Overexpression of Aurora-C interferes with the spindle checkpoint by promoting the degradation of Aurora-B

The chromosomal passenger complex (CPC) plays a pivotal role in controlling accurate chromosome segregation and cytokinesis during cell division. Aurora-B, one of the chromosomal passenger proteins, is important for the mitotic spindle assembly checkpoint (SAC). Previous reports noted that Aurora-C is predominantly expressed in male germ cells and has the same subcellular localization as Aurora-B. Increasing evidence indicates that Aurora-C is overexpressed in many somatic cancers, although its function is uncertain. Our previous study showed that the aberrant expression of Aurora-C increases the tumorigenicity of cancer cells. Here, we demonstrate that overexpressed Aurora-C displaces the centromeric localization of CPCs, including INCENP, survivin, and Aurora-B. When cells were treated with nocodazole to turn on SAC, both the Aurora-B protein stability and kinase activity were affected by overexpressed Aurora-C. As a result, the activation of spindle checkpoint protein, BubR1, and phosphorylation of histone H3 and MCAK were also eliminated in Aurora-C-overexpressing cells. Thus, our results suggest that aberrantly expressed Aurora-C in somatic cancer cells may impair SAC by displacing the centromeric localization of CPCs.     

Direct association with inner centromere protein (INCENP) activates the novel chromosomal passenger protein, Aurora-C

A family of serine/threonine kinase Aurora constitutes a key regulator in the orchestration of mitotic events. The human Aurora paralogues Aurora-A, Aurora-B, and Aurora-C have a highly conserved catalytic domain. Extensive studies on the role of Aurora-A and Aurora-B have revealed distinct localizations and functions in regulating mitotic processes, whereas little is known about Aurora-C. The present study shows that human Aurora-C is a chromosomal passenger protein that forms complexes with Aurora-B and inner centromere protein (INCENP), which are known passenger proteins. We show that INCENP binds and activates Aurora-C in vivo and in vitro. Furthermore, Aurora-C co-expressed with INCENP elicits the phosphorylation of endogenous histone H3 in mammalian cells, even though this phosphorylation is not sufficient to establish chromosome condensation in interphase cells. We therefore suggest that Aurora-C is a novel chromosomal passenger protein that cooperates with Aurora-B to regulate mitotic chromosome dynamics in mammalian cells.     

Centromere targeting of the chromosomal passenger complex requires a ternary subcomplex of Borealin, Survivin, and the N-terminal domain of INCENP

The chromosomal passenger complex (CPC), consisting of the serine/threonine kinase Aurora B, the inner centromere protein INCENP, Survivin, and Borealin/DasraB, has essential functions at the centromere in ensuring correct chromosome alignment and segregation. Despite observations that small interfering RNA-mediated knockdown of any one member of the CPC abolishes localization of the other subunits, it remains unclear how the complex is targeted to the centromere. We have now identified a ternary subcomplex of the CPC comprising Survivin, Borealin, and the N-terminal 58 amino acids of INCENP in vitro and in vivo. This subcomplex was found to be essential and sufficient for targeting to the centromere. Notably, Aurora B kinase, the enzymatic core of the CPC, was not required for centromere localization of the subcomplex. We demonstrate that CPC targeting to the centromere does not depend on CENP-A and hMis12, two core components for kinetochore/centromere assembly, and provide evidence that the CPC may be directed to centromeric DNA directly via the Borealin subunit. Our findings thus establish a functional module within the CPC that assembles on the N terminus of INCENP and controls centromere recruitment.     

Aurora-C kinase supports mitotic progression in the absence of Aurora-B

Aurora family kinases regulate numerous mitotic processes, and their dysfunction or overexpression can cause aneuploidy, a contributing factor for tumorigenesis. In vertebrates, the Aurora-B kinase regulates kinetochore maturation, destabilization of improper kinetochore-microtubule attachments, the spindle assembly checkpoint, central spindle organization, and cytokinesis. A gene duplication event created the related Aurora-C kinase in mammals. While Aurora-C function is unclear, it has similar structural and localization properties as Aurora-B. Inhibition of either Aurora-B or Aurora-C function causes aneuploidy, while simultaneous inhibition of both causes a higher frequency of aneuploidy. To determine if Aurora-C and –B have overlapping or unique complementary functions during mitosis, we created a system where Aurora-B is replaced by wild-type or kinase-defective mutant Aurora-C in HeLa cells. In this model, Aurora-B protein levels and mitotic functions were suppressed including the regulation of kinetochore-microtubule attachments, the spindle assembly checkpoint, and cytokinesis. Wild-type, but not kinase-defective Aurora-C expression, was able to rescue these functions. Therefore, Aurora-C can perform these essential functions of Aurora-B in mitosis.     

