Inner centromere protein


Cyclin B and the release of AuroraB and INCENP from kinetochores

Successful mitosis requires that anaphase chromosomes sustain a commitment to move to their assigned spindle poles. This requires stable spindle attachment of anaphase kinetochores. Prior to anaphase, stable spindle attachment depends on tension created by opposing forces on sister kinetochores. Because tension is lost when kinetochores disjoin, stable attachment in anaphase must have a different basis. After expression of nondegradable cyclin B (CYC-BS) in Drosophila embryos, sister chromosomes disjoined normally but their anaphase behavior is abnormal. Chromosomes exhibit cycles of reorientation from one pole to the other. Additionally, the unpaired kinetochores accumulate attachments to both poles (merotelic attachments), congress (again) to a pseudometaphase plate, and reacquire associations with checkpoint proteins more characteristic of prometaphase kinetochores. Unpaired prometaphase kinetochores, which occur in a mutant entering mitosis with unreplicated (unpaired) chromosomes, behave just like the anaphase kinetochores at the CYC-BS arrest. Finally, the normal anaphase release of AuroraB/INCENP from kinetochores is blocked by CYC-BS expression and, reciprocally, is advanced in a CycB mutant. Given its established role in destabilizing kinetochore-microtubule interactions, Aurora B dissociation is likely to be key to the change in kinetochore behavior. These findings show that, in addition to loss of sister chromosome cohesion, successful anaphase requires a kinetochore behavioral transition triggered by CYC-B destruction (Parry, 2003).

Stable cyclins have been shown to block mitotic exit in numerous systems, and detailed analyses of the cytological consequence of stabilization of each of the cognate mitotic cyclins of Drosophila have begun to reveal regulatory features that were not evident in other experimental systems. A group of chromosomal 'passenger proteins' that are localized between paired kinetochores at metaphase usually relocalizes to the central spindle upon onset of anaphase. This relocalization is blocked upon expression of stable sea urchin cyclin B in mammalian cells. In agreement with this, expression of Drosophila CYC-BS in Drosophila embryos blocks relocalization of two interacting passenger proteins, INCENP and Aurora B. Normal metaphase foci of INCENP split in two at anaphase, half segregating with each sister kinetochore without relocalization to the spindle. Failure to release kinetochore-localized AuroraB/INCENP and a slowing of anaphase A chromosome movements are the earliest perturbations of mitotic progression observed upon CYC-BS expression. The onset of these defects immediately follows or overlaps the time of destruction of normal CYC-B (Parry, 2003).

Embryos expressing a different stabilized mitotic cyclin, CYC-B3S, arrest with chromosomes at the spindle poles after normal anaphase movements and normal redistribution of AuroraB/INCENP from the kinetochore to the spindle midzone. Thus, CYC-BS and not CYC-B3S maintains kinetochore localization of AuroraB/INCENP (Parry, 2003).

As a result of partial redundancy among Drosophila cyclins, CycB null mutants undergo mitosis. As in wild-type, AuroraB/INCENP is associated with kinetochores in metaphase cells lacking CYC-B; however, its anaphase relocalization occurs prematurely. Thus, the endogenous CYC-B in the wild-type inhibits AuroraB/INCENP relocalization, and relocalization appears to await its destruction. Together, precocious relocalization in the CycB mutant, coincidence in the onset of relocalization and the time of CYC-B destruction, and the block to relocalization by persistent CYC-B lead to the conclusion that CYC-B destruction times AuroraB/INCENP relocalization (Parry, 2003).

The dramatic transition in kinetochore-protein interactions upon destruction of CYC-B might serve only to release the sequestered passenger proteins to play their important function at the spindle midzone in cytokinesis. However, elegant studies of Ipl1, the Aurora B kinase homolog of yeast, suggest that Ipl1 can destabilize kinetochore interactions with the spindle. These studies, as well as supporting work in vertebrate cells, suggest that loss of Aurora B function upon CYC-B destruction might alter kinetochore behavior. Indeed, the current results suggest that CYC-B destruction does have an important influence on anaphase chromosome behavior (Parry, 2003 and references therein).

When Drosophila cells enter anaphase in the presence of CYC-BS, poleward movement of unpaired chromosomes is abortive and chromosome behavior is unusual. It has been suggested that this chromosome behavior might represent an extension of prometaphase/metaphase behavior, differing only in so far as the loss of kinetochore pairing at metaphase/anaphase alters the behavior. The behavior of unpaired prometaphase kinetochores has been examined in a mutant in maize, exhibiting premature loss of chromosome pairing and after microsurgical production of single kinetochore chromosomes in mammalian cells. In these experiments, single-kinetochore chromosomes behaved much as the chromosomes of Drosophila cells that progress to anaphase (to produce unpaired kinetochores) in the presence of CYC-BS (Parry, 2003 and references therein).

To further test this parallel, the Drosophila mutant, double parked was examined, in which unpaired chromosomes exist in prometaphase. Double Parked is an essential replication protein that is also required for a checkpoint function that ordinarily prevents cells from entering mitosis with unreplicated DNA, and like analogous mutants in S. cerevisiae (e.g., cdc6), Drosophila cells lacking Double Parked enter mitosis with unreplicated DNA. When a maternal supply of Double Parked is depleted, replication fails in double parked embryos and cells accumulate in mitosis. The mitotic arrest occurs because unpaired chromosomes are incapable of normal bipolar alignment and consequently induce the spindle checkpoint (Parry, 2003).

In fixed images of the double parked arrest, most chromosomes are scattered along the spindle, with some clustered in a central pseudometaphase plate, just as in CYC-BS-arrested cells. Real-time analysis shows that this is a dynamic situation, with chromosomes making oscillatory movements between the poles. This chromosome movement between the poles resembles that observed during the CYC-BS block and is consistent with reorientation of the kinetochore from one pole to the other, as occurs for prometaphase chromosomes (Parry, 2003).

