Integration of the centrosome in cell cycle control, stress response and signal transduction pathways

Pyroptosis leads to loss of centrosomal integrity in macrophages

NLRP3 forms a multiprotein inflammasome complex to initiate the inflammatory response when macrophages sense infection or tissue damage, which leads to caspase-1 activation, maturation and release of the inflammatory cytokines interleukin-1β (IL-1β) and IL-18 and Gasdermin-D (GSDMD) mediated pyroptosis. NLRP3 inflammasome activity must be controlled as unregulated and chronic inflammation underlies inflammatory and autoimmune diseases. Several findings uncovered that NLRP3 inflammasome activity is under the regulation of centrosome localized proteins such as NEK7 and HDAC6, however, whether the centrosome composition or structure is altered during the inflammasome activation is not known. Our data show that levels of the centrosomal scaffold protein pericentrin (PCNT) are reduced upon NLRP3 inflammasome activation via different activators in human and murine macrophages. PCNT loss occurs in the presence of membrane stabilizer punicalagin, suggesting this is not a consequence of membrane rupture. We found that PCNT loss is dependent on NLRP3 and active caspases as MCC950 and pan caspase inhibitor ZVAD prevent its degradation. Moreover, caspase-1 and GSDMD are both required for this NLRP3-mediated PCNT loss because absence of caspase-1 or GSDMD triggers an alternative regulation of PCNT via its cleavage by caspase-3 in response to nigericin stimulation. PCNT degradation occurs in response to nigericin, but also other NLRP3 activators including lysomotropic agent L-Leucyl-L-Leucine methyl ester (LLOMe) and hypotonicity but not AIM2 activation. Our work reveals that the NLRP3 inflammasome activation alters centrosome composition highlighting the need to further understand the role of this organelle during inflammatory responses.

Centrosomal Proteins in Urothelial Tumors: New Pathways in Disease Pathogenesis

This commentary highlights the article by Li et al that identified the centrosomal protein 72 as a biomarker for prognosis of urothelial cancer.

Novel phosphorylation states of the yeast spindle pole body

Phosphorylation regulates yeast spindle pole body (SPB) duplication and separation and likely regulates microtubule nucleation. We report a phosphoproteomic analysis using tandem mass spectrometry of enriched Saccharomyces cerevisiae SPBs for two cell cycle arrests, G1/S and the mitotic checkpoint, expanding on previously reported phosphoproteomic data sets. We present a novel phosphoproteomic state of SPBs arrested in G1/S by a cdc4-1 temperature-sensitive mutation, with particular focus on phosphorylation events on the γ-tubulin small complex (γ-TuSC). The cdc4-1 arrest is the earliest arrest at which microtubule nucleation has occurred at the newly duplicated SPB. Several novel phosphorylation sites were identified in G1/S and during mitosis on the microtubule nucleating γ-TuSC. These sites were analyzed in vivo by fluorescence microscopy and were shown to be required for proper regulation of spindle length. Additionally, in vivo analysis of two mitotic sites in Spc97 found that phosphorylation of at least one of these sites is required for progression through the cell cycle. This phosphoproteomic data set not only broadens the scope of the phosphoproteome of SPBs, it also identifies several γ-TuSC phosphorylation sites that influence microtubule formation.

Summary: A phosphoproteome of yeast spindle pole bodies in G1/S or M phase identifies phosphorylation sites involved in spindle length control and provides direction for future phosphorylation analyses of spindle pole components.

Noncanonical Biogenesis of Centrioles and Basal Bodies

Centrioles and basal bodies (CBBs) organize centrosomes and cilia within eukaryotic cells. These organelles are composed of microtubules and hundreds of proteins performing multiple functions such as signaling, cytoskeleton remodeling, and cell motility. The CBB is present in all branches of the eukaryotic tree of life and, despite its ultrastructural and protein conservation, there is diversity in its function, occurrence (i.e., presence/absence), and modes of biogenesis across species. In this review, we provide an overview of the multiple pathways through which CBBs are formed in nature, with a special focus on the less studied, noncanonical ways. Despite the differences among each mechanism herein presented, we highlighted some of their common principles. These principles, governing different steps of biogenesis, ensure that CBBs may perform a multitude of functions in a huge diversity of organisms but yet retained their robustness in structure throughout evolution.

The Centrosome as the Main Integrator of Endothelial Cell Functional Activity

The centrosome is an intracellular structure of the animal cell responsible for organization of cytoplasmic microtubules. According to modern concepts, the centrosome is a very important integral element of the living cell whose functions are not limited to its ability to polymerize microtubules. The centrosome localization in the geometric center of the interphase cell, the high concentration of various regulatory proteins in this area, the centrosome-organized radial system of microtubules for intracellular transport by motor proteins, the centrosome involvement in the perception of external signals and their transmission - all these features make this cellular structure a unique regulation and distribution center managing dynamic morphology of the animal cell. In conjunction with the tissue-specific features of the centrosome structure, this suggests the direct involvement of the centrosome in execution of cell functions. This review discusses the involvement of the centrosome in the vital activity of endothelial cells, as well as its possible participation in the implementation of barrier function, the major function of endothelium.

The benefits of local depletion: The centrosome as a scaffold for ubiquitin-proteasome-mediated degradation

The centrosome is the major microtubule-organizing center in animal cells but is dispensable for proper microtubule spindle formation in many biological contexts and is thus thought to fulfill additional functions. Recent observations suggest that the centrosome acts as a scaffold for proteasomal degradation in the cell to regulate a variety of biological processes including cell fate acquisition, cell cycle control, stress response, and cell morphogenesis. Here, we review the body of studies indicating a role for the centrosome in promoting proteasomal degradation of ubiquitin-proteasome substrates and explore the functional relevance of this system in different biological contexts. We discuss a potential role for the centrosome in coordinating local degradation of proteasomal substrates, allowing cells to achieve stringent spatiotemporal control over various signaling processes.

JAK2 tyrosine kinase phosphorylates and is negatively regulated by centrosomal protein Ninein

JAK2 is a cytoplasmic tyrosine kinase critical for cytokine signaling. In this study, we have identified a novel centrosome-associated complex containing ninein and JAK2. We have found that active JAK2 localizes around the mother centrioles, where it partly colocalizes with ninein, a protein involved in microtubule (MT) nucleation and anchoring. We demonstrated that JAK2 is an important regulator of centrosome function. Depletion of JAK2 or use of JAK2-null cells causes defects in MT anchoring and increased numbers of cells with mitotic defects; however, MT nucleation is unaffected. We showed that JAK2 directly phosphorylates the N terminus of ninein while the C terminus of ninein inhibits JAK2 kinase activity in vitro. Overexpressed wild-type (WT) or C-terminal (amino acids 1179 to 1931) ninein inhibits JAK2. This ninein-dependent inhibition of JAK2 significantly decreases prolactin- and interferon gamma (IFN-γ)-induced tyrosyl phosphorylation of STAT1 and STAT5. Downregulation of ninein enhances JAK2 activation. These results indicate that JAK2 is a novel member of centrosome-associated complex and that this localization regulates both centrosomal function and JAK2 kinase activity, thus controlling cytokine-activated molecular pathways.

