InteractiveFly: GeneBrief

Aurora borealis: Biological Overview | Regulation | Developmental Biology | Effects of Mutation | Evolutionary Homologs | References

BIOLOGICAL OVERVIEW

The protein kinase Aurora-A is required for centrosome maturation, spindle assembly, and asymmetric protein localization during mitosis. Borealis (Bora, so named for aurora borealis to indicate its similarity with aurora-A) is a conserved protein that is required for the activation of Aurora-A at the onset of mitosis. In the Drosophila peripheral nervous system, bora mutants show defects during asymmetric cell division identical to those observed in aurora-A. Furthermore, overexpression of bora can rescue defects caused by mutations in aurora-A. Bora is conserved in vertebrates, and both Drosophila and human Bora can bind to Aurora-A and activate the kinase in vitro. In interphase cells, Bora is a nuclear protein, but upon entry into mitosis, Bora is excluded from the nucleus and translocates into the cytoplasm in a Cdc2-dependent manner. A model is presented here in which activation of Cdc2 initiates the release of Bora into the cytoplasm where it can bind and activate Aurora-A (Hutterer, 2006).

Cell division involves the coordinated execution of several distinct steps. First, chromosomes condense and the nuclear envelope breaks down. Then, the mitotic spindle forms, sister chromatids separate, and chromosomes segregate into the two daughter cells. Finally, mitosis finishes with cytokinesis, the actual division of the cell into two separate daughter cells. Mitosis involves the sequential activation of several protein kinases that are required for all or a subset of these mitotic events: while Cdc2 is a master regulator of mitosis and is required for the initiation of mitosis, kinases of the Aurora and Polo families are responsible for distinct subsets of mitotic events. How these kinases are activated and how they regulate individual mitotic events is not very well understood (Hutterer, 2006).

Aurora kinases were originally identified in Drosophila, but homologs were later found in all eukaryotic organisms. While yeast contains only a single Aurora kinase called Ipl1p, at least two families with distinct functions and subcellular localizations can be distinguished in multicellular organisms: Aurora-A is concentrated on the spindle and on centrosomes and is required for centrosome maturation and spindle assembly, while Aurora-B is localized on chromosomes and on the central spindle and is involved in chromosome condensation, kinetochore-microtubule attachment and cytokinesis. Aurora-B is part of a multimeric complex containing the so-called chromosome passenger proteins INCENP, survivin, and borealin. The individual members of that complex are codependent for their subcellular localization, and their role is to direct Aurora-B to its correct localization within the cell. Consistent with the conserved function and localization of Aurora-B, all members of the complex are conserved in evolution (in C. elegans, they are called ICP-1, BIR-1, and CSC-1. Binding partners have also been identified for Aurora-A, but in this case, their evolutionary conservation is less clear (Kufer, 2003). TPX2 is a microtubule binding protein required for spindle assembly. It can bind Aurora-A and activate the kinase via an N-terminal domain. Upon TPX2 RNAi, Aurora-A fails to localize to the spindle whereas its centrosome localization is unaffected (Kufer, 2002). Since the interaction of TPX2 with Aurora-A is stimulated by the small GTPase Ran, a model was proposed in which activated Ran is generated by condensed chromatin and locally activates Aurora-A, thereby stabilizing microtubules. Although a putative C. elegans TPX2 homolog was identified, the homology does not extend over the whole protein and no homologs are present in other invertebrates, including Drosophila. Another Aurora-A binding partner is the LIM domain protein Ajuba. Like TPX2, Ajuba can activate Aurora-A, but again, no homologs have been identified in invertebrates (Hutterer, 2006 and references therein).

Besides its role in centrosome maturation and spindle assembly, Aurora-A has a special function during asymmetric cell division (Berdnik, 2002). To divide asymmetrically, some cells are capable of segregating cell fate determinants into one of their two daughter cells (Betschinger, 2004). Asymmetric cell divisions are particularly well understood in Drosophila external sensory (ES) organs where they contribute to the formation of four different cell types from a single sensory organ precursor (SOP) cell. The SOP cell divides into a pIIa and a pIIb cell. Later, pIIa gives rise to the two outer cells, while pIIb generates the two inner cells of the organ. During each division, the cell fate determinant Numb localizes asymmetrically and segregates into one of the two daughter cells where it regulates cell fate by repressing Notch signaling. In numb mutants, Notch is not repressed and abnormal ES organs with too many outer and no inner cells are formed. A similar phenotype is observed in aurora-A mutants. In these mutants, Numb does not localize asymmetrically and is not segregated into one of the two daughter cells (Berdnik, 2002). Since asymmetric Numb localization requires actin, but not microtubules, this phenotype is not an indirect consequence of the centrosome maturation and spindle assembly defects that are also observed in aurora-A. Thus, besides its role in regulating microtubules, Aurora-A also regulates actin-dependent mitotic processes (Hutterer, 2006 and references therein).