Survivin mediates targeting of the chromosomal passenger complex to the centromere and midbody

The chromosomal passenger complex (CPC) coordinates chromosomal and cytoskeletal events of mitosis. The enzymatic core of this complex (Aurora-B) is guided through the mitotic cell by its companion chromosomal passenger proteins, inner centromere protein (INCENP), Survivin and Borealin/Dasra-B, thereby allowing it to act at the right place at the right time. Here, we addressed the individual contributions of INCENP, Survivin and Borealin to the proper functioning of this complex. We show that INCENP has an important role in stabilizing the complex, and that Borealin acts to promote binding of Survivin to INCENP. Importantly, when Survivin is directly fused to INCENP, this hybrid can restore CPC function at the centromeres and midbody, even in the absence of Borealin and the centromere-targeting domain of INCENP. Thus, Survivin is an important mediator of centromere and midbody docking of Aurora-B during mitosis.     

The Survivin-Crm1 interaction is essential for chromosomal passenger complex localization and function

The chromosomal passenger complex (CPC) of Aurora-B, Borealin, INCENP (inner centromere protein) and Survivin coordinates essential chromosomal and cytoskeletal events during mitosis. Here, we show that the nuclear export receptor Crm1 is crucially involved in tethering the CPC to the centromere by interacting with a leucine-rich nuclear export signal (NES), evolutionarily conserved in all mammalian Survivin proteins. We show that inhibition of the Survivin-Crm1 interaction by treatment with leptomycin B or by RNA-interference-mediated Crm1 depletion prevents centromeric targeting of Survivin. The genetic inactivation of the Survivin-Crm1 interaction by mutation of the NES affects the correct localization and function of Survivin and the CPC during mitosis. By contrast, CPC assembly does not seem to require the Survivin-Crm1 interaction. Our report shows the functional significance of the Survivin-Crm1 interface and provides a novel link between the mitotic effector Crm1 and the CPC.     

Perturbation of Incenp function impedes anaphase chromatid movements and chromosomal passenger protein flux at centromeres

Incenp is an essential mitotic protein that, together with Aurora B, Survivin, and Borealin, forms the core of the chromosomal passenger protein complex (CPC). The CPC regulates various mitotic processes and functions to maintain genomic stability. The proper subcellular localization of the CPC and its full catalytic activity require the presence of each core subunit in the complex. We have investigated the mitotic tasks of the CPC using a function blocking antibody against Incenp microinjected into cells at different mitotic phases. This method allowed temporal analysis of CPC functions without perturbation of complex assembly or activity prior to injection. We have also studied the dynamic properties of Incenp and Aurora B using fusion protein photobleaching. We found that in early mitotic cells, Incenp and Aurora B exhibit dynamic turnover at centromeres, which is prevented by the anti-Incenp antibody. In these cells, the loss of centromeric CPC turnover is accompanied by forced mitotic exit without the execution of cytokinesis. Introduction of anti-Incenp antibody into early anaphase cells causes abnormalities in sister chromatid separation through defects in anaphase spindle functions. In summary, our data uncovers new mitotic roles for the CPC in anaphase and proposes that CPC turnover at centromeres modulates spindle assembly checkpoint signaling. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Differential functions of the Aurora-B and Aurora-C kinases in mammalian spermatogenesis

The Aurora kinases are cell cycle-regulatory serine-threonine kinases that have been implicated in the function of the centrosomes, kinetechores, chromosome dynamics, and cytokinesis. In comparison with other tissues, there are high levels of expression of Aurora-B and -C in testis. What their respective roles in mammalian spermatogenesis are is an open question. Here we describe the expression and distribution patterns of the three kinases in mouse testis using in situ hybridization and immunohistochemistry. Importantly, the localization of Aurora-B is tightly regulated during spermatogenesis, whereas Aurora-C expression appears to be testis specific. To address the function of Aurora-B in spermatogenesis, we have generated transgenic mice using a pachytene-stage-specific promoter driving the expression of either wild-type Aurora-B or an inactive form of the kinase. Expression of the inactive Aurora-B results in abnormal spermatocytes, increased apoptosis, spermatogenic arrest, and subfertility defects. The function of Aurora-C may also be targeted in the Aurora-B transgenic mutants. To address the function of Aurora-C in testis, we generated Aurora-C knockout mice by homologous recombination. Remarkably, Aurora-C null mice were viable, yet the males had compromised fertility. Aurora-C mutant sperm display abnormalities that included heterogenous chromatin condensation, loose acrosomes, and blunted heads. These findings indicate that Aurora-B and Aurora-C serve specialized functions in mammalian spermatogenesis.     