Despite the absence of prior replication, INCENP and Aurora B localize to the unpaired kinetochores in the double parked arrest, as in the CYC-BS arrest. Furthermore, despite the presence of only a single kinetochore, many of the chromosomes congress to a pseudometaphase plate in double parked and CYC-BS arrests. It is concluded that, when CYC-B persists, unpaired chromosomes behave similarly before and after the metaphase/anaphase transition (Parry, 2003).

Although it was somewhat puzzling that some chromosomes congressed to a pseudometaphase plate in double parked embryos, a similar observation was made when single kinetochore chromosomes were present in prometaphase in mammals. These congressed single kinetochore chromosomes have attachments to both poles (merotelic attachment). Robust kinetochore fibers are observed in double parked spindles, and in cases that are not confounded by the clustering of chromosomes in the middle, it is apparent that kinetochore fibers from both poles impinge on single kinetochores. These observations are interpreted as an indication of frequent merotelic attachment in the double parked arrest; similar findings have been noted in the CYC-BS-arrested cells (Parry, 2003).

The finding that merotelic attachments accumulate in the double parked arrest suggests that kinetochore pairing normally helps to prevent merotelic attachments under prometaphase conditions. It is suggested that such an effect could be explained by an extension of the idea that trial and error processes contribute to bipolar attachment of paired kinetochores in prometaphase. Because kinetochore-spindle interactions are unstable in prometaphase, all modes of attachment can be sampled, at least transiently, but the most stable mode ultimately predominates. Consequently, the most stable (correct bipolar attachment) precludes less stable and incorrect attachments. Spindle tension stabilizes attachment, and it has been suggested that, upon bipolar arrangement, tension deforms the paired kinetochore, effectively 'pulling' the attachment sites away from a centrally localized destabilizing activity. Although tension also deforms a merotelically attached kinetochore, it is suggested that the distortion is not as orderly as in bipolar attachment and that the separation from the destabilizing activity is less effective. Consequently, when kinetochores are paired, bipolar attachments will accumulate as the most stable outcome and hence exclude merotelic attachments. When kinetochores are unpaired, the dynamics of formation and decay of merotelic attachments appears to favor their accumulation (Parry, 2003).

Prior to the time at which CYC-B is usually degraded, no defects are seen in mitotic progression in cells expressing CYC-BS. Sister chromatids separate from one another, and other substrates of the APC/C are degraded. The dissociation of BubR1 from kinetochores marks the release of checkpoint control. CYC-BS-expressing cells having an anaphase configuration (prior to final arrest) have a greatly decreased level of kinetochore staining. However, at the final arrest point, BubR1 again localizes to the kinetochores. BubR1 staining does not completely disappear during anaphase, and levels at final arrest do not match the highest levels at prometaphase. Nevertheless, since a return of BubR1 to the kinetochore after sister chromatid separation is never observed in wild-type cells, there appears to be some reactivation of the checkpoint at the CYC-BS arrest (Parry, 2003).

In conclusion, these results show that a change in kinetochore composition and behavior accompanies the metaphase/anaphase transition and that a change in kinetochore behavior is essential for the unerring commitment of chromosomes to their assigned poles. Because the success of mitosis depends on this change, the transition is thought of as an integral part of the metaphase/anaphase transition. Destruction of CYC-B triggers and times the kinetochore transition at the onset of anaphase. The kinetochore transition is coordinated with the disjunction of sister chromosomes as a result of their common regulation by APC/C, which promotes the destruction of CYC-B as well as the sister cohesion regulators, securin and cyclin A. The change in kinetochore behavior can be understood as a change from dynamically exchanging tension-stabilized attachment to fixed stable attachment. The striking coupling of this change with the release of Aurora B/INCENP from the kinetochore, and the identified role of Aurora B kinase in destabilizing kinetochore spindle attachments, suggests a plausible mechanism in which the dissociation of Aurora B stabilizes spindle attachments. However, a stable derivative of the sea urchin cyclin B does not produce similar modifications of chromosome behavior in mammalian cells despite blocking the release of GFP-Aurora B from the kinetochores. Clearly, further work is required to elucidate the regulatory paths connecting kinetochore behavior with CYC-B destruction (Parry, 2003).

It was found that unpaired chromosomes developed merotelic attachments whenever AuroraB/INCENP was associated with unpaired kinetochores, whether this occured in anaphase as a result of CYC-BS expression or in prophase as a result of a failure in DNA replication (in the double parked arrest). It is suggested that kinetochore pairing influences the outcome of dynamic reassortment of kinetochore attachments. Evidently, it is important to stabilize kinetochore-spindle attachments upon disjunction of sisters; otherwise attachments reequilibrate to the most stable states available to unpaired kinetochores, including merotelic attachments (Parry, 2003).

Borealin is required for Aurora B and Incenp localization to the mitotic spindle

The chromosomal passenger complex (CPC) is a key regulator of mitosis in many organisms, including yeast and mammals. Its components co-localise at the equator of the mitotic spindle and function interdependently to control multiple mitotic events such as assembly and stability of bipolar spindles, and faithful chromosome segregation into daughter cells. This study reports the first detailed characterisation of a CPC mutation in Drosophila, using a loss-of-function allele of borealin (borr). Like its mammalian counterpart, Borr colocalises with the CPC components Aurora B kinase and Incenp in mitotic Drosophila cells, and is required for their localisation to the mitotic spindle. borr mutant cells show multiple mitotic defects that are consistent with loss of CPC function. These include a drastic reduction of histone H3 phosphorylation at serine 10 (a target of Aurora B kinase), and a pronounced attenuation at prometaphase and multipolar spindles. The evidence suggests that borr mutant cells undergo multiple consecutive abnormal mitoses, producing large cells with giant nuclei and high ploidy that eventually apoptose. The delayed apoptosis of borr mutant cells in the developing wing disc appears to cause non-autonomous repair responses in the neighbouring wild-type epithelium. These responses involve Wingless signalling, which ultimately perturbs the tissue architecture of adult flies. Unexpectedly, during late larval development, cells survive loss of borr and develop giant bristles that may reflect their high degree of ploidy (Hanson, 2005).