Despite its functional conservation, a conserved pathway for the activation of Aurora-A is not known. This study describes the identification of Bora, an interaction partner of Aurora-A that is conserved from C. elegans to humans. Bora was identified due to its phenotypic similarity to aurora-A and it has been shown that bora overexpression can partially rescue aurora-A mutants. Bora binds to Aurora-A and can activate the kinase in vitro. Bora is a nuclear protein that translocates into the cytoplasm upon activation of Cdc2, suggesting that its subcellular localization might contribute to the regulation of Aurora-A. These results describe a regulator of Aurora-A that is conserved from Drosophila to humans and suggest a potential mechanism for the sequential activation of Cdc2 and Aurora-A (Hutterer, 2006).

Thus the conserved Aurora-A binding partner Bora is essential for Aurora-A to perform centrosome maturation, spindle assembly, and asymmetric protein localization during mitosis in Drosophila. Bora can activate Aurora-A in vitro. Bora is a nuclear protein that is excluded from the nucleus during prophase in a Cdc2-dependent manner. Nuclear retention of Bora might help to keep Aurora-A inactive during interphase. When Cdc2 becomes activated, Bora is released into the cytoplasm where it can bind and activate Aurora-A. This hypothesis could provide a molecular explanation for previous results which have demonstrated that Cdc2 is crucial for the activation of Aurora-A (Marumoto, 2003; Maton, 2003). Since Bora is a substrate for Cdc2 in vitro and—at least in vertebrates—a fraction of Cdc2 has been reported to be nuclear, it is conceivable that direct phosphorylation of Bora might facilitate its exclusion from the nucleus. However, nuclear release of Bora is not the only mechanism by which its activation of Aurora-A is regulated since the bora mutant phenotype can also be rescued by Bora fused to a myristylation signal, which keeps the protein in the cytoplasm, or fused to a nuclear localization signal, which retains the protein in the nucleus until nuclear envelope breakdown (Hutterer, 2006).

Although in Drosophila, Bora so far is the only known activator of Aurora-A, several in vitro activators of Aurora-A have been identified in other organisms. In vertebrates, TPX2 prevents PP1-dependent dephosphorylation and thereby locks the kinase in its active conformation (Bayliss, 2003). The activation of Aurora-A by Cdc2 is PP1 independent (Maton, 2005), and, therefore, TPX2 is unlikely to participate in this particular event. Furthermore, TPX2 is only required for a subset of Aurora-A-dependent processes: TPX2 inactivation by RNAi causes spindle defects and loss of Aurora-A from the mitotic spindle, but centrosome maturation is normal, and the centrosome pool of the kinase is unaffected. TPX2/Aurora-A binding is stimulated by the small GTPase Ran, which in turn is activated by RCC1, an exchange factor that is located on condensed chromatin and is involved in microtubule nucleation and spindle formation. Thus, unlike Bora, TPX2 seems to be specifically responsible for the spindle assembly function of Aurora-A. So far, no TPX2 homolog has been identified in Drosophila. Whether this is due to a low level of sequence similarity that escapes standard homology searches or whether it reflects a fundamental difference in Aurora-A function between organisms is currently unclear (Hutterer, 2006).

One protein that might be generally required for Aurora-A activation is Ajuba (Hirota, 2003). Upon Ajuba RNAi, Aurora-A fails to be activated. In HeLa cells, this leads to a cell-cycle block in G2 and prevents entry into mitosis. However, since ajuba null mutant mice are completely viable (Pratt, 2005) and keratinocytes from these mice have no cell-cycle block, the significance of these RNAi experiments is unclear. Furthermore, no Ajuba homologs are found in C. elegans or Drosophila, suggesting that a functional connection between Ajuba and Bora is unlikely (Hutterer, 2006).

