Centrosome composition and microtubule anchoring mechanisms

Rootletin forms centriole-associated filaments and functions in centrosome cohesion

After duplication of the centriole pair during S phase, the centrosome functions as a single microtubule-organizing center until the onset of mitosis, when the duplicated centrosomes separate for bipolar spindle formation. The mechanisms regulating centrosome cohesion and separation during the cell cycle are not well understood. In this study, we analyze the protein rootletin as a candidate centrosome linker component. As shown by immunoelectron microscopy, endogenous rootletin forms striking fibers emanating from the proximal ends of centrioles. Moreover, rootletin interacts with C-Nap1, a protein previously implicated in centrosome cohesion. Similar to C-Nap1, rootletin is phosphorylated by Nek2 kinase and is displaced from centrosomes at the onset of mitosis. Whereas the overexpression of rootletin results in the formation of extensive fibers, small interfering RNA-mediated depletion of either rootletin or C-Nap1 causes centrosome splitting, suggesting that both proteins contribute to maintaining centrosome cohesion. The ability of rootletin to form centriole-associated fibers suggests a dynamic model for centrosome cohesion based on entangling filaments rather than continuous polymeric linkers.

A Single-Cell Analysis of Myogenic Dedifferentiation Induced by Small Molecules

An important direction in chemical biology is the derivation of compounds that affect cellular differentiation or its reversal. The fragmentation of multinucleate myofibers into viable mononucleates (called cellularization) occurs during limb regeneration in urodele amphibians, and the isolation of myoseverin, a trisubstituted purine that could apparently activate this pathway of myogenic dedifferentiation in mammalian cells, generated considerable interest. We have explored the mechanism and outcome of cellularization at a single-cell level, and we report findings that significantly extend the previous work with myoseverin. Using a panel of compounds, including a triazine compound with structural similarity and comparable activity to myoseverin, we have identified microtubule disruption as critical for activation of the response. Time-lapse microscopy has enabled us to analyze the fate of identified mononucleate progeny, and directly assess the extent of dedifferentiation.

The focal adhesion scaffolding protein HEF1 regulates activation of the Aurora-A and Nek2 kinases at the centrosome

Although HEF1 has a well-defined role in integrin-dependent attachment signalling at focal adhesions, it relocalizes to the spindle asters at mitosis. We report here that overexpression of HEF1 causes an increase in centrosome numbers and multipolar spindles, resembling defects induced by manipulation of the mitotic regulatory kinase Aurora-A (AurA). We show that HEF1 associates with and controls activation of AurA. We also show that HEF1 depletion causes centrosomal splitting, mono-astral spindles and hyperactivation of Nek2, implying additional action earlier in the cell cycle. These results provide new insight into the role of an adhesion protein in coordination of cell attachment and division.

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.

Centrosome aberrations in chronic myeloid leukemia correlate with stage of disease and chromosomal instability

Centrosome abnormalities are hallmarks of various cancers and have been implicated in chromosome missegregation, chromosomal instability, and aneuploidy. Since genetic instability is a common feature in chronic myeloid leukemia (CML), we sought to investigate whether centrosome aberrations occur and correlate with disease stage and cytogenetic findings in CML. We examined 34 CML samples including CD 34+Ph+cells of 18 newly diagnosed patients (chronic phase (CP)) and 16 blast crisis (BC) specimens by using a centrosome-specific antibody to pericentrin. All CP and BC samples displayed centrosome alterations as compared with corresponding CD 34+control cells. Centrosome abnormalities were detected in 29.1+/-5.9% of CP blasts and in 54.3+/-4.8% of BC blasts, but in only 2.4+/-1.1% of controls (P<0.0001). Additional karyotypic alterations to the t(9;22) translocation were found in only 1/18 CML-CP patients. In contrast, 11/16 (73%) CML-BC patients displayed additional karyotype alterations in 48.7% of analyzed cells, correlating with an abnormal centrosome status (P=0.0005). Our results indicate that centrosome defects are a common and early detectable feature in CML that may contribute to acquisition of chromosomal aberrations and aneuploidy. They may be considered as the driving force of disease progression and could serve as future prognostic markers.