Evolutionary relationships of Aurora kinases: Implications for model organism studies and the development of anti-cancer drugs

Background  

As key regulators of mitotic chromosome segregation, the Aurora family of serine/threonine kinases play an important role in cell division. Abnormalities in Aurora kinases have been strongly linked with cancer, which has lead to the recent development of new classes of anti-cancer drugs that specifically target the ATP-binding domain of these kinases. From an evolutionary perspective, the species distribution of the Aurora kinase family is complex. Mammals uniquely have three Aurora kinases, Aurora-A, Aurora-B, and Aurora-C, while for other metazoans, including the frog, fruitfly and nematode, only Aurora-A and Aurora-B kinases are known. The fungi have a single Aurora-like homolog. Based on the tacit assumption of orthology to human counterparts, model organism studies have been central to the functional characterization of Aurora kinases. However, the ortholog and paralog relationships of these kinases across various species have not been rigorously examined. Here, we present comprehensive evolutionary analyses of the Aurora kinase family.     

Dynamic localization and functional implications of Aurora-C kinase during male mouse meiosis

Aurora-C was first identified during screening for kinases expressed in mouse sperm and eggs. Herein, we report for the first time the precise subcellular localization of endogenous Aurora-C during male meiotic division. The localization of Aurora-C was analyzed by immunofluorescence staining on chromosome spreads of mouse spermatocytes or in squashed seminiferous tubules. Aurora-C was first detected at clusters of chromocenters in diplotene spermatocytes and was concentrated at centromeres in metaphase I and II. Interestingly, Aurora-C was also found along the chromosome axes, including both the regions of centromeres and the chromosome arms in diakinesis. During the anaphase I/telophase I and anaphase II/telophase II transitions, Aurora-C was relocalized to the spindle midzone and midbody. A similar distribution pattern was also observed for Aurora-B during male meiotic divisions. Surprisingly, we detected no Aurora-C in mitotic spermatogonia. Furthermore, immunoprecipitation analyses revealed that INCENP associated with Aurora-C in the male testis. We propose that INCENP recruits Aurora-C (or some other factor(s) recruit INCENP and Aurora-C) to meiotic chromosomes, while Aurora-C may either work alone or cooperate with Aurora-B to regulate chromosome segregation during male meiosis.     

RanBP2 and SENP3 function in a mitotic SUMO2/3 conjugation-deconjugation cycle on Borealin

The ubiquitin-like SUMO system controls cellular key functions, and several lines of evidence point to a critical role of SUMO for mitotic progression. However, in mammalian cells mitotic substrates of sumoylation and the regulatory components involved are not well defined. Here, we identify Borealin, a component of the chromosomal passenger complex (CPC), as a mitotic target of SUMO. The CPC, which additionally comprises INCENP, Survivin, and Aurora B, regulates key mitotic events, including chromosome congression, the spindle assembly checkpoint, and cytokinesis. We show that Borealin is preferentially modified by SUMO2/3 and demonstrate that the modification is dynamically regulated during mitotic progression, peaking in early mitosis. Intriguingly, the SUMO ligase RanBP2 interacts with the CPC, stimulates SUMO modification of Borealin in vitro, and is required for its modification in vivo. Moreover, the SUMO isopeptidase SENP3 is a specific interaction partner of Borealin and catalyzes the removal of SUMO2/3 from Borealin. These data thus delineate a mitotic SUMO2/3 conjugation–deconjugation cycle of Borealin and further assign a regulatory function of RanBP2 and SENP3 in the mitotic SUMO pathway.     

