CP250, a Novel Acidic Coiled Coil Protein of the Dictyostelium centrosome,Affects Growth,Chemotaxis, and the Nuclear Envelope

The Dictyostelium centrosome is a nucleus associated body consisting of a box-shaped core surrounded by the corona, an amorphous matrix functionally equivalent to the pericentriolar material of animal centrosomes which is responsible for the nucleation and anchoring of microtubules. Here we describe CP250 a component of the corona, an acidic coiled coil protein that is present at the centrosome throughout interphase while disappearing during prophase and reappearing at the end of late telophase. Amino acids 756-1148 of the 2110 amino acids are sufficient for centrosomal targeting and cell cycle–dependent centrosome association. Mutant cells lacking CP250 are smaller in size, growth on bacteria is delayed, chemotaxis is altered, and development is affected, which, in general, are defects observed in cytoskeletal mutants. Furthermore, loss of CP250 affected the nuclear envelope and led to reduced amounts and altered distribution of Sun-1, a conserved nuclear envelope protein that connects the centrosome to chromatin.     

Centrosome separation: respective role of microtubules and actin filaments

In mammalian cells, the separation of centrosomes is a prerequisite for bipolar mitotic spindle assembly. We have investigated the respective contribution of the two cytoskeleton components, microtubules and actin filaments, in this process. Distances between centrosomes have been measured during cell cycle progression in Xenopus laevis XL2 cultured cells in the presence or absence of either network. We considered two stages in centrosome separation: the splitting stage, when centrosomes start to move apart (minimum distance of 1 microm), and the elongation stage (from 1 to 7 microm). In interphase, depolymerisation of microtubules by nocodazole significantly inhibited the splitting stage, while the elongation stage was, on the contrary, facilitated. In mitosis, while nocodazole treatment completely blocked spindle assembly, in prophase, we observed that 55% of the centrosomes separated, versus 94% in the control. Upon actin depolymerisation by latrunculin, splitting of the interphase centrosome was blocked, and cells entered mitosis with unseparated centrosomes. Cells compensated for this separation delay by increasing the length of both prophase and prometaphase stages to allow for centrosome separation until a minimal distance was reached. Then the cells passed through anaphase, performing proper chromosome separation, but cytokinesis did not occur, and binuclear cells were formed. Our results clearly show that the actin microfilaments participate in centrosome separation at the G2/M transition and work in synergy with the microtubules to accelerate centrosome separation during mitosis.     

Excess centrosomes disrupt endothelial cell migration via centrosome scattering

Supernumerary centrosomes contribute to spindle defects and aneuploidy at mitosis, but the effects of excess centrosomes during interphase are poorly understood. In this paper, we show that interphase endothelial cells with even one extra centrosome exhibit a cascade of defects, resulting in disrupted cell migration and abnormal blood vessel sprouting. Endothelial cells with supernumerary centrosomes had increased centrosome scattering and reduced microtubule (MT) nucleation capacity that correlated with decreased Golgi integrity and randomized vesicle trafficking, and ablation of excess centrosomes partially rescued these parameters. Mechanistically, tumor endothelial cells with supernumerary centrosomes had less centrosome-localized γ-tubulin, and Plk1 blockade prevented MT growth, whereas overexpression rescued centrosome γ-tubulin levels and centrosome dynamics. These data support a model whereby centrosome–MT interactions during interphase are important for centrosome clustering and cell polarity and further suggest that disruption of interphase cell behavior by supernumerary centrosomes contributes to pathology independent of mitotic effects.     