One crucial role of the CPC during mitosis is to mediate the H3 phosphorylation of serine 10 (P-H3) by Aurora B, as has been demonstrated in budding yeast, C. elegans and Drosophila. The numbers of P-H3-positive (dividing) cells are reduced in the VNC of borr mutant embryos. Furthermore, the P-H3 levels of individual borr mitotic nuclei are typically reduced compared with those of wild-type nuclei. Often, they exhibit blotchy P-H3 staining rather than the more 'structured' staining outlining condensed chromosomes as observed in the wild type. A similar loss of P-H3 staining has also been observed in borr RNAi-depleted Kc167 cells. This reduction of the P-H3 levels in borr mutant cells is consistent with a loss of Aurora B kinase activity and, thus, with a disruption of CPC function (Hanson, 2005).

Despite the strong reduction of the P-H3 levels in mitotic VNC cells of borr mutant embryos, these cells display only a slight undercondensation of their chromatin, although the degree of undercondensation is somewhat variable from cell to cell. These results suggest that borr may not be essential for chromatin condensation (Hanson, 2005).

To examine the effects of borr loss on actively dividing epithelial cells, FRT-FLP-mediated recombination was used to generate borr mutant clones in imaginal discs whose cells undergo cell divisions throughout larval development. If borr mutant clones are induced during early larval stages and examined in fully grown larval discs, these clones are rare and are much smaller than the corresponding wild-type twin spots, suggesting that a large fraction of the mutant cells die. Hoechst staining revealed that many of the surviving borr mutant cells are large, with giant but well-formed nuclei that appear healthy, and well integrated into the epithelial tissue (Hanson, 2005).

Imaginal discs bearing borr mutant clones were stained with antibodies against Incenp and Aurora B, to assess the effect of borr loss on these CPC components during mitosis. Wild-type cells in metaphase show characteristic well-ordered mitotic spindles, with distinct staining of Aurora B and Incenp at specific sites along condensed chromatin. By contrast, borr mutant cells invariably show abnormal mitotic spindles, including multipolar ones. Most of these mutant spindles do not show any chromatin-associated Incenp or Aurora B staining, although occasionally patches of Incenp staining can still be observed, but they do not seem to be associated with any of the spindle components. These staining patterns suggest that these CPC components fail to localise properly to mitotic spindles in the absence of borr (and their levels may also be reduced, though the low frequency of surviving borr mutant cells does not allow assessment of this quantitatively). Therefore, as in mammalian cells, the correct localisation of Incenp and Aurora B to mitotic spindles of dividing imaginal disc cells depends on Borr. This is further evidence that Borr is a CPC protein, and that it interacts functionally with other known CPC components (Hanson, 2005).

The NSL complex regulates housekeeping genes in Drosophila

MOF is the major histone H4 lysine 16-specific (H4K16) acetyltransferase in mammals and Drosophila. In flies, it is involved in the regulation of X-chromosomal and autosomal genes as part of the MSL and the NSL complexes, respectively. While the function of the MSL complex as a dosage compensation regulator is fairly well understood, the role of the Non-Specific Lethal (NSL) complex (Raja, 2010) in gene regulation is still poorly characterized. This study reports a comprehensive ChIP-seq analysis of four NSL complex members (NSL1, NSL3, MBD-R2, and MCRS2) throughout the Drosophila melanogaster genome. Strikingly, the majority (85.5%) of NSL-bound genes (a sample of which include Bap170, CG6506, sec5, CG15011, Ent2, Incenp, tho2 and Patj) are constitutively expressed across different cell types. An increased abundance of the histone modifications H4K16ac, H3K4me2, H3K4me3, and H3K9ac in gene promoter regions was found to be characteristic of NSL-targeted genes. Furthermore, these genes have a well-defined nucleosome free region and broad transcription initiation patterns. Finally, by performing ChIP-seq analyses of RNA polymerase II (Pol II) in NSL1- and NSL3-depleted cells, it was demonstrated that both NSL proteins are required for efficient recruitment of Pol II to NSL target gene promoters. The observed Pol II reduction coincides with compromised binding of TBP and TFIIB to target promoters, indicating that the NSL complex is required for optimal recruitment of the pre-initiation complex on target genes. Moreover, genes that undergo the most dramatic loss of Pol II upon NSL knockdowns tend to be enriched in DNA Replication-related Element (DRE). Taken together, these findings show that the MOF-containing NSL complex acts as a major regulator of housekeeping genes in flies by modulating initiation of Pol II transcription (Lam, 2012).

This study has revealed that the majority of the NSL-complex-bound targets are housekeeping genes in Drosophila. While chromatin-modifying complexes that regulate tissue-specific genes, such as SAGA, polycomb and trithorax complexes, have been studied extensively, global regulators of housekeeping genes are poorly understood. The NSL complex is the first identified major regulator of housekeeping genes (Feller, 2012; Lam, 2012).

The promoters of NSL target genes exhibit prominent enrichment of certain histone modifications (H4K16ac, H3K9ac, H3K4me2, H3K4me3) as well as specific core promoter elements (such as DRE, E-box and motif 1). Furthermore, these genes display distinct nucleosome occupancy and dispersed promoter configuration characterized by multiple transcription start sites. The correlation between these promoter characteristics (well-defined chromatin marks, TATA-less DNA sequences and broad initiation patterns) was previously identified for housekeeping genes in mammals and flies, but how these promoter features are translated into gene transcription had remained elusive. This study now conclusively demonstrates that the NSL complex modulates transcription at the level of transcription initiation by facilitating pre-initiation complex loading onto promoters. Therefore, it is proposed that the NSL complex is a key trans-acting factor that bridges the promoter architecture, defined by the DNA sequence, histone marks and higher chromatin structures with transcription regulation of constitutive genes in Drosophila (see Summary model: NSL-dependent Pol II recruitment to promoters of housekeeping genes) (Lam, 2012).