STK31 is a cell-cycle regulated protein that contributes to the tumorigenicity of epithelial cancer cells

Serine/threonine kinase 31 (STK31) is one of the novel cancer/testis antigens for which its biological functions remain largely unclear. Here, we demonstrate that STK31 is overexpressed in many human colorectal cancer cell lines and tissues. STK31 co-localizes with pericentrin in the centrosomal region throughout all phases of the cell cycle. Interestingly, when cells undergo mitosis, STK31 also localizes to the centromeres, central spindle, and midbody. This localization behavior is similar to that of chromosomal passenger proteins, which are known to be the important players of the spindle assembly checkpoint. The expression of STK31 is cell cycle-dependent through the regulation of a putative D-box near its C-terminal region. Ectopically-expressed STK31-GFP increases cell migration and invasive ability without altering the proliferation rate of cancer cells, whereas the knockdown expression of endogenous STK31 by lentivirus-derived shRNA results in microtubule assembly defects that prolong the duration of mitosis and lead to apoptosis. Taken together, our results suggest that the aberrant expression of STK31 contributes to tumorigenicity in somatic cancer cells. STK31 might therefore act as a potential therapeutic target in human somatic cancers.

CEP proteins: the knights of centrosome dynasty

Centrosome forms the backbone of cell cycle progression mechanism. Recent debates have occurred regarding the essentiality of centrosome in cell cycle regulation. CEP family protein is the active component of centrosome and plays a vital role in centriole biogenesis and cell cycle progression control. A total of 31 proteins have been categorized into CEP family protein category and many more are under candidate evaluation. Furthermore, by the recent advancements in genomics and proteomics researches, several new CEP proteins have also been characterized. Here we have summarized the importance of CEP family proteins and their regulation mechanism involved in proper cell cycle progression. Further, we have reviewed the detailed molecular mechanism behind the associated pathological phenotypes and the possible therapeutic approaches. Proteins such as CEP57, CEP63, CEP152, CEP164, and CEP215 have been extensively studied with a detailed description of their molecular mechanisms, which are among the primary targets for drug discovery. Moreover, CEP27, CEP55, CEP70, CEP110, CEP120, CEP135, CEP192, CEP250, CEP290, and CEP350 also seem promising for future drug discovery approaches. Since the overview implicates that the overall researches on CEP proteins are not yet able to present significant details required for effective therapeutics development, thus, it is timely to discuss the importance of future investigations in this field.

LGALS3BP regulates centriole biogenesis and centrosome hypertrophy in cancer cells

Centrosome morphology and number are frequently deregulated in cancer cells. Here, to identify factors that are functionally relevant for centrosome abnormalities in cancer cells, we established a protein-interaction network around 23 centrosomal and cell-cycle regulatory proteins, selecting the interacting proteins that are deregulated in cancer for further studies. One of these components, LGALS3BP, is a centriole- and basal body-associated protein with a dual role, triggering centrosome hypertrophy when overexpressed and causing accumulation of centriolar substructures when downregulated. The cancer cell line SK-BR-3 that overexpresses LGALS3BP exhibits hypertrophic centrosomes, whereas in seminoma tissues with low expression of LGALS3BP, supernumerary centriole-like structures are present. Centrosome hypertrophy is reversed by depleting LGALS3BP in cells endogenously overexpressing this protein, supporting a direct role in centrosome aberration. We propose that LGALS3BP suppresses assembly of centriolar substructures, and when depleted, causes accumulation of centriolar complexes comprising CPAP, acetylated tubulin and centrin.

CCDC41 is required for ciliary vesicle docking to the mother centriole

The initiation of primary cilium assembly entails the docking of ciliary vesicles presumably derived from the Golgi complex to the distal end of the mother centriole. Distal appendages, which anchor the mother centriole to the plasma membrane, are thought to be involved in the docking process. However, little is known about the molecular players and mechanisms that mediate the vesicle-centriole association. Here we report that coiled-coil domain containing 41 (CCDC41) is required for the docking of ciliary vesicles. CCDC41 specifically localizes to the distal end of the mother centriole and interacts with centrosomal protein 164 (Cep164), a distal appendage component. In addition, a pool of CCDC41 colocalizes with intraflagellar transport protein 20 (IFT20) subunit of the intraflagellar transport particle at the Golgi complex. Remarkably, knockdown of CCDC41 inhibits the recruitment of IFT20 to the centrosome. Moreover, depletion of CCDC41 or IFT20 inhibits ciliogenesis at the ciliary vesicle docking step, whereas intraflagellar transport protein 88 (IFT88) depletion interferes with later cilium elongation steps. Our results suggest that CCDC41 collaborates with IFT20 to support the vesicle-centriole association at the onset of ciliogenesis.

Clinical implication of centrosome amplification and expression of centrosomal functional genes in multiple myeloma

Background

Multiple myeloma (MM) is a low proliferative tumor of postgerminal center plasma cell (PC). Centrosome amplification (CA) is supposed to be one of the mechanisms leading to chromosomal instability. Also, CA is associated with deregulation of cell cycle, mitosis, DNA repair and proliferation. The aim of our study was to evaluate the prognostic significance and possible role of CA in pathogenesis and analysis of mitotic genes as mitotic disruption markers.

Design and methods

A total of 173 patients were evaluated for this study. CD138+ cells were separated by MACS. Immunofluorescent labeling of centrin was used for evaluation of centrosome amplification in PCs. Interphase FISH with cytoplasmic immunoglobulin light chain staining (cIg FISH) and qRT-PCR were performed on PCs.

Results

Based on the immunofluorescent staining results, all patients were divided into two groups: CA positive (38.2%) and CA negative (61.8%). Among the newly diagnosed patients, worse overall survival was indicated in the CA negative group (44/74) in comparison to the CA positive group (30/74) (P = 0.019).

Gene expression was significantly down-regulated in the CA positive group in comparison to CA negative in the following genes: AURKB, PLK4, TUBG1 (P < 0.05). Gene expression was significantly down-regulated in newly diagnosed in comparison to relapsed patients in the following genes: AURKA, AURKB, CCNB1, CCNB2, CETN2, HMMR, PLK4, PCNT, and TACC3 (P < 0.05).

Conclusions

Our findings indicate better prognosis for CA positive newly diagnosed patients. Considering revealed clinical and gene expression heterogeneity between CA negative and CA positive patients, there is a possibility to characterize centrosome amplification as a notable event in multiple myeloma pathogenesis.