Phosphorylation of centrin during the cell cycle and its role in centriole separation preceding centrosome duplication

Once during each cell cycle, mitotic spindle poles arise by separation of newly duplicated centrosomes. We report here the involvement of phosphorylation of the centrosomal protein centrin in this process. We show that centrin is phosphorylated at serine residue 170 during the G(2)/M phase of the cell cycle. Indirect immunofluorescence staining of HeLa cells using a phosphocentrin-specific antibody reveals intense labeling of mitotic spindle poles during prophase and metaphase of the cell division cycle, with diminished staining of anaphase and no staining of telophase and interphase centrosomes. Cultured cells undergo a dramatic increase in centrin phosphorylation following the experimental elevation of PKA activity, suggesting that this kinase can phosphorylate centrin in vivo. Surprisingly, elevated PKA activity also resulted intense phosphocentrin antibody labeling of interphase centrosomes and in the concurrent movement of individual centrioles apart from one another. Taken together, these results suggest that centrin phosphorylation signals the separation of centrosomes at prophase and implicates centrin phosphorylation in centriole separation that normally precedes centrosome duplication.     

Involvement of a centrosomal protein kendrin in the maintenance of centrosome cohesion by modulating Nek2A kinase activity

Centrosome cycle is strictly coordinated with chromosome duplication cycle to ensure the faithful segregation of chromosomes. Centrosome duplication occurs from the beginning of S phase, and the duplicated centrosomes are held together by centrosome cohesion to function as a single microtubule organizing center during interphase. At late G2 phase centrosome cohesion is disassembled by Nek2A kinase-mediated phosphorylation and, as a consequence, centrosomes are split and constitute spindle poles in mitosis. It has been reported that depletion of a centrosomal protein kendrin (also named pericentrin) induces premature centrosome splitting in interphase, however, it remains unknown how kendrin contributes to the maintenance of centrosome cohesion. Here we show that kendrin associates with Nek2A kinase, which exhibits considerably low activity. Nek2A kinase activity is inhibited in vitro by addition of the Nek2A-binding region of kendrin in a dose-dependent manner. Furthermore, ectopic expression of the same region decreases the number of the cells with split centrosomes at late G2 phase. Taken together, these results suggest that kendrin anchors Nek2A and suppresses its kinase activity at the centrosomes, and thus, is involved in the mechanism to prevent premature centrosome splitting during interphase.     

The Centrosomal Linker and Microtubules Provide Dual Levels of Spatial Coordination of Centrosomes

The centrosome is the principal microtubule organizing center in most animal cells. It consists of a pair of centrioles surrounded by pericentriolar material. The centrosome, like DNA, duplicates exactly once per cell cycle. During interphase duplicated centrosomes remain closely linked by a proteinaceous linker. This centrosomal linker is composed of rootletin filaments that are anchored to the centrioles via the protein C-Nap1. At the onset of mitosis the linker is dissolved by Nek2A kinase to support the formation of the bipolar mitotic spindle. The importance of the centrosomal linker for cell function during interphase awaits characterization. Here we assessed the phenotype of human RPE1 C-Nap1 knockout (KO) cells. The absence of the linker led to a modest increase in the average centrosome separation from 1 to 2.5 μm. This small impact on the degree of separation is indicative of a second level of spatial organization of centrosomes. Microtubule depolymerisation or stabilization in C-Nap1 KO cells dramatically increased the inter-centrosomal separation (> 8 μm). Thus, microtubules position centrosomes relatively close to one another in the absence of linker function. C-Nap1 KO cells had a Golgi organization defect with a two-fold expansion of the area occupied by the Golgi. When the centrosomes of C-Nap1 KO cells showed considerable separation, two spatially distinct Golgi stacks could be observed. Furthermore, migration of C-Nap1 KO cells was slower than their wild type RPE1 counterparts. These data show that the spatial organization of centrosomes is modulated by a combination of centrosomal cohesion and microtubule forces. Furthermore a modest increase in centrosome separation has major impact on Golgi organization and cell migration.     