Excitingly, the enrichment of DNA motifs on NSL target gene promoters in combination with the genome-wide Pol II binding data has established functional links between the motifs enriched on housekeeping genes and the NSL-dependent Pol II binding to promoters. The abundance of DRE motifs, for example, was found to be positively associated with the magnitude of Pol II loss upon NSL knockdowns. The DRE binding factor (DREF) interacts tightly with TRF2 to modulate the transcription of DRE-containing promoters in a TATA-box-independent fashion (Hochheimer, 2002). It is tempting to speculate that the NSL complex might also cooperate with the TRF2 complex to facilitate transcription in a specific manner, rendering DRE-containing promoters more sensitive to NSL depletions. As the NSL-bound promoters are associated with a large variety of transcription factors, it will be of great interest to study whether the NSL complex communicates with different transcription regulators, perhaps making use of distinct mechanisms (Lam, 2012).

In contrast to DRE, motif 1 showed an opposing effect on Pol II recruitment to NSL-complex-bound genes as the presence of strong motif 1 sequences was associated with decreased Pol II loss upon NSL depletion. The mechanistic reasons for this remain unclear. However, one can envisage several possible scenarios. It is possible that motif 1 may recruit another transcription factor, which can also function to recruit the transcription machinery. Alternatively, the turnover of the transcription machinery might be slower on promoters containing strong motif 1 sequences. There is precedent for the transcription machinery having various turnover rates on different promoters. For example, in yeast, it has been shown that TBP turnover is faster on TATA-containing than on TATA-less promoters. It is therefore possible that certain levels of the initiation complexes may still be maintained on motif-1-containing promoters, even though the recruitment of the transcription machinery will be compromised in the absence of NSL complex. Further work is required to understand the importance of sequence determinants for NSL complex recruitment and the analysis sets the grounds for targeted experiments in the future (Lam, 2012).

Taking MOF-mediated H4K16 acetylation into consideration, a putative role of the NSL complex might be to coordinate the opening of promoter architecture by histone acetylation and the assembly of PIC. Coupling of histone acetylation and PIC formation has been described before. For example, TAF1, a component of TFIID, is a histone aceyltransferase. The SAGA complex, which contains Gcn5 and can acetylate H3K9, is reported to interact with TBP and other PIC components to regulate tissue-specific genes and the recruitment of P300 to the promoter and H3 acetylation have been shown to proceed binding of TFIID in a coordinated manner. H4K16ac is also well-known for its role in transcription regulation of the male X chromosome, yet how H4K16 acetylation and PIC assembly are coordinated remains elusive. Interestingly, absence of the NSL complex does not severely abolish H4K16ac from target genes. Since the turnover of H4K16ac on target promoter is unknown, it remains possible that H4K16ac could remain for some time at the promoter after the NSL complex is depleted. Further studies will be crucial in unraveling the functional relevance of H4K16 acetylation and NSL complex function on housekeeping genes (Lam, 2012).

Protein Interactions

Essential roles of Drosophila Inner centromere protein (Incenp) and Aurora B in Histone H3 phosphorylation, metaphase chromosome alignment, kinetochore disjunction, and chromosome segregation

A biochemical and double-stranded RNA-mediated interference (RNAi) analysis of the role of two chromosomal passenger proteins, inner centromere protein (INCENP) and aurora B kinase, was performed in cultured cells of Drosophila melanogaster. INCENP and aurora B function is tightly interlinked. The two proteins bind to each other in vitro, and INCENP is required for Aurora B to localize properly in mitosis and function as a histone H3 kinase. Aurora B is required for INCENP accumulation at centromeres and transfer to the spindle at anaphase. RNAi for either protein dramatically inhibits the ability of cells to achieve a normal metaphase chromosome alignment. Cells were not blocked in mitosis, however, and entered an aberrant anaphase characterized by defects in sister kinetochore disjunction and the presence of large amounts of amorphous lagging chromatin. Anaphase A chromosome movement appeared to be normal, however cytokinesis often failed. INCENP and Aurora B are not required for the correct localization of the kinesin-like protein Pavarotti to the midbody at telophase. These experiments reveal that INCENP is required for aurora B kinase function and confirm that the chromosomal passengers have essential roles in mitosis (Adams, 2001b).

Aurora B is required for some, but not all, aspects of Incenp localization in mitosis. In the absence of Aurora B, Incenp localizes normally to chromosomes during pro-phase; however, it is subsequently unable to concentrate at centromeres and transfer to the central spindle or midbody. As predicted from previous studies, Incenp is essential for Aurora B targeting: after Incenp RNAi, Aurora B does not localize to chromosomes, midzone microtubules, or midbodies. Thus, the chromosomal passenger proteins are interdependent on one another for proper targeting during mitosis (Adams, 2001b).

This interdependence, plus the fact that the two proteins are stockpiled in an 11S complex in Xenopus eggs, suggests that they could function in vivo in a protein complex. Incenp binds microtubules in vitro and may be responsible for targeting Aurora B to the central spindle, as the kinase appears to lack microtubule binding activity of its own. However, the differences in centromere targeting in Drosophila early embryos suggest that the two proteins may not function in an obligate complex, at least during prophase (Adams, 2001b).

S. cerevisiae aurora/Ipl1p and C. elegans aurora B/AIR-2 are required for H3-serine10 phosphorylation in mitosis (Hsu, 2000). Not only is Incenp essential for the proper targeting of Aurora B in mitotic cells, but this targeting is required for normal levels of histone H3 phosphorylation on serine10. This is the first evidence that Incenp is an essential cofactor required for Aurora B kinase function in vivo (Adams, 2001b).