More recently, two other activation pathways for Aurora-A have been described. The focal adhesion protein HEF1 binds to Aurora-A and is required and sufficient for Aurora-A activation (Pugacheva, 2005). The protein kinase PAK relocalizes to centrosomes during mitosis where it is activated and in turn phosphorylates and activates Aurora-A (Zhao, 2005). Since PAK is a part of focal adhesion complexes, both pathways might be part of a mechanism establishing crosstalk between cell adhesion and the mitotic apparatus (Cotteret, 2005). However, PAK inhibition only delays centrosome maturation, suggesting that this pathway is not a crucial regulator of the G2/M functions of Aurora-A. In Drosophila, both PAK and HEF1 are conserved, but the PAK mutant phenotype does not suggest any requirement of the kinase for mitosis. Taken together, these observations suggest that Bora does not participate in any of the known pathways but is more globally involved in the activation of Aurora-A (Hutterer, 2006).

Like Aurora-A, Bora is required for actin-dependent asymmetric protein localization during mitosis (Berdnik, 2002). It is thought that the polarized localization of the kinase aPKC leads to asymmetric phosphorylation of the cytoskeletal protein Lgl. Since phosphorylation inactivates Lgl and Lgl is essential for establishing a cortical binding site for cell fate determinants, those determinants accumulate exclusively on the side of the cortex that is free of aPKC. Aurora-A could act at several points in this pathway: either the cortical binding site could already be polarized in interphase and activation of Aurora-A could establish its affinity for cell-fate determinants, or alternatively, Aurora-A could regulate the activity of aPKC. In this case, aPKC would be asymmetric but inactive in interphase and its activation in prophase would initiate asymmetric localization of cell-fate determinants. At the moment, it is not possible to distinguish between these possibilities, but identification of the Aurora-A substrates relevant for asymmetric protein localization should clarify its mode of action. In any case, the observation that Cdc2 is essential for asymmetric determinant localization as well (Tio, 2001) is consistent with a model where Cdc2 is required for the Bora-dependent activation of Aurora-A (Hutterer, 2006).

Aurora-A has been implicated in carcinogenesis. It is overexpressed in a number of cancers and its overexpression results in polyploidy or cells containing multiple centrosomes. Aurora-A has therefore been used as a drug target for cancer therapy, and the identification of Bora offers an alternative route for the discovery of Aurora-A selective inhibitors. The human Bora homolog (annotated as FLJ22624) is located on chromosome 13 in a region that contains a breast cancer susceptibility gene and has been implicated in a variety of malignant tumors. Future studies will reveal whether it is involved in carcinogenesis as well (Hutterer, 2006).

REGULATION

Protein Interactions

To test whether the genetic interaction reflects a physical interaction between Bora and Aurora-A, binding assays were performed in Drosophila tissue culture cells. Drosophila S2 cells were transfected with Aurora-A and Bora-GFP, and protein lysates were subjected to immunoprecipitation by anti-GFP. Since Aurora-A is specifically detected in the immunoprecipitate, it is concluded that Bora can bind to Aurora-A in vivo. To test whether this is due to a direct interaction, in vitro binding experiments were performed. In vitro translated Aurora-A binds to a GST-Bora fusion-protein but not to GST alone. While the nonconserved C terminus of Bora is dispensible for Aurora-A binding, the interaction is abrogated by deleting the conserved region (BoraΔ2) or a region N-terminal to the conserved part (BoraΔ1). Interestingly, the interaction is also observed between in vitro translated human Aurora-A and MBP-HsBora. Human Aurora-A can even bind to Drosophila MBP-Bora in vitro. The interaction with Aurora-A seems to be essential for Bora function since the N-terminal 404 amino acids of Bora (almost identical to BoraΔ3) can rescue the bora and aurA³⁷ mutant phenotypes, while the C terminus (amino acids 404–539) does not. Thus, Bora and its homologs act as binding partners of Aurora-A (Hutterer, 2006).