Suppression of centrosome duplication and amplification by deacetylases

Centrosome duplication is controlled both negatively and positively by a number of proteins. The activities and stabilities of those regulatory proteins are in many cases controlled by posttranslational modifications. Although acetylation and deacetylation are highly common posttranslational modifications, their roles in the regulation of centrosome duplication had not been closely examined. Here, through focusing on the deacetylases, we investigated the role of acetylation/deacetylation in the regulation of centrosome duplication and induction of abnormal amplification of centrosomes. We found that the deacetylation event negatively controls centrosome duplication and amplification. Of the 18 total known deacetylases (HDAC1–11, SIRT1–7), ten deacetylases possess the activity to suppress centrosome amplification, and their centrosome amplification suppressing activities are strongly associated with their abilities to localize to centrosomes. Among them, HDAC1, HDAC5 and SIRT1 show the highest suppressing activities, but each of them suppresses centrosome duplication and/or amplification with its unique mechanism.     

LIN-41 inactivation leads to delayed centrosome elimination and abnormal chromosome behavior during female meiosis in Caenorhabditis elegans

During oogenesis, two successive meiotic cell divisions occur without functional centrosomes because of the inactivation and subsequent elimination of maternal centrosomes during the diplotene stage of meiosis I. Despite being a conserved phenomenon in most metazoans, the means by which this centrosome behavior is controlled during female meiosis remain elusive. Here, we conducted a targeted RNAi screening in the Caenorhabditis elegans gonad to identify novel regulators of centrosome behavior during oogenesis. We screened 513 genes known to be essential for embryo production and directly visualized GFP–γ-tubulin to monitor centrosome behavior at all stages of oogenesis. In the screening, we found that RNAi-mediated inactivation of 33 genes delayed the elimination of GFP–γ-tubulin at centrosomes during oogenesis, whereas inactivation of nine genes accelerated the process. Depletion of the TRIM-NHL protein LIN-41 led to a significant delay in centrosome elimination and to the separation and reactivation of centrosomes during oogenesis. Upon LIN-41 depletion, meiotic chromosomes were abnormally condensed and pulled toward one of the two spindle poles around late pachytene even though the spindle microtubules emanated from both centrosomes. Overall, our work provides new insights into the regulation of centrosome behavior to ensure critical meiotic events and the generation of intact oocytes.     

The fission yeast transforming acidic coiled coil-related protein Mia1p/Alp7p is required for formation and maintenance of persistent microtubule-organizing centers at the nuclear envelope

Microtubule-organizing centers (MTOCs) concentrate microtubule nucleation, attachment and bundling factors and thus restrict formation of microtubule arrays in spatial and temporal manner. How MTOCs occur remains an exciting question in cell biology. Here, we show that the transforming acidic coiled coil–related protein Mia1p/Alp7p functions in emergence of large MTOCs in interphase fission yeast cells. We found that Mia1p was a microtubule-binding protein that preferentially localized to the minus ends of microtubules and was associated with the sites of microtubule attachment to the nuclear envelope. Cells lacking Mia1p exhibited less microtubule bundles. Microtubules could be nucleated and bundled but were frequently released from the nucleation sites in mia1Δ cells. Mia1p was required for stability of microtubule bundles and persistent use of nucleation sites both in interphase and postanaphase array dynamics. The γ-tubulin–rich material was not organized in large perinuclear or microtubule-associated structures in mia1Δ cells. Interestingly, absence of microtubules in dividing wild-type cells prevented appearance of large γ-tubulin–rich MTOC structures in daughters. When microtubule polymerization was allowed, MTOCs were efficiently assembled de novo. We propose a model where MTOC emergence is a self-organizing process requiring the continuous association of microtubules with nucleation sites.     

Occludin localizes to centrosomes and modifies mitotic entry

Proper control of cell cycle progression and barrier function are essential processes to the maintenance of epithelial cell homeostasis. The contribution of tight junction proteins to barrier function is well established, whereas their contribution to cell cycle control is only beginning to be understood. Centrosomes are the principal microtubule organizing centers in eukaryotic cells and centrosome duplication and separation are linked to the cell cycle and mitotic entry. Here we demonstrate that occludin localizes with centrosomes in Madin-Darby canine kidney cells. Immunocytochemistry and biochemical fractionation studies reveal occludin localizes with centrosomes during interphase and occludin Ser-490 phosphorylation at centrosomes increases with mitotic entry. Stable expression of aspartic acid phosphomimetic (S490D) results in centrosomal localization of occludin and increases cell numbers. Furthermore, we provide evidence that occludin regulates centrosome separation and mitotic entry as the nonphosphorylatable alanine mutation (S490A) impedes centrosome separation, delays mitotic entry, and reduces proliferation. Collectively, these studies demonstrate a novel location and function for occludin in centrosome separation and mitosis.     