Functional analysis of centrosomal kinase substrates in Drosophila melanogaster reveals a new function of the nuclear envelope component otefin in cell cycle progression

Phosphorylation is one of the key mechanisms that regulate centrosome biogenesis, spindle assembly, and cell cycle progression. However, little is known about centrosome-specific phosphorylation sites and their functional relevance. Here, we identified phosphoproteins of intact Drosophila melanogaster centrosomes and found previously unknown phosphorylation sites in known and unexpected centrosomal components. We functionally characterized phosphoproteins and integrated them into regulatory signaling networks with the 3 important mitotic kinases, cdc2, polo, and aur, as well as the kinase CkIIβ. Using a combinatorial RNA interference (RNAi) strategy, we demonstrated novel functions for P granule, nuclear envelope (NE), and nuclear proteins in centrosome duplication, maturation, and separation. Peptide microarrays confirmed phosphorylation of identified residues by centrosome-associated kinases. For a subset of phosphoproteins, we identified previously unknown centrosome and/or spindle localization via expression of tagged fusion proteins in Drosophila SL2 cells. Among those was otefin (Ote), an NE protein that we found to localize to centrosomes. Furthermore, we provide evidence that it is phosphorylated in vitro at threonine 63 (T63) through Aurora-A kinase. We propose that phosphorylation of this site plays a dual role in controlling mitotic exit when phosphorylated while dephosphorylation promotes G(2)/M transition in Drosophila SL2 cells.

γ-Tubulin plays a key role in inactivating APC/C(Cdh1) at the G(1)-S boundary

Failure to inactivate APC/C^CdhA at the G₁–S boundary of the cell cycle as a result of a γ-tubulin mutation that disrupts the APC/C^CdhA localization prevents cell cycle progression.

A γ-tubulin mutation in Aspergillus nidulans, mipA-D159, causes failure of inactivation of the anaphase-promoting complex/cyclosome (APC/C) in interphase, resulting in failure of cyclin B (CB) accumulation and removal of nuclei from the cell cycle. We have investigated the role of CdhA, the A. nidulans homologue of the APC/C activator protein Cdh1, in γ-tubulin–dependent inactivation of the APC/C. CdhA was not essential, but it targeted CB for destruction in G₁, and APC/C^CdhA had to be inactivated for the G₁–S transition. mipA-D159 altered the localization pattern of CdhA, and deletion of the gene encoding CdhA allowed CB to accumulate in all nuclei in strains carrying mipA-D159. These data indicate that mipA-D159 causes a failure of inactivation of APC/C^CdhA at G₁–S, perhaps by altering its localization to the spindle pole body, and, thus, that γ-tubulin plays an important role in inactivating APC/C^CdhA at this point in the cell cycle.

Effects of seawater acidification on cell cycle control mechanisms in Strongylocentrotus purpuratus embryos

Previous studies have shown fertilization and development of marine species can be significantly inhibited when the pH of sea water is artificially lowered. Little mechanistic understanding of these effects exists to date, but previous work has linked developmental inhibition to reduced cleavage rates in embryos. To explore this further, we tested whether common cell cycle checkpoints were involved using three cellular biomarkers of cell cycle progression: (1) the onset of DNA synthesis, (2) production of a mitotic regulator, cyclin B, and (3) formation of the mitotic spindle. We grew embryos of the purple sea urchin, Strongylocentrotus purpuratus, in seawater artifically buffered to a pH of ∼7.0, 7.5, and 8.0 by CO(2) infusion. Our results suggest the reduced rates of mitotic cleavage are likely unrelated to common cell cycle checkpoints. We found no significant differences in the three biomarkers assessed between pH treatments, indicating the embryos progress through the G(1)/S, G(2)/M and metaphase/anaphase transitions at relatively similar rates. These data suggest low pH environments may not impact developmental programs directly, but may act through secondary mechanisms such as cellular energetics.

Gene ontology analysis of the centrosome proteomes of Drosophila and human

The centrosome is a complex cell organelle in higher eukaryotic cells that functions in microtubule organization and is integrated into major cellular signaling pathways.1-3 For example, a tight link exists between cell cycle regulation and centrosome duplication, as centrosome numbers must be precisely controlled to ensure high fidelity of chromosome segregation.4 The analysis of the centrosome's protein composition provides the opportunity for a better understanding of centrosome function and to identify possible links to cellular signaling pathways.5,6 Our proteomics study of the Drosophila centrosome recently identified 251 centrosome candidate proteins that we subsequently characterized by RNAi in Drosophila SL2 cells and classified according to their function in centrosome duplication/segregation, structure maintenance and cell cycle regulation.7 Interestingly, functional characterization of their human orthologous proteins revealed the highest functional conservation in the process of centrosome duplication and separation. To analyze functional and biochemical interdependencies further, we carried out an analysis of the gene ontology (GO) annotation of the identified Drosophila centrosome proteins, as well as of the human centrosome proteome.5 The GO analysis of the group of proteins that did not show a centrosome, chromosome segregation or cell cycle related phenotype in our RNAi assays suggests that these molecules may constitute linker proteins to other cellular signaling pathways. Furthermore, the results of our GO analysis of components of the human and of the Drosophila centrosome reflect the somatic and embryonic origin, respectively, of the isolated centrosomes, implicating the Drosophila centrosome proteins in developmental signaling and cell differentiation.

Proteomic and functional analysis of the mitotic Drosophila centrosome

Regulation of centrosome structure, duplication and segregation is integrated into cellular pathways that control cell cycle progression and growth. As part of these pathways, numerous proteins with well-established non-centrosomal localization and function associate with the centrosome to fulfill regulatory functions. In turn, classical centrosomal components take up functional and structural roles as part of other cellular organelles and compartments. Thus, although a comprehensive inventory of centrosome components is missing, emerging evidence indicates that its molecular composition reflects the complexity of its functions. We analysed the Drosophila embryonic centrosomal proteome using immunoisolation in combination with mass spectrometry. The 251 identified components were functionally characterized by RNA interference. Among those, a core group of 11 proteins was critical for centrosome structure maintenance. Depletion of any of these proteins in Drosophila SL2 cells resulted in centrosome disintegration, revealing a molecular dependency of centrosome structure on components of the protein translation machinery, actin- and RNA-binding proteins. In total, we assigned novel centrosome-related functions to 24 proteins and confirmed 13 of these in human cells.

Gamma-tubulin regulates the anaphase-promoting complex/cyclosome during interphase

Activation of the APC/C requires microtubule-nucleating independent aspects of γ-tubulin function.