Several Aurora-A regulators—like TPX2 also act as substrates for the kinase. To test whether Bora can be phosphorylated by Aurora-A, in vitro kinase assays were performed. Drosophila Aurora-A expressed and purified from E. coli can phosphorylate bacterially expressed myelin basic protein tagged Bora (MBP-Bora) but not MBP alone. Interestingly, the kinase activity of Aurora-A toward Bora is as potent as toward myelin basic protein, which is often used as a model substrate. Similarly, human Aurora-A can phosphorylate the human Bora homolog. To test which region of Bora is phosphorylated, Bora deletions were used in the kinase assay. Deletion of 125 amino acids from the N terminus of Bora (BoraΔ2) eliminates phosphorylation by Aurora-A, while deletion of the C terminus from amino acid 209 onward (BoraΔ5) does not affect it. Interestingly, Bora is still phosphorylated when the N-terminal 67 amino acids are deleted (BoraΔ1), suggesting that direct binding to Aurora-A is not necessary for Bora to act as a substrate. These experiments suggest that the N terminus of Bora is phosphorylated by Aurora-A (Hutterer, 2006).

To test whether Bora can influence the kinase activity of Aurora-A, recombinant human Bora was used in an in vitro kinase assay with myelin basic protein as a substrate. Addition of Bora increases Aurora-A activity in a dose-dependent manner, and a 2.5-fold maximum increase in kinase activity was observed. Aurora-A is regulated by phosphorylation in the activation loop of the kinase. Since Aurora-A can autophosphorylate, any kinase preparation may be partially active, and this might explain the modest degree of activation by recombinant Bora. Consistent with this, when Aurora-A is inactivated by pretreatment with protein phosphatase 1 (PP1), addition of Bora induces an over 7-fold increase in kinase activity. Analogous experiments with the Drosophila homologs reveal that Drosophila Bora similarly activates the Drosophila kinase, showing that it acts as a kinase activator as well. Taken together, these results demonstrate that Bora is an activator of Aurora-A (Hutterer, 2006).

Mutation of the autophosphorylation site of Aurora-A to alanine renders the kinase inactive, and an interesting question is whether the stimulation of Aurora-A by Bora bypasses the need for autophosphorylation. It was found that addition of Bora does not restore activity to the mutant kinase, suggesting that activation by Bora requires autophosphorylation of Aurora-A (Hutterer, 2006).

To determine the subcellular localization of Bora in SOP cells, live imaging was performed of a Bora-GFP fusion protein, which can rescue both bora and aurA³⁷ mutant phenotypes. Histone-RFP is used to label chromosomes and indicates the cell-cycle stage. Constructs were specifically expressed by neuralized-Gal4 in SOP cells and dividing cells were imaged in whole living pupae. In interphase, Bora is a nuclear protein. When chromosomes condense, however, Bora is released from the nucleus. It is completely excluded from the nucleus by late prophase and is uniformly distributed in the cytoplasm after nuclear envelope breakdown. In telophase, Bora enters both daughter cells where it relocates into the nucleus. Bora does not have an obvious nuclear localization signal. However, it was found that the first 125 amino acids of the protein are sufficient for nuclear retention, suggesting that they contain the sequence that mediates nuclear import. Live imaging of GFP-Aurora-A together with Histone-RFP allows correlation of the localization of Aurora-A with Bora. In interphase, the two proteins are in distinct compartments. Nuclear release of Bora coincides with centrosome separation and strong recruitment of Aurora-A to the maturing centrosomes. Since both centrosome separation and maturation defects are observed in aurora-A mutants, these results suggest that release of Bora coincides with Aurora-A activation (Hutterer, 2006).

While Aurora-A is required for a subset of mitotic events, Cdc2 is essential for all steps of mitosis. How Cdc2 activates Aurora-A is unclear. To test whether Cdc2 regulates the release of Bora into the cytoplasm, Bora localization was examined in string mutants. String is the Drosophila homolog of the Cdc25 phosphatase, and in string mutants, Cdc2 is not activated. Antibody staining of Drosophila embryos reveals that endogenous Bora shows the same dynamic localization during the cell cycle as the functional GFP fusion protein. In string mutant embryos, however, Bora was never observed in the cytoplasm, indicating that Cdc2 activation is required for the release of Bora from the nucleus. To test whether Cdc2 might directly phosphorylate Bora, in vitro kinase assays were performed. Both Bora and HsBora are phosphorylated by recombinant Cdk1. Although the in vivo relevance of Cdk1 phosphorylation remains to be tested, these experiments show that Bora is released into the cytoplasm at the onset of mitosis in a Cdc2-dependent manner (Hutterer, 2006).