Protein 4.1R regulates interphase microtubule organization at the centrosome

In human red blood cells, protein 4.1 (4.1R) stabilizes the spectrin-actin network and anchors it to the plasma membrane. To contribute to the characterization of functional roles of 4.1R in nonerythroid cells, we analysed the effect of ectopic expression of 4.1R isoforms on interphase microtubules in fibroblastic cells. We found that specific 4.1R isoforms disturbed the microtubule architecture but not the actin cytoskeleton. Biochemical sedimentation and/or confocal microscopy analyses showed that the pericentriolar components gamma-tubulin and pericentrin remained at centrosomes, whereas the distributions of proteins p150Glued and the dynein intermediate chain were altered. Remarkably, 4.1R was displaced from the centrosome. In microtubule depolymerizing-repolymerizing assays, 4.1R-transfected cells showed an ability to depolymerize and nucleate microtubules that was similar to that of untransfected cells; however, microtubules became disorganized soon after regrowth. In microtubule-depolymerized transfected cells and during the initial steps of microtubule regrowth, centrosomal 4.1R localized with gamma-tubulin but did not when microtubules became disorganized. To learn more about centrosomal 4.1R function, isolated centrosomes were examined by confocal microscopy, western blot and in vitro microtubule aster-assembly assays. The experiments showed that 4.1R was present in isolated centrosome preparations, that it remained in the center of in-vitro-assembled microtubule asters and that more asters were assembled by the addition of protein 4.1R fused to glutathione-S-transferase. Together, these results indicate that 4.1R plays a key role at the centrosome, contributing to the maintenance of a radial microtubule organization.

CLIPs and CLASPs and cellular dynamics

The dynamic properties of microtubules are regulated by plus-end tracking proteins (+TIPs), which associate with the distal ends of microtubules. Among the +TIPs are cytoplasmic linker proteins (CLIPs), which promote microtubule growth and regulate dynein-dynactin localization, and CLIP-associating proteins (CLASPs), which stabilize specific subsets of microtubules on reception of signalling cues. CLIPs and CLASPs interact and cooperate to direct the microtubule network, thereby regulating cellular asymmetry.

Cell polarity during folliculogenesis and oogenesis

Polarity is an important aspect of oogenesis and early development for many animal groups, but only recently it has become relevant to the study of mammals. Mammalian oocyte development occurs through tight coordination and interaction between all ovarian structures. In fact, bi-directional communication between the oocyte and its companion granulosa cells (GC) in the ovarian follicle seems essential for GC proliferation, differentiation, and production of a functional female gamete. The transzonal projections (TZP), which are specialized extensions from granulosa cells that terminate on the oolema after crossing the zona pellucida, are major structural components necessary for oocyte-GC interaction. Granulosa cell polarity seems to be a necessary requisite for appropriate function of TZP, and the role of FSH as modulator of a polarized phenotype on GC is discussed. This article also discusses oocyte polarity with special reference to the partial loss of polarity that occurs during in-vitro oocyte maturation and possible implications in the modulation of oocyte competencies. Cytoskeletal markers that may account for oocyte quality were defined and found to be distinct in in-vivo and in-vitro matured oocytes. Implications of partial loss of oocyte polarity during in-vitro maturation, reflected by distinct distribution of these markers, are further discussed. It is also proposed that expression of both somatic and germ cell polarity in the ovarian follicle will ultimately determine acquisition of meiotic, fertilization and developmental competences by the oocyte.

Liver-Specific pRB Loss Results in Ectopic Cell Cycle Entry and Aberrant Ploidy

The liver exhibits an exquisitely controlled cell cycle, wherein hepatocytes are maintained in quiescence until stimulated to proliferate. The retinoblastoma tumor suppressor, pRB, plays a central role in proliferative control by inhibiting inappropriate cell cycle entry. In many cases, liver cancer arises due to aberrant cycles of proliferation, and correspondingly, pRB is functionally inactivated in the majority of hepatocellular carcinomas. Therefore, to determine how pRB loss may provide conditions permissive for deregulated hepatocyte proliferation, we investigated the consequence of somatic pRB inactivation in murine liver. We show that liver-specific pRB loss results in E2F target gene deregulation and elevated cell cycle progression during post-natal growth. However, in adult livers, E2F targets are repressed and hepatocytes become quiescent independent of pRB, suggesting that other factors may compensate for pRB loss. Therefore, to probe the consequences of acute pRB inactivation in livers of adult mice, we gave adenoviral-Cre by i.v. injection. We show that acute pRB loss is sufficient to elicit E2F target gene expression and cell cycle entry in adult liver, demonstrating a critical role for pRB in maintaining hepatocyte quiescence. Finally, we show that liver-specific pRB loss results in the development of nuclear pleomorphism associated with elevated ploidy that is evident in adult mice harboring both acute and chronic pRB loss. Together, these results show the crucial role played by pRB in maintaining hepatocyte quiescence and ploidy in adult liver in vivo and underscore the critical importance of delineating the consequences of acute pRB loss in adult animals.