A cold-sensitive γ-tubulin allele of Aspergillus nidulans, mipAD159, causes defects in mitotic and cell cycle regulation at restrictive temperatures that are apparently independent of microtubule nucleation defects. Time-lapse microscopy of fluorescently tagged mitotic regulatory proteins reveals that cyclin B, cyclin-dependent kinase 1, and the Ancdc14 phosphatase fail to accumulate in a subset of nuclei at restrictive temperatures. These nuclei are permanently removed from the cell cycle, whereas other nuclei, in the same multinucleate cell, cycle normally, accumulating and degrading these proteins. After each mitosis, additional daughter nuclei fail to accumulate these proteins, resulting in an increase in noncycling nuclei over time and consequent inhibition of growth. Extensive analyses reveal that these noncycling nuclei result from a nuclear autonomous, microtubule-independent failure of inactivation of the anaphase-promoting complex/cyclosome. Thus, γ-tubulin functions to regulate this key mitotic and cell cycle regulatory complex.

Role of Filia, a maternal effect gene, in maintaining euploidy during cleavage-stage mouse embryogenesis

During oogenesis, mammalian eggs accumulate proteins required for early embryogenesis. Although limited data suggest a vital role of these maternal factors in chromatin reprogramming and embryonic genome activation, the full range of their functions in preimplantation development remains largely unknown. Here we report a role for maternal proteins in maintaining chromosome stability and euploidy in early-cleavage mouse embryogenesis. Filia, expressed in growing oocytes, encodes a protein that binds to MATER and participates in a subcortical maternal complex essential for cleavage-stage embryogenesis. The depletion of maternal stores of Filia impairs preimplantation embryo development with a high incidence of aneuploidy that results from abnormal spindle assembly, chromosome misalignment, and spindle assembly checkpoint (SAC) inactivation. In helping to ensure normal spindle morphogenesis, Filia regulates the proper allocation of the key spindle assembly regulators (i.e., AURKA, PLK1, and gamma-tubulin) to the microtubule-organizing center via the RhoA signaling pathway. Concurrently, Filia is required for the placement of MAD2, an essential component of the SAC, to kinetochores to enable SAC function. Thus, Filia is central to integrating the spatiotemporal localization of regulators that helps ensure euploidy and high-quality cell cycle progression in preimplantation mouse development. Defects in the well-conserved human homologue could play a similar role and account for recurrent human fetal wastage.

The expression of tubulin polymerization promoting protein TPPP/p25alpha is developmentally regulated in cultured rat brain oligodendrocytes and affected by proteolytic stress

The tubulin polymerization-promoting protein (TPPP)/p25alpha was identified as a brain specific protein, is associated with microtubules (MTs) in vitro and can promote abnormal MT assembly. Furthermore it has aggregation promoting properties and is a constituent in pathological protein deposits of neurodegenerative diseases. In the brain, TPPP/p25alpha is present in myelinating oligodendrocytes. Here we show, using cultured rat brain oligodendrocytes, that TPPP/p25alpha expression is increasing during development in culture, and particularly in immature cells is associated with the centrosome. MT binding properties in oligodendrocytes are rather low, however, when MTs are disassembled by nocodazole, TPPP/p25alpha accumulates in the perinuclear region. Treatment of oligodendrocytes with the proteasomal inhibitor MG-132 (1 micaroM; 18 h) caused an increase in the amount of TPPP/p25alpha by about 40%, a decrease in its solubility, and led to the appearance of TPPP/p25alpha-positive cytoplasmic inclusions, which stained with thioflavin S and resembled inclusion bodies. Hence, it might be speculated that acute or chronic malfunction of the proteasomal degradation system, leading to the accumulation of aggregation prone proteins and the pro-aggregatory protein TPPP/p25alpha or to the aggregation of TPPP/p25alpha on its own, is causally related to the protein aggregation process in a variety of neurodegenerative diseases.

Ptpcd-1 is a novel cell cycle related phosphatase that regulates centriole duplication and cytokinesis

Proper progression of mitosis requires spatio-temporal regulation of protein phosphorylation by orchestrated activities of kinases and phosphatases. Although many kinases, such as Aurora kinases, polo-like kinases (Plks), and cyclin B-Cdk1 are relatively well characterized in the context of their physiological functions at mitosis and regulation of their enzymatic activities during mitotic progression, phosphatases involved are largely unknown. Here we identified a novel protein tyrosine phosphatase containing domain 1 (Ptpcd 1) as a mitotic phosphatase, which shares sequence homology to Cdc14. Immunofluorescence studies revealed that Ptpcd1 partially colocalized with gamma-tubulin, an archetypical centrosomal marker. Overexpression of this phosphatase prevented unscheduled centrosomal amplification in hydroxyurea arrested U2OS cells. Intriguingly, Ptpcd 1-associated and colocalized with polo-like kinase 1(Plk1). Hence, overexpression of Ptpcd1 rescued prometaphase arrest of Plk-1 depleted cells, but resulted in aberrant cytokinesis as did as Plk1 overexpression. These results suggested that Ptpcd1 is involved in centrosomal duplication and cytokinesis.

The sphingosine 1-phosphate receptor 5 and sphingosine kinases 1 and 2 are localised in centrosomes: possible role in regulating cell division

We show here that the endogenous sphingosine 1-phosphate 5 receptor (S1P(5), a G protein coupled receptor (GPCR) whose natural ligand is sphingosine 1-phosphate (S1P)) and sphingosine kinases 1 and 2 (SK1 and SK2), which catalyse formation of S1P, are co-localised in the centrosome of mammalian cells, where they may participate in regulating mitosis. The centrosome is a site for active GTP-GDP cycling involving the G-protein, G(i) and tubulin, which are required for spindle pole organization and force generation during cell division. Therefore, the presence of S1P(5) (which normally functions as a plasma membrane guanine nucleotide exchange factor, GEF) and sphingosine kinases in the centrosome might suggest that S1P(5) may function as a ligand activated GEF in regulating G-protein-dependent spindle formation and mitosis. The addition of S1P to cells inhibits trafficking of S1P(5) to the centrosome, suggesting a dynamic shuttling endocytic mechanism controlled by ligand occupancy of cell surface receptor. We therefore propose that the centrosomal S1P(5) receptor might function as an intracellular target of S1P linked to regulation of mitosis.

Fluorescence imaging of the centrosome cycle in mammalian cells

The formation of a bipolar spindle is essential for the equal segregation of duplicated DNA into two daughter cells during mitosis. Spindle bipolarity is largely dependent on the mitotic cell possessing two centrosomes that can each establish one spindle pole. The centrosome is also now known to regulate many other aspects of cell cycle progression, including G1/S progression, spindle orientation and symmetry, cytokinesis, and checkpoint signalling. As a result, defects in centrosome arrangement or number can lead to loss of cell polarity, defective cell division, and abnormal chromosome segregation, all events that are typical of cancer cells. Indeed, cancer cells often exhibit overduplicated centrosomes and multipolar spindles. Here, we outline a number of fluorescence imaging methodologies that can be used to study events of the centrosome duplication cycle, as well as the dynamics of individual centrosome proteins. Specifically, we discuss the generation and imaging of cell lines with fluorescently labelled centrosomes, the use of photobleaching methods to measure the dynamics of centrosome proteins, and assays for observing centrosome overduplication and centrosome separation in fixed and live cells. These experimental approaches can provide important information on the regulation of centrosomes, their role in normal cell cycle progression and how their deregulation might contribute to the deleterious phenotypes of malignant cancer cells.