The availability of mitotic cells containing chromosomes with a range of levels of H3 phosphorylated on serine10 has enabled an assessment of the widely held hypothesis that H3 phosphorylation is correlated with the degree of chromatin condensation. When phospho-H3 levels and the degree of chromatin compaction were compared by quantitative fluorescence microscopy, only a weak correlation between the two values was observed. Instead, interference with Incenp and Aurora B function appears to correlate much more strongly with difficulties in assembling mitotic chromosomes of normal morphology. Mitotic chromosomes deficient in phospho-H3 have a characteristic dumpy morphology, with no evidence of resolved sister chromatids. This resembles the defects seen in Drosophila mutants in the SMC4 subunit of condensin (Steffensen, 2001) and also those of a ts mutant in C. elegans aurora B/AIR-2 when it enters mitosis at nonpermissive temperature (Severson, 2000). Phosphorylation of histone H3 or another chromosomal substrate by Aurora B might be required for the binding of condensins or other chromosomal proteins that give mitotic chromosomes their characteristic morphology (Adams, 2001b).

At later times, after addition of dsRNA, a dramatic increase is seen in the percentage of mitotic cells in prometaphase coupled with a corresponding decrease in the number of metaphase cells. This is particularly dramatic in the Incenp RNAi, where no Incenp-negative cells in metaphase are seen. Surprisingly, this did not lead to an increase in the mitotic index of the cultures. Therefore, in the absence of Incenp and/or Aurora B function, Drosophila Dmel2 cells must exit mitosis from prometaphase. Elimination of Incenp and Aurora B function does not trigger a mitotic checkpoint in Dmel2 cells. However, since these cells do not arrest in mitosis in colcemid, they appear to lack a robust metaphase checkpoint in any case (Adams, 2001b).

What is the ultimate fate of these prometaphase cells? They are not removed by cell death. An alternative explanation for the lack of an increase in mitotic index would be if the cells transit directly from prometaphase into anaphase or telophase, as is the case for budding yeast cells mutant in the essential kinetochore protein Ndc10p. Consistent with this, a variety of striking abnormalities were seen in cells either undergoing anaphase, or early in the next cell cycle. Although anaphase/telophase cells with kinetochores at opposite poles of the chromatin mass could be seen, the kinetochores were often not clustered as tightly as normal This may reflect the initiation of anaphase movement without prior alignment of the chromosomes at a metaphase plate (Adams, 2001b).

Why does abrogation of Incenp and/or Aurora B function prevent cells from attaining a stable metaphase chromosome alignment? One obvious possibility is that kinetochore function is impaired. In budding yeast, the aurora kinase Ipl1p phosphorylates the essential kinetochore component Ndc10p (Biggins, 1999). It is therefore possible that, in metazoans, one or more kinetochore components must be phosphorylated by Aurora B in order for kinetochores to function in mitosis. An obvious candidate for this is CENP-A/Cid. CENP-A retains a site homologous to serine10, which is serine5 in Cid. It will be important to determine whether CENP-A/Cid is phosphorylated in an Aurora B kinase-dependent manner (Adams, 2001b).

Arguing against this model is the observation that kinetochores assemble correctly, at least as far as CENP-A/Cid binding is concerned, and move to the spindle poles at anaphase/telophase. This implies that the ability of kinetochores to bind microtubules and to undergo anaphase A movement are preserved after abrogation of Incenp and Aurora B function. However, other aspects of kinetochore function, namely the ability to form bipolar spindle attachments and disjoin at anaphase, appear to be defective. How RNAi of Incenp or Aurora B leads to defects in chromosome biorientation is unknown, but this is unlikely to be a result of interference with binding of the condensin subunit barren, since barren mutants successfully complete metaphase chromosome alignment. Furthermore, the possibility that some of the abnormal aspects of chromosome behavior reflect an impairment of microtubule and/or spindle function cannot be excluded. The detailed behavior of the mitotic spindle after RNAi of Incenp and Aurora B requires further analysis (Adams, 2001b).

Anaphase/telophase cells after RNAi for Incenp or Aurora B exhibit three highly unusual chromosomal phenotypes: (1) they often have one or more pairs of sister kinetochores located in the central spindle or flanking the midbody; (2) the foci of CENP-A/Cid staining at or near the spindle poles is often present as pairs, suggesting that sister kinetochores remain paired despite having undergone anaphase A-like poleward movement; (3) separated masses of chromatin are often connected by a mass of lagging chromatin. This is referred to as chromatin and not chromosomes because the material is amorphous, and little or no evidence of a condensed mitotic chromosome morphology can be observed (Adams, 2001b).

The first two phenotypes can be explained if centromeres fail to disjoin at anaphase onset. Under these circumstances, centromeres of bioriented chromosomes would tend to accumulate near the spindle equator -- later, near the midbody -- and be stretched apart by the spindle forces. Mono-oriented kinetochores would move as pairs to one or the other spindle pole. If this occurred in cells that had attained metaphase, then the bulk of kinetochores would remain as pairs in the spindle midzone. However, abrogation of Incenp and/or Aurora B function prevents cells from reaching metaphase and would therefore be expected to lead to the observed phenotype, with most centromeres at the poles and only a few remaining in the midzone. Defects in sister kinetochore disjunction could arise if Incenp and/or Aurora B were involved in regulation of the cohesin complex at centromeres; experiments are under way to determine whether cohesin components are substrates for Aurora B (Adams, 2001b).