Nuclear targeting of adenovirus type 2 requires CRM1-mediated nuclear export

Incoming adenovirus type 2 (Ad2) and Ad5 shuttle bidirectionally along microtubules, biased to the microtubule-organizing center by the dynein/dynactin motor complex. It is unknown how the particles reach the nuclear pore complex, where capsids disassemble and viral DNA enters the nucleus. Here, we identified a novel link between nuclear export and microtubule-mediated transport. Two distinct inhibitors of the nuclear export factor CRM1, leptomycin B (LMB) and ratjadone A (RJA) or CRM1-siRNAs blocked adenovirus infection, arrested cytoplasmic transport of viral particles at the microtubule-organizing center or in the cytoplasm and prevented capsid disassembly and nuclear import of the viral genome. In mitotic cells where CRM1 is in the cytoplasm, adenovirus particles were not associated with microtubules but upon LMB treatment, they enriched at the spindle poles implying that CRM1 inhibited microtubule association of adenovirus. We propose that CRM1, a nuclear factor exported by CRM1 or a protein complex containing CRM1 is part of a sensor mechanism triggering the unloading of the incoming adenovirus particles from microtubules proximal to the nucleus of interphase cells.

The Golgi-associated protein GRASP65 regulates spindle dynamics and is essential for cell division

At the onset of mitosis, the pericentriolar Golgi apparatus of mammalian cells is converted into small fragments, which are dispersed throughout the cytosol. The Golgi-associated protein GRASP65 is involved in this process. To address the role of GRASP65 in mitotic Golgi fragmentation, we depleted the protein from HeLa cells by RNAi. In the absence of GRASP65, the number of cisternae per Golgi stack is reduced without affecting the overall organization of Golgi membranes and protein transport. GRASP65-depleted cells entered mitosis, but accumulated in metaphase with condensed chromatin and multiple aberrant spindles and eventually died. Although Centrin2 and g-tubulin were detected in two of the spindle poles, the other spindle poles contained g-tubulin, but not Centrin2. Furthermore, we provide evidence that the expression of the C-terminus of GRASP65 interferes with entry of cells into mitosis. Our results suggest the requirement for GRASP65 in the regulation of spindle dynamics rather than a direct role in the stacking of Golgi cisternae. This novel function is in addition to the previously established negative role of GRASP65 at the G2/M transition, which is mediated by its C-terminus.

Odf2-deficient mother centrioles lack distal/subdistal appendages and the ability to generate primary cilia

Outer dense fibre 2 (Odf2; also known as cenexin) was initially identified as a main component of the sperm tail cytoskeleton, but was later shown to be a general scaffold protein that is specifically localized at the distal/subdistal appendages of mother centrioles. Here we show that Odf2 expression is suppressed in mouse F9 cells when both alleles of Odf2 genes are deleted. Unexpectedly, the cell cycle of Odf2(-/-) cells does not seem to be affected. Immunofluorescence and ultrathin-section electron microscopy reveals that in Odf2(-/-) cells, distal/subdistal appendages disappear from mother centrioles, making it difficult to distinguish mother from daughter centrioles. In Odf2(-/-) cells, however, the formation of primary cilia is completely suppressed, although approximately 25% of wild-type F9 cells are ciliated under the steady-state cell cycle. The loss of primary cilia in Odf2(-/-) F9 cells can be rescued by exogenous Odf2 expression. These findings indicate that Odf2 is indispensable for the formation of distal/subdistal appendages and the generation of primary cilia, but not for other cell-cycle-related centriolar functions.

African swine fever virus infection disrupts centrosome assembly and function

African swine fever virus (ASFV) is a large, enveloped DNA virus that assembles in perinuclear sites located close to the centrosome. It is reported here that the microtubule network becomes disorganized soon after the onset of viral DNA replication and formation of assembly sites. ASFV infection resulted in loss of gamma-tubulin and pericentrin at the centrosome; this was due to protein relocalization, but not degradation. ASFV infection also inhibited the ability of the centrosome to nucleate microtubules. The reorganization of microtubules seen in ASFV-infected cells may therefore be mediated by gamma-tubulin and pericentrin redistribution, and consequent disruption of centrosome assembly and function.