Identification of a novel centrosomal protein CrpF46 involved in cell cycle progression and mitosis

A novel centrosome-related protein CrpF46 was detected using a serum F46 from a patient suffering from progressive systemic sclerosis. We identified the protein by immunoprecipitation and Western blotting followed by tandem mass spectrometry sequencing. The protein CrpF46 has an apparent molecular mass of ~60 kDa, is highly homologous to a 527 amino acid sequence of the C-terminal portion of the protein Golgin-245, and appears to be a splice variant of Golgin-245. Immunofluorescence microscopy of synchronized HeLa cells labeled with an anti-CrpF46 monoclonal antibody revealed that CrpF46 localized exclusively to the centrosome during interphase, although it dispersed throughout the cytoplasm at the onset of mitosis. Domain analysis using CrpF46 fragments in GFP-expression vectors transformed into HeLa cells revealed that centrosomal targeting is conferred by a C-terminal coiled-coil domain. Antisense CrpF46 knockdown inhibited cell growth and proliferation and the cell cycle typically stalled at S phase. The knockdown also resulted in the formation of poly-centrosomal and multinucleate cells, which finally became apoptotic. These results suggest that CrpF46 is a novel centrosome-related protein that associates with the centrosome in a cell cycle-dependent manner and is involved in the progression of the cell cycle and M phase mechanism.

PACSIN proteins bind tubulin and promote microtubule assembly

PACSINs are intracellular adapter proteins involved in vesicle transport, membrane dynamics and actin reorganisation. In this study, we report a novel role for PACSIN proteins as components of the centrosome involved in microtubule dynamics. Glutathione S-transferase (GST)-tagged PACSIN proteins interacted with protein complexes containing alpha- and gamma-tubulin in brain homogenate. Analysis of cell lysates showed that all three endogenous PACSINs co-immunoprecipitated dynamin, alpha-tubulin and gamma-tubulin. Furthermore, PACSINs bound only to unpolymerised tubulin, not to microtubules purified from brain. In agreement, the cellular localisation of endogenous PACSIN 2 was not affected by the microtubule depolymerising reagent nocodazole. By light microscopy, endogenous PACSIN 2 localised next to gamma-tubulin at purified centrosomes from NIH 3T3 cells. Finally, reduction of PACSIN 2 protein levels with small-interfering RNA (siRNA) resulted in impaired microtubule nucleation from centrosomes, whereas microtubule centrosome splitting was not affected, suggesting a role for PACSIN 2 in the regulation of tubulin polymerisation. These findings suggest a novel function for PACSIN proteins in dynamic microtubuli nucleation.

Centrosomal localization of DNA damage checkpoint proteins

During mitosis, the phosphatidylinositol-3 (PI-3) family-related DNA damage checkpoint kinases ATM and ATR were found on the centrosomes of human cells. ATRIP, an interaction partner of ATR, as well as Chk1 and Chk2, the downstream targets of ATR or ATM, were also localized to the centrosomes. Surprisingly, the DNA-PK inhibitor vanillin enhanced the level of ATM on centrosomes. Accordingly, DNA-PKcs, the catalytic subunit of DNA-PK, was also found on the centrosomes. Vanillin altered the phosphorylation of Chk2 in the centrosomes and in whole cell extracts. Nucleoplasmic ATM co-immunoprecipitated with Ku70/86, the DNA binding subunits of DNA-PK, while vanillin diminished this association. Vanillin did not affect microtubule polymerization at the centrosomes but, surprisingly, caused a transient enhancement of alpha-tubulin foci in the nucleus. Interestingly, gamma-tubulin was also present in the nucleus and co-immunoprecipitated with ATR or BRCA1. DNA damage led to a reduction of the mentioned checkpoint proteins on the centrosomes but increased the level of gamma-tubulin at this organelle. Taken together, these results indicate that DNA damage checkpoint proteins may control the formation of gamma-tubulin and/or the kinetics of microtubule formation at the centrosomes, and thereby couple them to the DNA damage response.

Anchoring of protein kinase A-regulatory subunit IIalpha to subapically positioned centrosomes mediates apical bile canalicular lumen development in response to oncostatin M but not cAMP

Oncostatin M and cAMP signaling stimulate apical surface-directed membrane trafficking and apical lumen development in hepatocytes, both in a protein kinase A (PKA)-dependent manner. Here, we show that oncostatin M, but not cAMP, promotes the A-kinase anchoring protein (AKAP)-dependent anchoring of the PKA regulatory subunit (R)IIalpha to subapical centrosomes and that this requires extracellular signal-regulated kinase 2 activation. Stable expression of the RII-displacing peptide AKAP-IS, but not a scrambled peptide, inhibits the association of RIIalpha with centrosomal AKAPs and results in the repositioning of the centrosome from a subapical to a perinuclear location. Concomitantly, common endosomes, but not apical recycling endosomes, are repositioned from a subapical to a perinuclear location, without significant effects on constitutive or oncostatin M-stimulated basolateral-to-apical transcytosis. Importantly, however, the expression of the AKAP-IS peptide completely blocks oncostatin M-, but not cAMP-stimulated apical lumen development. Together, the data suggest that centrosomal anchoring of RIIalpha and the interrelated subapical positioning of these centrosomes is required for oncostatin M-, but not cAMP-mediated, bile canalicular lumen development in a manner that is uncoupled from oncostatin M-stimulated apical lumen-directed membrane trafficking. The results also imply that multiple PKA-mediated signaling pathways control apical lumen development and that subapical centrosome positioning is important in some of these pathways.

RINT-1 serves as a tumor suppressor and maintains Golgi dynamics and centrosome integrity for cell survival

Faithful mitotic partitioning of the Golgi apparatus and the centrosome is critical for proper cell division. Although these two cytoplasmic organelles are probably coordinated during cell division, supporting evidence of this coordination is still largely lacking. Here, we show that the RAD50-interacting protein, RINT-1, is localized at the Golgi apparatus and the centrosome in addition to the endoplasmic reticulum. To examine the biological roles of RINT-1, we found that the homozygous deletion of Rint-1 caused early embryonic lethality at embryonic day 5 (E5) to E6 and the failure of blastocyst outgrowth ex vivo. About 81% of the Rint-1 heterozygotes succumbed to multiple tumor formation with haploinsufficiency during their average life span of 24 months. To pinpoint the cellular function of RINT-1, we found that RINT-1 depletion by RNA interference led to the loss of the pericentriolar positioning and dispersal of the Golgi apparatus and concurrent centrosome amplification during the interphase. Upon mitotic entry, RINT-1-deficient cells exhibited multiple abnormalities, including aberrant Golgi dynamics during early mitosis and defective reassembly at telophase, increased formation of multiple spindle poles, and frequent chromosome missegregation. Mitotic cells often underwent cell death in part due to the overwhelming cellular defects. Taken together, these findings suggest that RINT-1 serves as a novel tumor suppressor essential for maintaining the dynamic integrity of the Golgi apparatus and the centrosome, a prerequisite to their proper coordination during cell division.