The presence of massive amounts of lagging chromatin is highly characteristic of anaphase/telophase after loss of Incenp and/or Aurora B function. This lagging chromatin might arise from difficulties in sister chromatid disjunction, but it is more likely that it represents a failure of the chromosomes to move as integral units under the physical stress of anaphase movement. If the dumpy chromosomes observed in prometaphase cells lack an organized infrastructure then when centromeres begin to move polewards, the chromatin of the arms might simply unravel and be left behind as a smear of amorphous chromatin. This would be consistent with the observation that interference with Aurora B function in Drosophila cells interferes with the binding of the condensin subunit barren to mitotic chromosomes (Giet, 2001). Indeed, barren mutants exhibit dramatic chromatin bridges during syncytial mitosis, however such a dramatic defect was not seen in mutants affecting the condensin subunit SMC4 in Drosophila (Steffensen, 2001). It is possible that action of Incenp/aurora B on other chromosomal components, in addition to condensin subunits, contributes to a loss of chromosomal integrity during anaphase (Adams, 2001b).

In the oocytes of many species, bipolar spindles form in the absence of centrosomes. Drosophila oocyte chromosomes have a major role in nucleating microtubules, a process that precedes the bundling and assembly of these microtubules into a bipolar spindle. Evidence is presented that a region similar to the anaphase central spindle functions to organize acentrosomal spindles. subito mutants are characterized by the formation of tripolar or monopolar spindles and nondisjunction of homologous chromosomes at meiosis I. subito encodes a kinesinlike protein and associates with the meiotic central spindle, consistent with its classification in the Kinesin 6/MKLP1 family. This class of proteins is known to be required for cytokinesis, but the current results suggest a new function in spindle formation. The meiotic central spindle appears during prometaphase and includes passenger complex proteins such as AurB and Incenp. Unlike mitotic cells, the passenger proteins do not associate with centromeres before anaphase. In the absence of Subito, central spindle formation is defective and AurB and Incenp fail to properly localize. It is proposed that Subito is required for establishing and/or maintaining the central spindle in Drosophila oocytes, and this substitutes for the role of centrosomes in organizing the bipolar spindle (Jang, 2005).

The chromosomal passenger complex is required for meiotic acentrosomal spindle assembly and chromosome biorientation

During meiosis in the females of many species, spindle assembly occurs in the absence of the microtubule-organizing centers called centrosomes. In the absence of centrosomes, the nature of the chromosome-based signal that recruits microtubules to promote spindle assembly as well as how spindle bipolarity is established and the chromosomes orient correctly toward the poles is not known. To address these questions, this study focused on the chromosomal passenger complex (CPC). The CPC localizes in a ring around the meiotic chromosomes that is aligned with the axis of the spindle at all stages. Using new methods that dramatically increase the effectiveness of RNA interference in the germline, it was shown that the CPC interacts with Drosophila oocyte chromosomes and is required for the assembly of spindle microtubules. Furthermore, chromosome biorientation and the localization of the central spindle kinesin-6 protein Subito, which is required for spindle bipolarity, depend on the CPC components Aurora B and Incenp. Based on these data it is proposed that the ring of CPC around the chromosomes regulates multiple aspects of meiotic cell division including spindle assembly, the establishment of bipolarity, the recruitment of important spindle organization factors, and the biorientation of homologous chromosomes (Radford, 2012).

Previous work using Xenopus egg extracts demonstrated that both RanGTP and the CPC are required for chromatin-induced spindle assembly. In contrast, RanGTP appears not to be required for acentrosomal spindle assembly in Drosophila (Cesario, 2011) and mouse oocytes. This study has shown that the CPC is essential for the accumulation of microtubules around the chromosomes in Drosophila oocytes, suggesting that in vivo the CPC is the critical factor for regulating acentrosomal spindle assembly. A model is presented for acentrosomal spindle assembly with implications for how the CPC simultaneously promotes bipolarity and homolog bi-orientation (Radford, 2012).

The results support a model in which the primary step in the establishment of meiotic spindle bipolarity is the accumulation of the CPC in a ring encircling the chromosomes. The enrichment of CPC proteins in a ring around the karyosome may provide the increased local concentration of Aurora B that has been postulated to be necessary to activate the Aurora B kinase for chromosome-based spindle assembly in Xenopus egg extracts. It is proposed that the CPC has two critical functions in Drosophila oocytes: it promotes microtubule accumulation near the chromosomes and also constrains microtubule growth into two poles by establishing the spindle axis. This replaces two functions of the centrosomes: recruitment of microtubules and organizing a bipolar spindle. Previous studies have suggested that the CPC promotes spindle assembly by suppressing the microtubule-depolymerizing activity of a kinesin-13 protein near the chromosomes. In contrast, this study has shown that down-regulating KLP10A, a Drosophila kinesin-13 protein known to regulate spindle length, is not a sufficient explanation for the activity of the CPC. While a role for the CPC in regulating two additional kinesin-13s encoded by the Drosophila genome, KLP59C and KLP59D cannot be ruled out, during acentrosomal spindle assembly, evidence summarized below suggests that the CPC positively regulates spindle assembly factors (Radford, 2012).

For the second function, constraining microtubule assembly towards two poles, a simple model is suggested by the shape of the ring: the ring may act like a tube that restricts microtubules to assemble in only two directions. Additionally, the CPC ring establishes the location for recruitment of other spindle assembly factors that regulate bipolarity, including Subito. A direct physical interaction between Subito and Incenp would be consistent with results showing that the mammalian Subito ortholog MKLP2 physically interacts with Aurora B and Incenp (Gruneberg, 2004). This must depend on Aurora B activity since no Subito localization was observed in plI-aurora-like kinase RNAi oocytes even though Incenp was associated with the chromatin. It is suggested that the CPC interacts with chromosomes in a ring, promotes microtubule accumulation, and recruits proteins like Subito to these microtubules, which results in the establishment or stabilization of antiparallel microtubules, spindle bipolarity, and the formation of two poles (Radford, 2012).

Subito and the CPC appear to have a mutual dependency. It has been shown previously that the meiotic central spindle localization of the CPC depended on Subito (Jang, 2005). To explain these results, it is suggested that the CPC is first recruited to the chromosomes, and then moves to the central spindle microtubules. In the absence of Subito and the central spindle microtubules, the interaction of Incenp with the chromosomes persists and the CPC does not move to the microtubules. While interacting with the chromosomes the CPC can apparently promote spindle assembly, but not bi-orientation (Radford, 2012).

