. We accomplished this by titrating Plk1 activity in RPE1 human epithelial cells using chemical genetics and verifying results in additional lines. We found that distinct activity thresholds are required for known functions of Plk1 including (from low to high activity) bipolar spindle formation, timely mitotic entry, and formation of a cytokinesis cleavage furrow. Subtle losses in Plk1 activity impaired chromosome congression and produced severe anaphase dysfunction characterized by poor separation of chromosome masses. These two phenotypes were separable, suggesting that they stem from distinct phosphorylation events. Impaired chromosome segregation in anaphase was the most sensitive to modest loss in Plk1 activity. Mechanistically, it was associated with unpaired sister chromatids with stretched kinetochores, suggestive of merotelic attachments. The C-terminal Polo box domain of Plk1 was required for its anaphase function, although it was dispensable for forming a bipolar spindle. The ultimate effect of partial inhibition of Plk1 was the formation of micronuclei, an increase in tetraploid progeny, and senescence. These results demonstrate that different thresholds of Plk1 activity can elicit distinct phenotypes, illustrating a general method for separating pleiotropic functions of a protein kinase even when these are executed close in time.

Balancing the kinetochore ledger

Reduction of polo-like kinase-1 (Plk1) at kinetochores as cells progress from prometaphase to metaphase is surprising given that the kinase is thought to stabilize kinetochore–microtubule (kt–MT) attachments. In this issue, Liu et al. (2012. J. Cell Biol. doi:10.1083/jcb.201205090) demonstrate that kinetochore-associated Plk1 is a potent suppressor of microtubule plus-end dynamics. The authors propose that Plk1 activity facilitates the establishment of kt–MT attachments in prometaphase by stabilizing microtubules and that reduction of the kinase in metaphase promotes force generation by dynamic microtubules.