EFFECTS OF MUTATION

In a genetic screen for mutations affecting the development of Drosophila external sensory (ES) organs, mutations were identified in aurora-A (Berdnik, 2002). In these mutants, Numb fails to localize asymmetrically and the proteins γ-Tubulin and Centrosomin are not recruited to centrosomes during mitosis, leading to spindle abnormalities. Two other mutations from the same screen caused similar phenotypes but are not allelic to aurora-A. Both alleles affect the same gene, which has been named bora (for aurora borealis) to indicate its similarity with aurora-A. Flies that are homozygous for bora on the head and eye were generated by the ey-Flp/FRT system. These flies frequently show duplicated hairs and sockets, a phenotype indicative of defects in asymmetric cell division. To determine whether this morphological defect results from cell-fate transformations, the SOP cell progeny were analyzed by using different molecular markers. The socket cell expresses the transcription factor Suppressor of Hairless (Su(H)), whereas the sheath cell can be recognized by expression of Prospero. All four cells express the transcription factor Cut, and the hair cell can be distinguished from the neuron based on its larger size. In bora mutant ES organs, four equally sized Cut-positive cells are found, two of which express Su(H), while no Prospero-positive cell can be detected. Thus in bora mutants, inner cells are transformed into additional outer cells, which is a phenotype characteristic of a defect in Numb localization (Berdnik, 2002; Bhalerao, 2005). Indeed, whereas in wild-type SOP cells Numb localizes asymmetrically into a crescent in mitosis and segregates into one of the two daughter cells, in bora mutant SOP cells, the protein is uniformly cortical in metaphase and equally distributed into both daughter cells. Defects in asymmetric localization (although at lower frequency) are also observed for the Numb binding partner Pon (Partner of Numb), but localization of Gαi and Pins is normal. Gαi and Pins are required for Numb localization and can act as markers for the polarization of SOP cells, which already occurs in interphase. Thus, bora is required for the asymmetric localization of cell fate determinants during mitosis but is not essential for polarization of SOP cells in general (Hutterer, 2006).

To further explore the phenotypic similarity with aurora-A, centrosome maturation was analyzed in bora mutants. In wild-type SOP cells, several proteins including γ-Tubulin and Centrosomin are recruited to centrosomes during mitosis. In bora mutant SOP cells, however, Centrosomin recruitment is either weak or not detected at all. Frequently, only one or two closely spaced Centrosomin dots were detected, indicating defects in centrosome separation. Thus, bora mutants recapitulate all aspects of the aurora-A mutant phenotype in SOP cells (Hutterer, 2006).

To test whether Aurora-A is active in bora mutants, phosphospecific antibodies were used against D-TACC, a substrate of Aurora-A. In wild-type cells, phosphorylated D-TACC is found at centrosomes and on the mitotic spindle. In both aurA³⁷ and bora mutants, however, P-D-TACC staining is significantly reduced and not enriched on any intracellular structures. These results suggest that Bora is required for the activation of Aurora-A during mitosis (Hutterer, 2006).

To determine which gene is affected in bora mutants, the mutation was narrowed down to the cytological interval 75B-C by P-element and deficiency mapping. Single-nucleotide polymorphism (SNP) mapping was used for further refinement and sequencing of candidate genes in the respective region. This revealed that both mutants carry lesions in a transcript that has been annotated as CG6897 by the Drosophila sequencing consortium. bora¹⁵ is a 14 base-pair out-of-frame deletion in the coding region, which introduces a stop codon after amino acid 162, while bora¹⁸ is a G-to-A transition that affects a splice-acceptor site. Both alleles are lethal during pupal stages when homozygous, transheterozygous, or hemizygous over Df(3L)Cat, suggesting that they are either null or strong hypomorphic alleles. Flies carrying large bora¹⁵ or bora¹⁸ mutant clones frequently show duplication of hairs and sockets. These defects can be rescued by expression of a Bora-GFP fusion-protein under the control of scabrous-Gal4, indicating that CG6897 is indeed responsible for the bora mutant phenotype (Hutterer, 2006).