Mto2p, a novel fission yeast protein required for cytoplasmic microtubule organization and anchoring of the cytokinetic actin ring

Microtubules regulate diverse cellular processes, including chromosome segregation, nuclear positioning, and cytokinesis. In many organisms, microtubule nucleation requires gamma-tubulin and associated proteins present at specific microtubule organizing centers (MTOCs). In fission yeast, interphase cytoplasmic microtubules originate from poorly characterized interphase MTOCs and spindle pole body (SPB), and during late anaphase from the equatorial MTOC (EMTOC). It has been previously shown that Mto1p (Mbo1p/Mod20p) function is important for the organization/nucleation of all cytoplasmic microtubules. Here, we show that Mto2p, a novel protein, interacts with Mto1p and is important for establishing a normal interphase cytoplasmic microtubule array. In addition, mto2Delta cells fail to establish a stable EMTOC and localize gamma-tubulin complex members to this medial structure. As predicted from these functions, Mto2p localizes to microtubules, the SPB, and the EMTOC in an Mto1p-dependent manner. mto2Delta cells fail to anchor the cytokinetic actin ring in the medial region of the cell and under conditions that mildly perturb actin structures, these rings unravel in mto2Delta cells. Our results suggest that the Mto2p and the EMTOC are critical for anchoring the cytokinetic actin ring to the medial region of the cell and for proper coordination of mitosis with cytokinesis.

Microtubule nucleation and anchoring at the centrosome are independent processes linked by ninein function

The centrosome organizes microtubules by controlling nucleation and anchoring processes. In mammalian cells, subdistal appendages of the mother centriole are major microtubule-anchoring structures of the centrosome. It is not known how newly nucleated microtubules are anchored to these appendages. We show here that ninein, a component of subdistal appendages, localizes to the centriole via its C-terminus and interacts with gamma-tubulin-containing complexes via its N-terminus. Expression of a construct encoding the ninein C-terminus displaced endogenous ninein and the gamma-tubulin ring complex (gamma-TuRC) from the centrosome, leading to microtubule nucleation and anchoring defects. By contrast, expression of a fusion consisting of the N- and C-terminal domains (lacking the central coiled-coil region) displaced endogenous ninein without perturbing gamma-TuRC localization. Accordingly, only anchoring defects were observed in this case. Therefore, expression of this fusion appeared to uncouple microtubule nucleation and anchorage activities at the centrosome. Our results suggest that ninein has a role not only in microtubule anchoring but also in promoting microtubule nucleation by docking the gamma-TuRC at the centrosome. In addition, we show that the gamma-TuRC might not be sufficient to anchor microtubules at the centrosome in the absence of ninein. We therefore propose that ninein constitutes a molecular link between microtubule-nucleation and -anchoring activities at the centrosome.

Localization of centrins in the hypotrich ciliate Paraurostyla weissei

Centrins are ubiquitous cytoskeletal proteins that are generally associated with the centrosome and form large cytoskeletal networks in protists. To obtain more data on the respective role of different centrin proteins, we studied their distribution and behavior in one ciliate species, Paraurostyla weissei, using specific antibodies. In this species, only two major proteins of 21 and 24 kDa corresponding to centrins, were identified by 1D and 2D electrophoresis. Immunofluorescence analysis showed that these two proteins displayed non-overlapping localization in the interphase cell and during morphogenesis. Both centrin proteins localize on the fibrous network linking the oral basal bodies in the interphase cell and in the form of marginal dots, which correspond to the proximal ends of the striated rootlets; the 21 kDa centrin was also detected within the basal bodies, whereas the 24 kDa centrin allowed identifying new structures, the frontal dashes. During morphogenesis, the 21 kDa centrin locates at the basal bodies, while the 24 kDa centrin is detected along the striated rootlets and in close association with the basal bodies pairs. These data are discussed in terms of the potential roles of the two centrins in different cellular functions.

The centrosome in human genetic disease

The centrosome is an indispensable component of the cell-cycle machinery of eukaryotic cells, and the perturbation of core centrosomal or centrosome-associated proteins is linked to cell-cycle misregulation and cancer. Recent work has expanded our understanding of the functional complexity and importance of this organelle. The centrosomal localization of proteins that are involved in human genetic disease, and the identification of novel centrosome-associated proteins, has shown that numerous, seemingly unrelated, cellular processes can be perturbed by centrosomal dysfunction. Here, we review the mechanistic relationship between human disease phenotypes and the function of the centrosome, and describe some of the newly-appreciated functions of this organelle in animal cells.