Lamins position the nuclear pores and centrosomes by modulating dynein

Lamins, the type V nuclear intermediate filament proteins, are reported to function in both interphase and mitosis. For example, lamin deletion in various cell types can lead to an uneven distribution of the nuclear pore complexes (NPCs) in the interphase nuclear envelope, whereas deletion of B-type lamins results in spindle orientation defects in mitotic neural progenitor cells. How lamins regulate these functions is unknown. Using mouse cells deleted of different combinations or all lamins, we show that lamins are required to prevent the aggregation of NPCs in the nuclear envelope near centrosomes in late G2 and prophase. This asymmetric NPC distribution in the absence of lamins is caused by dynein forces acting on NPCs via the dynein adaptor BICD2. We further show that asymmetric NPC distribution upon lamin depletion disrupts the distribution of BICD2 and p150 dynactin on the nuclear envelope at prophase, which results in inefficient dynein-driven centrosome separation during prophase. Therefore lamins regulate microtubule-based motor forces in vivo to ensure proper NPC distribution in interphase and centrosome separation in the mitotic prophase.     

Centrioles resist forces applied on centrosomes during G2/M transition

BACKGROUND INFORMATION: Centrosome movements at the onset of mitosis result from a balance between the pulling and pushing forces mediated by microtubules. The structural stability of the centrosome core structure, the centriole pair, is correlated with a heavy polyglutamylation of centriole tubulin. RESULTS: Using HeLa cells stably expressing centrin-green fluorescent protein as a centriole marker, we monitored the effect of microinjecting an anti-(polyglutamylated tubulin) monoclonal antibody, GT335, in G1/S or G2 cells. In contrast with the slow effect of the monoclonal antibody GT335 during interphase, a dramatic and rapid centrosome fragmentation occurred in cells microinjected in G2 that was both Eg5- and dynein-dependent. Inhibition of either one of these two motors significantly decreased the scattering of centrosome fragments, and inhibition of centrosome segregation by impairing microtubule dynamics abolished centrosome fragmentation. CONCLUSIONS: Our results demonstrate that the compact structure of the mitotic centrosome is capable of absorbing most of the pulling and pushing forces during G2/M transition and suggest that centrosomes could act as mechanosensors integrating tensions during cell division.     

The effect of the UV microirradiation of the centrosome on cell behavior. III. The ultrastructure of the centrosome after irradiation]

One of the spindle poles of mitotic PK cells was irradiated with UV microbeam in metaphase or in anaphase. Electron microscopy showed that immediately after irradiation the microtubules around the centrosome were maintained, and that the ultrastructure of both irradiated and nonirradiated poles was similar. After microirradiation of the centrosome in metaphase, the mitotic halo around this centrosome was retained, but in due time the number of microtubules was getting less compared to that around the nonirradiated centrosome. When daughter cells with irradiated centrosomes are passing into the interphase, their centrioles are not separated from each other, no primary cilia are formed, and no replication of centrioles occurs. In the interphase cells with irradiated centrosomes, satellites are formed on the active centriole, but centrosome-attached microtubules are practically absent.     

MSA-35: a protein identified by human autoantibodies that colocalizes with microtubules.