What controls the localization of the CPC ring and how it gets targeted to the region between bi-oriented centromeres remains to be uncovered. In the absence of Aurora B, the localization pattern of Incenp within the karyosome is disorganized, suggesting that the kinase activity of the CPC may play a role in shaping the ring, but underlying features of the chromosomes may also be important. It is intriguing that the passenger proteins are not detected in the centromere regions as they are in mitotic and centrosomal meiotic cells. The results are consistent with data from C. elegans oocytes, showing that the CPC interacts with non-centromeric chromatin at metaphase of meiosis I. In C. elegans, the CPC forms a ring at the center of each bivalent that colocalizes with cohesion proteins distal to chiasmata. The C. elegans CPC ring is a complex structure which, like in Drosophila, contains motor proteins (Klp-19) and is required for segregation of homologs at meiosis I. The importance of non-centromeric CPC in a variety of organisms suggests that the unique demands of acentrosomal meiosis have resulted in a meiosis-specific CPC/central spindle localization pattern with a conserved role in spindle assembly and chromosome segregation. Finding out the identity or structural features of the chromosome locations to which the CPC ring localizes will be critical to understanding how the chromosomes organize acentrosomal spindles (Radford, 2012).

Centromeres are paired in Drosophila oocytes prior to NEB. Based on examination of oocytes depleted of the CPC and spindle assembly motors Subito and NCD, the following pathway leading to homolog bi-orientation is proposed. First, the CPC binds in a ring to the chromosomes and recruits spindle assembly factors such as Subito. This stage is defined by the observation that the CPC can bind chromosomes independent of microtubules and, in its absence, the microtubules and Subito fail to accumulate around the chromosomes. Second, microtubules with attachments to the chromosomes provide a poleward force on the centromeres. This stage is defined by the observation that, in the absence of the CPC, and consequently the absence of microtubules, the homologous centromeres fail to separate. Third, the homologs bi-orient through interactions with the central spindle microtubules. This stage is defined by the observation that, in sub mutants, the central spindle is absent but microtubules with attachments to the chromosomes still form and the homologous centromeres separate but fail to bi-orient (Radford, 2012).

The nature of the microtubule attachments to the chromosomes that lead to centromere separation is not known. Some previous studies have suggested that chromosome alignment depends on lateral interactions during acentrosomal meiosis. However, an alternative model incorporates an important role for kinetochore microtubules). Kinetochore microtubules in oocytes have been inferred by Hughes (2011) and could be the cold-resistant karyosome-associated microtubules observed in previous studies. Whether the microtubules connect to the chromosomes though traditional end-on kinetochore attachments or lateral attachments, it is proposed that these microtubules are bundled with central spindle microtubules to achieve bi-orientation. Interactions between central spindle microtubules and the microtubules with attachments to the chromosomes could be mediated by the kinesin-5 KLP61F or the kinesin-14 NCD. Indeed, this study has shown that NCD is required for homolog bi-orientation. The frayed spindles that are typical of ncd mutants could be explained by the loss of bundling between chromosome and central spindle microtubules (Radford, 2012).

A possible mechanism for how the CPC ring may facilitate bi-orientation at meiosis is suggested by two recent studies in mammalian mitotic and meiotic cells (Kitajima, 2011; Magidson, 2011). In both systems, prometaphase chromosomes move towards the outside edges of the developing spindle and then congress via lateral interactions to a ring around the central part of the spindle. This 'prometaphase belt' facilitates and enhances the rate of bi-orientation by bringing kinetochores into the vicinity of a high density of microtubules, which leads to stable kinetochore-microtubule attachments. It is proposed that the ring of CPC protein promotes a prometaphase belt-like organization to enhance the interaction of centromeres with a high density of microtubules in Drosophila oocytes (Radford, 2012).

Chromosome-based spindle assembly is a well described phenomenon, but the responsible chromatin-based factors in intact oocytes have not been previously identified. The current data suggests that the CPC interacts with noncentromeric chromatin and not only promotes the accumulation of microtubules around the chromosomes, but also regulates multiple aspects of spindle function, including the establishment of bipolarity and bi-orientation of homologs. Indeed, the localization to a central spindle ring and not centromeres may be critical for these functions. At this location, the CPC could regulate several different types of target protein that organize microtubules. One type is represented by Subito, which is required for spindle bipolarity, perhaps through the stabilization of antiparallel microtubules in the central spindle. Another type of target protein may function to promote microtubule attachment to the chromosomes. Indeed, these results provide the starting point for investigating what controls the localization of the CPC and what are its critical targets during acentrosomal meiosis (Radford, 2012). PubMed ID: 20929775

The chromosomal passenger complex activates Polo kinase at centromeres

The coordinated activities at centromeres of two key cell cycle kinases, Polo and Aurora B, are critical for ensuring that the two sister kinetochores of each chromosome are attached to microtubules from opposite spindle poles prior to chromosome segregation at anaphase. Initial attachments of chromosomes to the spindle involve random interactions between kinetochores and dynamic microtubules, and errors occur frequently during early stages of the process. The balance between microtubule binding and error correction (e.g., release of bound microtubules) requires the activities of Polo and Aurora B kinases, with Polo promoting stable attachments and Aurora B promoting detachment. This study concerns the coordination of the activities of these two kinases in vivo. INCENP, a key scaffolding subunit of the chromosomal passenger complex (CPC), which consists of Aurora B kinase, INCENP, Survivin, and Borealin/Dasra B, also interacts with Polo kinase in Drosophila cells. It was known that Aurora A/Bora activates Polo at centrosomes during late G2. However, the kinase that activates Polo on chromosomes for its critical functions at kinetochores was not known. This study shows that Aurora B kinase phosphorylates Polo on its activation loop at the centromere in early mitosis. This phosphorylation requires both INCENP and Aurora B activity (but not Aurora A activity) and is critical for Polo function at kinetochores. The results demonstrate clearly that Polo kinase is regulated differently at centrosomes and centromeres and suggest that INCENP acts as a platform for kinase crosstalk at the centromere. This crosstalk may enable Polo and Aurora B to achieve a balance wherein microtubule mis-attachments are corrected, but proper attachments are stabilized allowing proper chromosome segregation (Carmena, 2012).