Survivin’s Dual Role: An Export’s View

Survivin is proposed to function as a mitotic regulator and an apoptosis inhibitor duringdevelopment and pathogenesis. As such, survivin has aroused keen interest in disparateareas of basic and translational research. Survivin acts as a subunit of the chromosomalpassenger complex (CPC), composed of the mitotic kinase Aurora-B, Borealin 5 and INCENP,and is essential for proper chromosome segregation and cytokinesis. Our recent findingsindicate that the nuclear export receptor Crm1 is critically involved in tethering the CPC to thecentromere by interacting with a leucine-rich nuclear export signal (NES), evolutionaryconserved in all mammalian survivin proteins. In addition, the survivin/Crm1 interaction10 seems to be required for the cytoprotective activity of survivin, because export deficientsurvivin fails to protect tumor cells against cancer therapy-induced apoptosis. These findingsappear of clinical relevance since preferential nuclear localization of survivin turned out to bea favorable prognostic factor in cancer patients. Besides emphasizing the functionalsignificance of the Crm1/survivin interface, we suggest to exploit the pharmacogenetic15 interference with survivin’s export as a novel strategy to antagonize survivin’s activity.     

Overexpression of active Aurora-C kinase results in cell transformation and tumour formation

Aurora kinases belong to a conserved family of serine/threonine kinases key regulators of cell cycle progression. Aurora-A and Aurora-B are expressed in somatic cells and involved mainly in mitosis while Aurora-C is expressed during spermatogenesis and oogenesis and is involved in meiosis. Aurora-C is hardly detectable in normal somatic cells. However all three kinases are overexpressed in many cancer lines. Aurora-A possesses an oncogenic activity while Aurora-B does not. Here we investigated whether Aurora-C possesses such an oncogenic activity. We report that overexpression of Aurora-C induces abnormal cell division resulting in centrosome amplification and multinucleation in both transiently transfected cells and in stable cell lines. Only stable NIH3T3 cell clones overexpressing active Aurora-C formed foci of colonies when grown on soft agar, indicating that a gain of Aurora-C activity is sufficient to transform cells. Furthermore, we reported that NIH-3T3 stable cell lines overexpressing Aurora-C induced tumour formation when injected into nude mice, demonstrating the oncogenic activity of enzymatically active Aurora kinase C. Interestingly enough tumor aggressiveness was positively correlated with the quantity of active kinase, making Aurora-C a potential anti-cancer therapeutic target.     

The chromosomal passenger complex: guiding Aurora-B through mitosis

During mitosis, the chromosomal passenger complex (CPC) orchestrates highly different processes, such as chromosome alignment, histone modification, and cytokinesis. Proper and timely localization of this complex is the key to precise control over the enzymatic core of the CPC, the Aurora-B kinase. We discuss the molecular mechanisms by which the CPC members direct the dynamic localization of the complex throughout cell division. Also, we summarize posttranslational modifications that occur on the CPC and discuss their roles in regulating localization and function of this mitotic complex.     

Overexpression of an Aurora-C kinase-deficient mutant disrupts the Aurora-B/INCENP complex and induces polyploidy

Aurora kinases are emerging as key regulators of centrosome function, chromosome segregation and cytokinesis. We previously isolated Aurora-C (Aie1), a third type of Aurora kinase, in a screen for kinases expressed in mouse sperm and eggs. Currently, we know very little about the precise localization and function of Aurora-C. Immunofluorescence analysis of ectopically expressed GFP-Aurora-C has revealed that Aurora-C is a new member of the chromosomal passenger proteins localizing first to the centromeres and then to the central spindles during cytokinesis. In order to study the potential role of Aurora-C, we examined the effects of a kinase-deficient (KD) mutant (AurC-KD) in HeLa Tet-Off cells under tetracycline control. Our results showed that overexpression of AurC-KD causes defects in cell division and induces polyploidy and apoptosis. Interestingly, AurC-KD overexpression also inhibits centromere/kinetochore localization of Aurora-B, Bub1, and BubR1, reduces histone H3 phosphorylation, and disrupts the association of INCENP with Aurora-B. Together, our results showed that Aurora-C is a chromosomal passenger protein, which may serve as a key regulator in cell division.     

Identification of V23RalA-Ser194 as a critical mediator for Aurora-A-induced cellular motility and transformation by small pool expression screening

Human Aurora kinases have three gene family members: Aurora-A, Aurora-B, and Aurora-C. It is not yet established what the specificity of these kinases are and what signals relayed by their reactions. Therefore, we employed small pool expression screening to search for downstream substrates of Aurora-A. Interestingly, all of the identified Aurora-A substrates were resistant to serve as substrates for Aurora-B or Aurora-C, suggesting that these Aurora family members may have distinct substrate specificity for propagation of diverse signaling pathways, even though they share a conserved catalytic kinase domain. Of the candidate substrates, Aurora-A could increase the functional activity of RalA. Mutational analysis revealed that RalA-Ser194 was the phosphorylation site for Aurora-A. Ectopic expression of V23RalA-WT could enhance collagen I-induced cell migration and anchorage-independent growth in Madin-Darby canine kidney (MDCK) Aurora-A stable cell lines. In contrast, overexpression of V23RalA-S194A in MDCK Aurora-A-stable cell lines abolished the intrinsic migration and transformation abilities of Aurora-A. To our knowledge, this is the first systematic search for the downstream substrates of Aurora-A kinase. Moreover, these results support the notion that Aurora-A may act in concert with V23RalA through protein phosphorylation on Ser194 to promote collagen I-induced cell motility and anchorage-independent growth in MDCK epithelial cells.     