We have identified a putative 35-kilodalton protein that colocalizes with microtubules and displays a unique spatial and temporal distribution during the cell cycle of HeLa cells. This protein has been given the designation MSA-35. MSA-35 first appears in association with microtubules and centrosomes of interphase cells exhibiting centrosome separation as a prelude to cell division. This protein is found in conjunction with kinetochore microtubules throughout their appearance. MSA-35 transiently associates with interpolar microtubules following anaphase and the pattern of MSA-35 reactivity in telophase cells suggests that there are at least seven domains within the intercellular bridge. The distribution of MSA-35 during and following recovery from mitotic arrest with nocodazole suggest that it is also present at low levels in interphase cells, can associate with interphase centrosomes, and colocalizes with nascent microtubules. The complex spatial and temporal distribution of MSA-35 indicates that it may be necessary for a series of events in the mitotic process such as the bundling of microtubules.     

Cytoplasmic dynein-mediated assembly of pericentrin and gamma tubulin onto centrosomes

Centrosome assembly is important for mitotic spindle formation and if defective may contribute to genomic instability in cancer. Here we show that in somatic cells centrosome assembly of two proteins involved in microtubule nucleation, pericentrin and gamma tubulin, is inhibited in the absence of microtubules. A more potent inhibitory effect on centrosome assembly of these proteins is observed after specific disruption of the microtubule motor cytoplasmic dynein by microinjection of dynein antibodies or by overexpression of the dynamitin subunit of the dynein binding complex dynactin. Consistent with these observations is the ability of pericentrin to cosediment with taxol-stabilized microtubules in a dynein- and dynactin-dependent manner. Centrosomes in cells with reduced levels of pericentrin and gamma tubulin have a diminished capacity to nucleate microtubules. In living cells expressing a green fluorescent protein-pericentrin fusion protein, green fluorescent protein particles containing endogenous pericentrin and gamma tubulin move along microtubules at speeds of dynein and dock at centrosomes. In Xenopus extracts where gamma tubulin assembly onto centrioles can occur without microtubules, we find that assembly is enhanced in the presence of microtubules and inhibited by dynein antibodies. From these studies we conclude that pericentrin and gamma tubulin are novel dynein cargoes that can be transported to centrosomes on microtubules and whose assembly contributes to microtubule nucleation.     

ANKRD26 recruits PIDD1 to centriolar distal appendages to activate the PIDDosome following centrosome amplification

Centriole copy number is tightly maintained by the once‐per‐cycle duplication of these organelles. Centrioles constitute the core of centrosomes, which organize the microtubule cytoskeleton and form the poles of the mitotic spindle. Centrosome amplification is frequently observed in tumors, where it promotes aneuploidy and contributes to invasive phenotypes. In non‐transformed cells, centrosome amplification triggers PIDDosome activation as a protective response to inhibit cell proliferation, but how extra centrosomes activate the PIDDosome remains unclear. Using a genome‐wide screen, we identify centriole distal appendages as critical for PIDDosome activation in cells with extra centrosomes. The distal appendage protein ANKRD26 is found to interact with and recruit the PIDDosome component PIDD1 to centriole distal appendages, and this interaction is required for PIDDosome activation following centrosome amplification. Furthermore, a recurrent ANKRD26 mutation found in human tumors disrupts PIDD1 localization and PIDDosome activation in cells with extra centrosomes. Our data support a model in which ANKRD26 initiates a centriole‐derived signal to limit cell proliferation in response to centrosome amplification.     

PLK1 regulates centrosome migration and spindle dynamics in male mouse meiosis

Cell division requires the regulation of karyokinesis and cytokinesis, which includes an essential role of the achromatic spindle. Although the functions of centrosomes are well characterised in somatic cells, their role during vertebrate spermatogenesis remains elusive. We have studied the dynamics of the meiotic centrosomes in male mouse during both meiotic divisions. Results show that meiotic centrosomes duplicate twice: first duplication occurs in the leptotene/zygotene transition, while the second occurs in interkinesis. The maturation of duplicated centrosomes during the early stages of prophase I and II are followed by their separation and migration to opposite poles to form bipolar spindles I and II. The study of the genetic mouse model Plk1(Δ/Δ) indicates a central role of Polo‐like kinase 1 in pericentriolar matrix assembly, in centrosome maturation and migration, and in the formation of the bipolar spindles during spermatogenesis. In addition, in vitro inhibition of Polo‐like kinase 1 and Aurora A in organotypic cultures of seminiferous tubules points out to a prominent role of both kinases in the regulation of the formation of meiotic bipolar spindles.     