Coordination of Polo and Aurora B activity at kinetochores is critical in early mitosis, as the two kinases play potentially antagonistic but complementary roles in regulating kinetochore-microtubule interactions. Aurora B is essential for the correction of aberrant attachments, and indeed, tethering Aurora B too close to kinetochores interferes with the formation of stable attachments. In contrast, Plk1 activity is required for initial stabilisation of microtubule attachments to kinetochores. It is suggested that interactions with INCENP may provide a mechanism to coordinate the activities of these two essential kinases during early mitosis (Carmena, 2012).

Recent studies suggest that Plk1 is activated at centrosomes when its T-loop (T210) is phosphorylated by Aurora A kinase-Bora, and that this promotes the G2/M transition upstream of Cdk1, although Polo activity is not required for mitotic entry. How Plk1 is activated at kinetochores remained an important unsolved question. The present results show that Aurora B and INCENP, which are concentrated at inner centromeres, function there to activate Polo by phosphorylating its T-loop (Carmena, 2012).

Plk1 recruitment to centromeres in late G2 has been variously proposed to be mediated by Bub1, INCENP, and BubR1. Another report implicated the self-primed interaction of Plk1 with PBIP1/CENP-U. This could potentially explain why Plk1 activity is reportedly required for its localisation to kinetochores in human cells (Carmena, 2012).

The current RNAi studies confirmed that Plk1 is partially dependent on the CPC for its centromeric localization in human cells. However, this appears not to be the case in Drosophila, where Polo is present at centromeres before NEB, at a time when INCENP is not yet concentrated at inner centromeres and before PoloT182ph, the active form of the kinase, is detected there. Indeed, no significant decrease was observed in kinetochore-associated Polo levels after INCENP RNAi in Drosophila cells (Carmena, 2012).

Although Polo targeting to kinetochores is independent of the CPC in Drosophila, its activation there does require the CPC with active Aurora B. The data suggest that INCENP binding to Polo facilitates its subsequent activation by Aurora B kinase. Indeed, INCENP and Polo interact physically in vitro and co-immunoprecipitate in mitotic cell extracts. Although most centromeric Polo kinase is concentrated in the outer kinetochore in prophase and prometaphase, active Polo (PoloT182ph) is also found in inner centromeres, where it overlaps with INCENP as confirmed by a proximity ligation assay (PLA)(Carmena, 2012).

A range of evidence presented in this study suggests that Aurora B is the upstream kinase responsible for Polo kinase activation at centromeres. Firstly, Aurora B phosphorylates Polo at Thr182 in vitro. Secondly, RNAi depletion of INCENP or Aurora B, but not Aurora A, reduces levels of active PoloT182ph at kinetochores. Thirdly, tissue culture cells and third larval instar neuroblasts treated with a specific inhibitor of Drosophila Aurora B kinase show decreased levels of PoloT182ph at kinetochores. In all of the preceding experiments, PoloT182ph levels are affected at kinetochores but not at centrosomes, where Polo is presumably activated by Aurora A. Importantly, this involvement of Aurora B in Polo activation at centromeres discovered in Drosophila is conserved for Plk1 in human cells (Carmena, 2012).

The current results suggest a model for interactions between Polo kinase and the CPC at centromeres (see Model for the interactions between the CPC and Polo kinase at the centromere/kinetochore). In Drosophila cells, Polo targets to centromeres before the CPC is recruited by Survivin binding to histone H3T3ph (Yamagishi, 2010: see Schematic depiction of the pathways that regulate CPC targeting to centromeres). At inner centromeres of chromosomes whose kinetochores are not under tension, Polo now binds to INCENP. This promotes Polo kinase activation, as Aurora B phosphorylates PoloT182. It is suggested that interactions with INCENP link the two complementary kinase activities, thereby potentially creating a microtubule attachment/detachment cycle at kinetochores. Such a cycle would not be possible without a balancing phosphatase activity, and PP2A-B56 has recently been shown to oppose both Aurora B and Plk1 activities at kinetochores to promote stable attachments (Carmena, 2012).

At metaphase, when chromosomes are bioriented and under tension, the CPC and Polo kinase exhibit only a partial overlap. A weakening of the INCENP/Polo PLA signals in metaphase suggests that Polo may be released from INCENP after its activation—possibly moving to the outer kinetochore. During metaphase, the CPC localizes in the inner centromere, stretching between sister kinetochores, whereas Polo and PoloT182ph concentrate mainly at the kinetochores. This separation may be necessary to allow Polo-mediated stabilisation of kinetochore-microtubule attachments. The coordinated activities of both kinases at kinetochores and their tension-mediated separation might facilitate a dynamic equilibrium between attached and unattached kinetochores, selectively stabilizing proper chromosome attachments (Carmena, 2012).

In summary, the results reveal that INCENP and Aurora B are responsible for Polo kinase activation at centromeres but not at centrosomes during mitosis. These experiments support the hypothesis that INCENP acts as a scaffold integrating the cross-talk between these two important mitotic kinases (Carmena, 2012).

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. Previous work has 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. This report shows 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. These results shed new light on the mechanisms that control the activity of Aurora B in meiosis and mitosis (Carmena, 2014).

Inner centromere protein : Biological Overview | Evolutionary Homologs | Developmental Biology | Effects of Mutation | References

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