Mitotic Phosphorylation of Histone H3: Spatio-Temporal Regulation by Mammalian Aurora Kinases

Phosphorylation at a highly conserved serine residue (Ser-10) in the histone H3 tail is considered to be a crucial event for the onset of mitosis. This modification appears early in the G(2) phase within pericentromeric heterochromatin and spreads in an ordered fashion coincident with mitotic chromosome condensation. Mutation of Ser-10 is essential in Tetrahymena, since it results in abnormal chromosome segregation and extensive chromosome loss during mitosis and meiosis, establishing a strong link between signaling and chromosome dynamics. Although mitotic H3 phosphorylation has been long recognized, the transduction routes and the identity of the protein kinases involved have been elusive. Here we show that the expression of Aurora-A and Aurora-B, two kinases of the Aurora/AIK family, is tightly coordinated with H3 phosphorylation during the G(2)/M transition. During the G(2) phase, the Aurora-A kinase is coexpressed while the Aurora-B kinase colocalizes with phosphorylated histone H3. At prophase and metaphase, Aurora-A is highly localized in the centrosomic region and in the spindle poles while Aurora-B is present in the centromeric region concurrent with H3 phosphorylation, to then translocate by cytokinesis to the midbody region. Both Aurora-A and Aurora-B proteins physically interact with the H3 tail and efficiently phosphorylate Ser10 both in vitro and in vivo, even if Aurora-A appears to be a better H3 kinase than Aurora-B. Since Aurora-A and Aurora-B are known to be overexpressed in a variety of human cancers, our findings provide an attractive link between cell transformation, chromatin modifications and a specific kinase system.     

CENP-A is phosphorylated by Aurora B kinase and plays an unexpected role in completion of cytokinesis.

Aurora B is a mitotic protein kinase that phosphorylates histone H3, behaves as a chromosomal passenger protein, and functions in cytokinesis. We investigated a role for Aurora B with respect to human centromere protein A (CENP-A), a centromeric histone H3 homologue. Aurora B concentrates at centromeres in early G2, associates with histone H3 and centromeres at the times when histone H3 and CENP-A are phosphorylated, and phosphorylates histone H3 and CENP-A in vitro at a similar target serine residue. Dominant negative phosphorylation site mutants of CENP-A result in a delay at the terminal stage of cytokinesis (cell separation). The only molecular defects detected in analysis of 22 chromosomal, spindle, and regulatory proteins were disruptions in localization of inner centromere protein (INCENP), Aurora B, and a putative partner phosphatase, PP1gamma1. Our data support a model where CENP-A phosphorylation is involved in regulating Aurora B, INCENP, and PP1gamma1 targeting within the cell. These experiments identify an unexpected role for the kinetochore in regulation of cytokinesis.     

Aurora-C interacts with and phosphorylates the transforming acidic coiled-coil 1 protein

Aurora-C, a member of the Aurora kinase family, is implicated in the regulation of mitosis. In contrast to Aurora-A and Aurora-B its cellular localization and functions are poorly characterized. TACC1 protein belongs to the transforming acidic coiled-coil family shown to interact with the Aurora kinases. In the present study we analyzed the interaction between Aurora-C and TACC1 by means of immunofluorescence (IF), co-immunoprecipitation (IP) and in vitro phosphorylation experiments. We demonstrated that Aurora-C and TACC1 proteins co-localize to the midbody of HeLa cells during cytokinesis. Immunoprecipitated TACC1 from HeLa cell extracts was associated with Aurora-C. In addition, the interaction of the two proteins was tested by analyzing the phosphorylation of TACC1 in vitro. The results demonstrated that TACC1 is phosphorylated by Aurora-C on a serine at position 228. In conclusion, the study demonstrated that TACC1 localizes at the midbody during cytokinesis and interacts with and is a substrate of Aurora-C, which warrant further investigation in order to elucidate the functional significance of this interaction.