Recruitment of the γ-Tubulin Ring Complex to Drosophila Salt-stripped Centrosome Scaffolds

Extracting isolated Drosophila centrosomes with 2 M KI generates salt-resistant scaffolds that lack the centrosomal proteins CP190, CP60, centrosomin, and γ-tubulin. To clarify the role of these proteins in microtubule nucleation by centrosomes and to identify additional centrosome components required for nucleation, we have developed an in vitro complementation assay for centrosome function. Centrosome aster formation is reconstituted when these inactive, salt-stripped centrosome scaffolds are supplemented with a soluble fraction of a Drosophila embryo extract. The CP60 and CP190 can be removed from this extract without effect, whereas removing the γ-tubulin destroys the complementing activity. Consistent with these results, we find no evidence that these three proteins form a complex together. Instead, γ-tubulin is found in two distinct protein complexes of 240,000 and ∼3,000,000 D. The larger complex, which is analogous to the Xenopus γ-tubulin ring complex (γTuRC) (Zheng, Y., M.L. Wong, B. Alberts, and T. Mitchison. 1995. Nature. 378:578–583), is necessary but not sufficient for complementation. An additional factor found in the extract is required. These results provide the first evidence that the γTuRC is required for microtubule nucleation at the centrosome.     

ATX-2, the C. elegans Ortholog of Human Ataxin-2, Regulates Centrosome Size and Microtubule Dynamics

Centrosomes are critical sites for orchestrating microtubule dynamics, and exhibit dynamic changes in size during the cell cycle. As cells progress to mitosis, centrosomes recruit more microtubules (MT) to form mitotic bipolar spindles that ensure proper chromosome segregation. We report a new role for ATX-2, a C. elegans ortholog of Human Ataxin-2, in regulating centrosome size and MT dynamics. ATX-2, an RNA-binding protein, forms a complex with SZY-20 in an RNA-independent fashion. Depleting ATX-2 results in embryonic lethality and cytokinesis failure, and restores centrosome duplication to zyg-1 mutants. In this pathway, SZY-20 promotes ATX-2 abundance, which inversely correlates with centrosome size. Centrosomes depleted of ATX-2 exhibit elevated levels of centrosome factors (ZYG-1, SPD-5, γ-Tubulin), increasing MT nucleating activity but impeding MT growth. We show that ATX-2 influences MT behavior through γ-Tubulin at the centrosome. Our data suggest that RNA-binding proteins play an active role in controlling MT dynamics and provide insight into the control of proper centrosome size and MT dynamics.     

Deregulated human Cdc14A phosphatase disrupts centrosome separation and chromosome segregation

We show that human Cdc14A phosphatase interacts with interphase centrosomes, and that this interaction is independent of microtubules and Cdc14A phosphatase activity, but requires active nuclear export. Disrupting the nuclear export signal (NES) led to Cdc14A being localized in nucleoli, which in unperturbed cells selectively contain Cdc14B (ref. 1). Conditional overproduction of Cdc14A, but not its phosphatase-dead or NES-deficient mutants, or Cdc14B, resulted in premature centrosome splitting and formation of supernumerary mitotic spindles. In contrast, downregulation of endogenous Cdc14A by short inhibitory RNA duplexes (siRNA) induced mitotic defects including impaired centrosome separation and failure to undergo productive cytokinesis. Consequently, both overexpression and downregulation of Cdc14A caused aberrant chromosome partitioning into daughter cells. These results indicate that Cdc14A is a physiological regulator of the centrosome duplication cycle, which, when disrupted, can lead to genomic instability in mammalian cells.