A centrosome-independent role for gamma-TuRC proteins in the spindle assembly checkpoint

The spindle assembly checkpoint guards the fidelity of chromosome segregation. It requires the close cooperation of cell cycle regulatory proteins and cytoskeletal elements to sense spindle integrity. The role of the centrosome, the organizing center of the microtubule cytoskeleton, in the spindle checkpoint is unclear. We found that the molecular requirements for a functional spindle checkpoint included components of the large gamma-tubulin ring complex (gamma-TuRC). However, their localization at the centrosome and centrosome integrity were not essential for this function. Thus, the spindle checkpoint can be activated at the level of microtubule nucleation.

Cyclin G2 is a centrosome-associated nucleocytoplasmic shuttling protein that influences microtubule stability and induces a p53-dependent cell cycle arrest

Cyclin G2 is an atypical cyclin that associates with active protein phosphatase 2A. Cyclin G2 gene expression correlates with cell cycle inhibition; it is significantly upregulated in response to DNA damage and diverse growth inhibitory stimuli, but repressed by mitogenic signals. Ectopic expression of cyclin G2 promotes cell cycle arrest, cyclin dependent kinase 2 inhibition and the formation of aberrant nuclei [Bennin, D. A., Don, A. S., Brake, T., McKenzie, J. L., Rosenbaum, H., Ortiz, L., DePaoli-Roach, A. A., and Horne, M. C. (2002). Cyclin G2 associates with protein phosphatase 2A catalytic and regulatory B' subunits in active complexes and induces nuclear aberrations and a G(1)/S-phase cell cycle arrest. J Biol Chem 277, 27449-67]. Here we report that endogenous cyclin G2 copurifies with centrosomes and microtubules (MT) and that ectopic G2 expression alters microtubule stability. We find exogenous and endogenous cyclin G2 present at microtubule organizing centers (MTOCs) where it colocalizes with centrosomal markers in a variety of cell lines. We previously reported that cyclin G2 forms complexes with active protein phosphatase 2A (PP2A) and colocalizes with PP2A in a detergent-resistant compartment. We now show that cyclin G2 and PP2A colocalize at MTOCs in transfected cells and that the endogenous proteins copurify with isolated centrosomes. Displacement of the endogenous centrosomal scaffolding protein AKAP450 that anchors PP2A at the centrosome resulted in the depletion of centrosomal cyclin G2. We find that ectopic expression of cyclin G2 induces microtubule bundling and resistance to depolymerization, inhibition of polymer regrowth from MTOCs and a p53-dependent cell cycle arrest. Furthermore, we determined that a 100 amino acid carboxy-terminal region of cyclin G2 is sufficient to both direct GFP localization to centrosomes and induce cell cycle inhibition. Colocalization of endogenous cyclin G2 with only one of two GFP-centrin-tagged centrioles, the mature centriole present at microtubule foci, indicates that cyclin G2 resides primarily on the mother centriole. Copurification of cyclin G2 and PP2A subunits with microtubules and centrosomes, together with the effects of ectopic cyclin G2 on cell cycle progression, nuclear morphology and microtubule growth and stability, suggests that cyclin G2 may modulate the cell cycle and cellular division processes through modulation of PP2A and centrosomal associated activities.

Interaction between HP1alpha and replication proteins in mammalian cells

HP1 is an essential heterochromatin-associated protein known to play an important role in the organization of heterochromatin as well as in the transcriptional regulation of heterochromatic and euchromatic genes both in repression and activation. Using the yeast two-hybrid system and immunoprecipitation, we report here that murine HP1alpha interacts with the preRC proteins ORC1, ORC2 and CDC6. Immunofluorescence staining and EGFP/DsRed fusion proteins revealed a colocalization of HP1alpha with ORC1, ORC2 and CDC6 in heterochromatin, supporting the notion that ORC and probably CDC6 play an important role in murine HP1alpha function. Besides that, we also observed a colocalization of HP1alpha with gamma-tubulin suggesting a centrosomal localization of HP1alpha in murine cells. To gain insight into HP1alpha function, we applied the RNAi technique. Depletion of HP1alpha leads to a slow down of cell proliferation, an aberrant cell cycle progression as well as to multinucleated cells with insufficiently organized microtubule. These results together indicate that HP1alpha exerts functions in mitosis and cytokinesis.

The centrosome and the DNA damage induced checkpoint

The centrosome, the microtubule-organizing center of the cell, acts as a localization point, where signaling molecules are able to interact. Many kinases and phosphatases critical for regulation of DNA damage signaling pathways localize to the centrosome. This review will discuss the possible involvement of the centrosome in mediating DNA damage checkpoint control, in particular the effect of DNA damage signaling pathways involved in initiation or maintenance of cell cycle arrest on the centrosome. The mechanisms that lead to centrosome abnormalities such as centrosome hyperamplification and multipolarity in response to DNA damage will also be addressed.

Induction of Centrosome Amplification during Arsenite-Induced Mitotic Arrest in CGL-2 Cells

Arsenite-induced mitotic abnormalities result in mitotic death in several cancer cell lines. However, how arsenite induces these effects is not known. We have previously shown that arsenite induces mitotic arrest, mitotic abnormalities, and mitotic death in CGL-2 cells. To further delineate the mechanism of action of arsenite, we examined its effect on centrosome duplication and the possible link between centrosome dysregulation and arsenite-induced mitotic death. Immunofluorescence staining of gamma-tubulin revealed that centrosome amplification was induced in arsenite-arrested mitotic cells but not in nocodazole-arrested cells. When S phase-enriched cells were treated with arsenite, they progressed into and arrested at mitosis and then formed supernumerary centrosomes. A further increase in arsenite-induced centrosome amplification was seen during the prolonged mitotic arrest. The arsenite-induced supernumerary centrosomes might result from uneven fragmentation of centrosome, overexpression of pericentriolar materials, and inhibition of centrosomal coalescence during mitosis. Furthermore, termination of mitotic arrest by treatment of arsenite-arrested mitotic cells with cyclin-dependent kinase 1 inhibitors or by suppression of spindle checkpoint function by small interfering RNA-mediated silencing of BubR1 or Mad2 markedly reduced the induction of centrosome amplification and mitotic death in arsenite-treated cells. These results indicate that centrosome amplification is induced in arsenite-arrested mitotic CGL-2 cells in a spindle checkpoint-dependent manner and is involved in the induction of arsenite-induced mitotic death.