The forkhead-associated domain protein Cep170 interacts with Polo-like kinase 1 and serves as a marker for mature centrioles

We report the characterization of Cep170, a forkhead-associated (FHA) domain protein of previously unknown function. Cep170 was identified in a yeast two-hybrid screen for interactors of Polo-like kinase 1 (Plk1). In human cells, Cep170 is constantly expressed throughout the cell cycle but phosphorylated during mitosis. It interacts with Plk1 in vivo and can be phosphorylated by Plk1 in vitro, suggesting that it is a physiological substrate of this kinase. Both overexpression and small interfering RNA (siRNA)-mediated depletion studies suggest a role for Cep170 in microtuble organization and cell morphology. Cep170 associates with centrosomes during interphase and with spindle microtubules during mitosis. As shown by immunoelectron microscopy, Cep170 associates with subdistal appendages, typical of the mature mother centriole. Thus, anti-Cep170 antibodies stain only one centriole during G1, S, and early G2, but two centrioles during late G2 phase of the cell cycle. We show that Cep170 labeling can be used to discriminate bona fide centriole overduplication from centriole amplification that results from aborted cell division.

Kinetochore-microtubule interactions during cell division

Proper segregation of chromosomes during cell division is essential for the maintenance of genetic stability. During this process chromosomes must establish stable functional interactions with microtubules through the kinetochore, a specialized protein structure located on the surface of the centromeric heterochromatin. Stable attachment of kinetochores to a number of microtubules results in the formation of a kinetochore fibre that mediates chromosome movement. How the kinetochore fibre is formed and how chromosome motion is produced and regulated remain major questions in cell biology. Here we look at some of the history of research devoted to the study of kinetochore-microtubule interaction and attempt to identify significant advances in the knowledge of the basic processes. Ultrastructural work has provided substantial insights into the structure of the kinetochore and associated microtubules during different stages of mitosis. Also, recent in-vivo studies have probed deep into the dynamics of kinetochore-attached microtubules suggesting possible models for the way in which kinetochores harness the capacity of microtubules to do work and turn it into chromosome motion. Much of the research in recent years suggests that indeed multiple mechanisms are involved in both formation of the k-fibre and chromosome motion. Thus, rather than moving to a unified theory, it has become apparent that most cell types have the capacity to build the spindle using multiple and probably redundant mechanisms.

Glutamylated and glycylated tubulin isoforms in the aberrant sperm axoneme of the gall-midge fly, Asphondylia ruebsaameni

The axonemal organization expressed in the sperm flagella of the cecidomyiid dipteran Asphondylia ruebsaameni is unconventional, being characterized by the presence of an exceedingly high number of microtubular doublets and by the absence of both the inner dynein arms and the central pair/radial spoke complex. Consequently, its motility, both in vivo and in vitro, is also peculiar. Using monoclonal antibodies directed against posttranslational modifications, we have analyzed the presence and distribution of glutamylated and glycylated tubulin isoforms in this aberrant axonemal structure, and compared them with those of a reference insect species (Apis mellifera), endowed with a conventional axoneme. Our results have shown that the unorthodox structure and motility of the Asphondylia axoneme are concomitant with: (1). a very low glutamylation extent in the alpha-tubulin subunit, (2). a high level of glutamylation in the beta-subunit, (3). an extremely low total extent of glycylation, with regard to both monoglycylated and polyglycylated sites, either in alpha- or in beta-tubulin, (4). the presence of a strong labeling of glutamylated tubulin isoforms at the proximal end of the axoneme, and (5). a uniform distribution of glutamylated as well as glycylated isoforms along the rest of the axoneme. Thus, our data indicate that tubulin molecular heterogeneity is much lower in the Asphondylia axoneme than in the conventional 9+2 axoneme with regard to both isoform content and isoform distribution along the axoneme.

The budding yeast spindle pole body: structure, duplication, and function

Nucleation of microtubules by eukaryotic microtubule organizing centers (MTOCs) is required for a variety of functions, including chromosome segregation during mitosis and meiosis, cytokinesis, fertilization, cellular morphogenesis, cell motility, and intracellular trafficking. Analysis of MTOCs from different organisms shows that the structure of these organelles is widely varied even though they all share the function of microtubule nucleation. Despite their morphological diversity, many components and regulators of MTOCs, as well as principles in their assembly, seem to be conserved. This review focuses on one of the best-characterized MTOCs, the budding yeast spindle pole body (SPB). We review what is known about its structure, protein composition, duplication, regulation, and functions. In addition, we discuss how studies of the yeast SPB have aided investigation of other MTOCs, most notably the centrosome of animal cells.