The phenotypic similarity suggests a close connection between Bora and Aurora-A. To test whether bora and aurora-A interact genetically, rescue experiments were performed with the hypomorphic aurora-A allele aurA³⁷ (Berdnik, 2002). Overexpression of Bora-GFP with scabrous-Gal4 does not cause a phenotype by itself but can rescue the bristle duplications, which are observed in aurA³⁷ mutants. Antibody staining reveals that both the defects in Numb localization and the centrosome defects are rescued by Bora-GFP. While Numb is mislocalized and centrosome maturation is impaired in all SOP cells of aurA³⁷ mutant flies, asymmetric Numb localization is rescued to 77% in metaphase SOP cells and centrosome maturation to 35% upon overexpression of Bora-GFP. In contrast to aurA³⁷ clones, eyFlp/FRT clones of aurora-A null mutants die early after clone induction. Overexpression of Bora-GFP cannot inhibit this cell lethal effect suggesting that Bora can increase the activity of Aurora-A but not compensate for the complete loss of kinase activity. Taken together, these results suggest that Bora is a rate-limiting regulator of Aurora-A activity (Hutterer, 2006).

REFERENCES

Search PubMed for articles about Drosophila Aurora borealis

Bayliss, R., Sardon, T., Vernos, I. and Conti, E. (2003). Structural basis of Aurora-A activation by TPX2 at the mitotic spindle. Mol. Cell 12(4): 851-62. 14580337

Berdnik, D. and Knoblich, J. (2002). Drosophila Aurora-A is required for centrosome maturation and actin-dependent asymmetric protein localization during mitosis. Curr. Biol. 12: 640-647. 11967150

Bhalerao, S., Berdnik, D., Torok, T. and Knoblich, J. A. (2005). Localization-dependent and -independent roles of numb contribute to cell-fate specification in Drosophila. Curr Biol. 15(17): 1583-90. 16139215

Betschinger, J. and Knoblich, J. A. (2004). Dare to be different: asymmetric cell division in Drosophila, C. elegans and vertebrates. Curr. Biol. 14: R674-R685. 15324689

Cotteret, S. and Chernoff, J. (2005). Pak GITs to Aurora-A. Dev. Cell 9: 573-574. Medline abstract: 16256730

Hirota, T., et al. (2003). Aurora-A and an interacting activator, the LIM protein Ajuba, are required for mitotic commitment in human cells. Cell 114: 585-598. 13678582

Hutterer, A., Berdnik, D., Wirtz-Peitz, F., Zigman, M., Schleiffer, A., Knoblich, J. A. (2006). Mitotic activation of the kinase Aurora-A requires its binding partner Bora. Dev. Cell 11(2): 147-57. 16890155

Kufer, T. A., et al. (2002). Human TPX2 is required for targeting Aurora-A kinase to the spindle. J. Cell Biol. 158(4): 617-23. 12177045

Kufer, T., Nigg, E. and Sillje, H. (2003), Regulation of Aurora-A kinase on the mitotic spindle. Chromosoma 112: 159-163. 14634755

Marumoto, T., et al. (2003). Aurora-A kinase maintains the fidelity of early and late mitotic events in HeLa cells. J. Biol. Chem. 278: 51786-51795. 14523000

Maton, G., et al. (2003). Cdc2-cyclin B triggers H3 kinase activation of Aurora-A in Xenopus oocytes. J. Biol. Chem. 278: 21439-21449. 12670933

Maton, G., et al. (2005). Differential regulation of Cdc2 and Aurora-A in Xenopus oocytes: a crucial role of phosphatase 2A, J. Cell Sci. 118: 2485-2494. 15923661

Pratt, S., et al. (2005). The LIM protein Ajuba influences p130Cas localization and Rac1 activity during cell migration. J. Cell Biol. 168: 813-824. 15728191

Pugacheva, E. and Golemis, E. (2005). The focal adhesion scaffolding protein HEF1 regulates activation of the Aurora-A and Nek2 kinases at the centrosome. Nat. Cell Biol. 7: 937-946. 16184168

Tio, M., et al. (2001). cdc2 links the Drosophila cell cycle and asymmetric division machineries. Nature 409: 1063-1067. 11234018

Zhao, Z., et al. (2005). The GIT-associated kinase PAK targets to the centrosome and regulates Aurora-A. Mol. Cell 237-249. 16246726

date revised: 22 February 2007

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