The Syk tyrosine kinase localizes to the centrosomes and negatively affects mitotic progression

We showed previously that the spleen tyrosine kinase Syk is expressed by mammary epithelial cells and that it suppresses malignant growth of breast cancer cells. The exact molecular mechanism of its tumor-suppressive activity remains, however, to be identified. Here, we show that Syk colocalizes and copurifies with the centrosomal component gamma-tubulin and exhibits a catalytic activity within the centrosomes. Moreover, its centrosomal localization depends on its intact kinase activity. Centrosomal Syk expression is persistent in interphase but promptly drops during mitosis, obviously resulting from its ubiquitinylation and proteasomal degradation. Conversely, unrestrained exogenous expression of a fluorescently tagged Discosoma sp. red fluorescent protein (DsRed)-Syk chimera engenders abnormal cell division and cell death. Transient DsRed-Syk overexpression triggers an abrupt cell death lacking hallmarks of classic apoptosis but reminiscent of mitotic catastrophe. Surviving stable DsRed-Syk-transfected cells exhibit multipolar mitotic spindles and contain multiple abnormally sized nuclei and supernumerary centrosomes, revealing anomalous cell division. Taken together, these results show that Syk is a novel centrosomal kinase that negatively affects cell division. Its expression is strictly controlled in a spatiotemporal manner, and centrosomal Syk levels need to decline to allow customary progression of mitosis.

The novel centrosomal associated protein CEP55 is present in the spindle midzone and the midbody

Centrosomes are the major microtubule nucleating center in the cell; they also contribute to spindle pole organization and play a role in cell cycle progression as well as completing cytokinesis. Here we describe the molecular characterization of a novel human gene, CEP55, located in 10q23.33 that is expressed in multiple tissues and various cancer cell lines. Sequence analysis of the cDNA predicted a protein of 464 amino acids with several putative coiled-coil domains that are responsible for protein-protein interactions. Indeed, we found homodimerization of CEP55 by coimmunoprecipitation. Subcellular localization analysis revealed that endogenous CEP55 as well as an EGFP-CEP55 fusion protein is present at the centrosome throughout mitosis, whereas it also appears at the cleavage furrow in late anaphase and in the midbody in cytokinesis. Neither nocodazole nor taxol interfered with centrosome association of endogenous CEP55, suggesting that it directly interacts with centrosome components rather than with microtubules. In microtubule regrowth assays, overexpression of CEP55 did not enhance or inhibit microtubule nucleation. Together, these data suggest a possible involvement of CEP55 in centrosome-dependent cellular functions, such as centrosome duplication and/or cell cycle progression, or in the regulation of cytokinesis.

Loading and unloading: orchestrating centrosome duplication and spindle assembly by Ran/Crm1

The cell cycle is an intricate process of DNA replication and cell division that concludes with the formation of two genetically equivalent daughter cells. In this progression, the centrosome is duplicated once and only once during the G1/S transition to produce the bipolar spindle and ensure proper chromosome segregation. The presence of multiple centrosomes in cancer cells suggests that this process is mis-regulated during carcinogenesis. This suggests that certain factors exist that license the progression of centrosome duplication and serve to inhibit further duplications during a single cell cycle. Recent studies suggest that the Ran/Crm1 complex not only regulates nucleocytoplasmic transport but is also independently involved in mitotic spindle assembly. Factors that are capable of interacting with Ran/Crm1 through their nuclear export sequences, such as cyclins/cdks, p53 and Brca1/2, may potentially function as centrosome checkpoints akin to the G1/S and G2/M checkpoints of the cell cycle. Our recent findings indicate that nucleophosmin, a protein whose trafficking is mediated by the Ran/Crm1 network, is one of such checkpoint factors for maintaining proper centrosome duplication. We propose that Ran/Crm1 may act as a 'loading dock' to coordinate various checkpoint factors in regulating the fidelity of centrosome duplication during cell cycle progression, and the disruption of these processes may lead to genomic instability and an acceleration of oncogenesis.

Spindle pole fragmentation due to proteasome inhibition

During interphase, the centrosome concentrates cell stress response molecules, including chaperones and proteasomes, into a proteolytic center. However, whether the centrosome functions as proteolytic center during mitosis is not known. In this study, cultured mammalian cells were treated with the proteasome inhibitor MG 132 and spindle morphology in mitotic cells was characterized in order to address this issue. Proteasome inhibition during mitosis leads to the formation of additional asters that cause the assembly of multipolar spindles. The cause of this phenomenon was investigated by inhibiting microtubule-based transport and protein synthesis. These experimental conditions prevented the formation of supernumerary asters during mitosis. In addition, the expression of dsRed without proteasome inhibition led to the fragmentation of spindle poles. These experiments showed that the formation of extra asters depends on intact microtubule-based transport and protein synthesis. These results suggest that formation of supernumerary asters is due to excessive accumulation of proteins at the spindle poles and consequently fragmentation of the centrosome. Together, this leads to the conclusion that the centrosome functions as proteolytic center during mitosis and proteolytic activity at the spindle poles is necessary for maintaining spindle pole integrity.

Two-way traffic: centrosomes and the cell cycle

The well recognized activities of the mammalian centrosome--microtubule nucleation, duplication, and organization of the primary cilium--are under the control of the cell cycle. However, the centrosome is more than just a follower of the cell cycle; it can also be essential for the cell to transit G1 and enter S phase. How the centrosome influences G1 progression is a mystery.

Farnesylated RhoB inhibits radiation-induced mitotic cell death and controls radiation-induced centrosome overduplication

Our previous results demonstrated that expressing the GTPase ras homolog gene family, member B (RhoB) in radiosensitive NIH3T3 cells increases their survival following 2 Gy irradiation (SF2). We have first demonstrated here that RhoB expression inhibits radiation-induced mitotic cell death. RhoB is present in both a farnesylated and a geranylgeranylated form in vivo. By expressing RhoB mutants encoding for farnesylated (RhoB-F cells), geranylgeranylated or the CAAX deleted form of RhoB, we have then shown that only RhoB-F expression was able to increase the SF2 value by reducing the sensitivity of these cells to radiation-induced mitotic cell death. Moreover, RhoB-F cells showed an increased G2 arrest and an inhibition of centrosome overduplication following irradiation mediated by the Rho-kinase, strongly suggesting that RhoB-F may control centrosome overduplication during the G2 arrest after irradiation. Overall, our results for the first time clearly implicate farnesylated RhoB as a crucial protein in mediating cellular resistance to radiation-induced nonapoptotic cell death.

Spatial organization and isotubulin composition of microtubules in epidermal tendon cells of Artemia franciscana

Epidermally derived tendon cells attach the exoskeleton (cuticle) of the Branchiopod crustacean, Artemia franciscana, to underlying muscle in the hindgut, while the structurally similar transalar tendon (epithelial) cells, which also arise from the epidermis and are polarized, connect dorsal and ventral exopodite surfaces. To establish these latter attachments the transalar tendon cells interact with cuticles on opposite sides of the exopodite by way of their apical surfaces and with one another via basal regions, or the cuticle attachments may be mediated through linkages with phagocytic storage cells found in the hemolymph. In some cases, phyllopod tendon cells attach directly to muscle cells. Tendon cells in the hindgut of Artemia possess microtubule bundles, as do the transalar cells, and they extend from the basal myotendinal junction to the apical domain located near the cuticle. The bundled microtubules intermingle with thin filaments reminiscent of microfilaments, but intermediate filament-like structures are absent. Microtubule bundles converging at apical cell surfaces contact structures termed apical invaginations, composed of cytoplasmic membrane infoldings associated with electron-dense material. Intracuticular rods protrude from apical invaginations, either into the cuticle during intermolt or the molting fluid in premolt. Confocal microscopy of immunofluorescently stained samples revealed tyrosinated, detyrosinated, and acetylated tubulins, the first time posttranslationally modified isoforms of this protein have been demonstrated in crustacean tendon cells. Microfilaments, as shown by staining with phalloidin, coincided spatially with microtubule bundles. Artemia tendon cells clearly represent an interesting system for study of cytoskeleton organization within the context of cytoplasmic polarity and the results in this article indicate functional cooperation of microtubules and microfilaments. These cytoskeletal elements, either acting independently or in concert, may transmit tension from muscle to cuticle in the hindgut and resist compression when connecting exopodite cuticular surfaces.

Identification of a novel centrosome/microtubule-associated coiled-coil protein involved in cell-cycle progression and spindle organization

Here we describe the identification of a novel vertebrate-specific centrosome/spindle pole-associated protein (CSPP) involved in cell-cycle regulation. The protein is predicted to have a tripartite domain structure, where the N- and C-terminal domains are linked through a coiled-coil mid-domain. Experimental analysis of the identified domains revealed that spindle association is dependent on the N-terminal and the coiled-coil mid domain. The expression of CSPP at the mRNA level was detected in all tested cell lines and in testis tissue. Ectopic expression of CSPP in HEK293T cells blocked cell-cycle progression in early G1 phase and in mitosis in a dose-dependent manner. Interestingly, mitosis-arrested cells contained aberrant spindles and showed impairment of chromosome congression. Inhibition of CSPP gene expression by small interfering RNAs induced cell-cycle arrest/delay in S phase. This phenotype was characterized by elevated levels of cyclin A, decreased levels of cyclin E and hyperphosphorylation of the S-phase checkpoint kinase Chk1. The activation of Chk1 may indicate a replication stress response due to an inappropriate G1/S-phase transition. Taken together, we demonstrate that CSPP is associated with centrosomes and microtubules and may play a role in the regulation of G(1)/S-phase progression and spindle assembly.

Functional role of centrosomes in spindle assembly and organization

The centrosome is the main MT organizing center in animal cells, and has traditionally been regarded as essential for organization of the bipolar spindle that facilitates chromosome segregation during mitosis. Centrosomes are associated with the poles of the mitotic spindle, and several cell types require these organelles for spindle formation. However, most plant cells and some female meiotic systems get along without this organelle, and centrosome-independent spindle assembly has now been identified within some centrosome containing cells. How can such observations, which point to mutually incompatible conclusions regarding the requirement of centrosomes in spindle formation, be interpreted? With emphasis on the functional role of centrosomes, this article summarizes the current models of spindle formation, and outlines how observations obtained from spindle assembly assays in vitro may reconcile conflicting opinions about the mechanism of spindle assembly. It is further described how Drosophila mutants are used to address the functional interrelationships between individual centrosomal proteins and spindle formation in vivo.

Dynactin

Dynactin is a multisubunit protein complex that is required for most, if not all, types of cytoplasmic dynein activity in eukaryotes. Dynactin binds dynein directly and allows the motor to traverse the microtubule lattice over long distances. A single dynactin subunit, p150Glued, is sufficient for both activities, yet dynactin contains several other subunits that are organized into an elaborate structure. It is currently believed that the bulk of the dynactin structure participates in interactions with a wide range of cellular structures, many of which are cargoes of the dynein motor. Genetic studies verify the importance of all elements of dynactin structure to its function. Although dynein can bind some membranous cargoes independently of dynactin, establishment of a fully functional dynein-cargo link appears to depend on dynactin. In this review, I summarize what is presently known about dynactin structure, the cellular structures with which it associates, and the intermolecular interactions that underlie and regulate binding. Although the molecular details of dynactin's interactions with membranous organelles and other molecules are complex, the framework provided here is intended to distill what is presently known and to be of use to dynactin specialists and beginners alike.

Centrosomal amplification and spindle multipolarity in cancer cells

Recent developments have highlighted the important role centrosomal defects play in the cellular changes associated with tumorigenesis. This article reviews recent developments addressing the impact of numerical centrosomal amplification on chromosomal segregational defects in the cancer cell. Probably, the most significant is the change to the structure of the spindle that leads to increased numbers of spindle poles and abnormal partitioning of the chromosomes in mitosis. I address how centrosomal changes are initiated and how they may lead to spindle multipolarity.

The Deubiquitinating Enzyme mUBPy Interacts with the Sperm-Specific Molecular Chaperone MSJ-1: The Relation with the Proteasome, Acrosome, and Centrosome in Mouse Male Germ Cells1

The mouse USP8/mUBPy gene codifies a deubiquitinating enzyme expressed preferentially in testis and brain. While the ubiquitin-specific processing proteases (UBPs) are known to be important for the early development in invertebrate organisms, their specific functions remain still unclear in mammals. Using specific antibodies, raised against a recombinant mUBPy protein, we studied mUBPy in mouse testis. The mUBPy is expressed exclusively by the germ cell component and is maintained in epididymal spermatozoa. The enzyme is functionally active, being able to detach ubiquitin moieties from endogenous protein substrates. Protein interaction assays showed that sperm UBPy interacts with MSJ-1, the sperm-specific DnaJ protein evolutionarily conserved for spermiogenesis. Immunocytochemistry revealed that mUBPy shares with MSJ-1 the intracellular localization during spermatid cell differentiation; intriguingly, we show here that the proteasomes also locate in mUBPy/MSJ-1-positive sites, such as the cytoplasmic surface of the developing acrosome and the centrosomal region. These colocalization sites are maintained in epididymal spermatozoa. The demonstration of a protein interaction between a deubiquitinating enzyme and a molecular chaperone and the documentation on the proteasomes in both differentiating and mature mouse male germ cells suggest that members of the chaperone and ubiquitin/proteasome systems could cooperate in the fine control of protein quality to yield functional spermatozoa.