Ase1p organizes antiparallel microtubule arrays during interphase and mitosis in fission yeast

Proper microtubule organization is essential for cellular processes such as organelle positioning during interphase and spindle formation during mitosis. The fission yeast Schizosaccharomyces pombe presents a good model for understanding microtubule organization. We identify fission yeast ase1p, a member of the conserved ASE1/PRC1/MAP65 family of microtubule bundling proteins, which functions in organizing the spindle midzone during mitosis. Using fluorescence live cell imaging, we show that ase1p localizes to sites of microtubule overlaps associated with microtubule organizing centers at both interphase and mitosis. ase1Delta mutants fail to form overlapping antiparallel microtubule bundles, leading to interphase nuclear positioning defects, and premature mitotic spindle collapse. FRAP analysis revealed that interphase ase1p at overlapping microtubule minus ends is highly dynamic. In contrast, mitotic ase1p at microtubule plus ends at the spindle midzone is more stable. We propose that ase1p functions to organize microtubules into overlapping antiparallel bundles both in interphase and mitosis and that ase1p may be differentially regulated through the cell cycle.

Coordinate regulation of the mother centriole component nlp by nek2 and plk1 protein kinases

Mitotic entry requires a major reorganization of the microtubule cytoskeleton. Nlp, a centrosomal protein that binds gamma-tubulin, is a G(2)/M target of the Plk1 protein kinase. Here, we show that human Nlp and its Xenopus homologue, X-Nlp, are also phosphorylated by the cell cycle-regulated Nek2 kinase. X-Nlp is a 213-kDa mother centriole-specific protein, implicating it in microtubule anchoring. Although constant in abundance throughout the cell cycle, it is displaced from centrosomes upon mitotic entry. Overexpression of active Nek2 or Plk1 causes premature displacement of Nlp from interphase centrosomes. Active Nek2 is also capable of phosphorylating and displacing a mutant form of Nlp that lacks Plk1 phosphorylation sites. Importantly, kinase-inactive Nek2 interferes with Plk1-induced displacement of Nlp from interphase centrosomes and displacement of endogenous Nlp from mitotic spindle poles, while active Nek2 stimulates Plk1 phosphorylation of Nlp in vitro. Unlike Plk1, Nek2 does not prevent association of Nlp with gamma-tubulin. Together, these results provide the first example of a protein involved in microtubule organization that is coordinately regulated at the G(2)/M transition by two centrosomal kinases. We also propose that phosphorylation by Nek2 may prime Nlp for phosphorylation by Plk1.

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.

Roles of polo-like kinase 1 in the assembly of functional mitotic spindles

BACKGROUND

The stable association of chromosomes with both poles of the mitotic spindle (biorientation) depends on spindle pulling forces. These forces create tension across sister kinetochores and are thought to stabilize microtubule-kinetochore interactions and to silence the spindle checkpoint. Polo-like kinase 1 (Plk1) has been implicated in regulating centrosome maturation, mitotic entry, sister chromatid cohesion, the anaphase-promoting complex/cyclosome (APC/C), and cytokinesis, but it is unknown if Plk1 controls chromosome biorientation.

RESULTS

We have analyzed Plk1 functions in synchronized mammalian cells by RNA interference (RNAi). Plk1-depleted cells enter mitosis after a short delay, accumulate in a preanaphase state, and subsequently often die by apoptosis. Spindles in Plk1-depleted cells lack focused poles and are not associated with centrosomes. Chromosomes attach to these spindles, but the checkpoint proteins Mad2, BubR1, and CENP-E are enriched at many kinetochores. When Plk1-depleted cells are treated with the Aurora B inhibitor Hesperadin, which silences the spindle checkpoint by stabilizing microtubule-kinetochore interactions, cells degrade APC/C substrates and exit mitosis without chromosome segregation and cytokinesis. Experiments with monopolar spindles that are induced by the kinesin inhibitor Monastrol indicate that Plk1 is required for the assembly of spindles that are able to generate poleward pulling forces.

CONCLUSIONS

Our results imply that Plk1 is not essential for mitotic entry and APC/C activation but is required for proper spindle assembly and function. In Plk1-depleted cells spindles may not be able to create enough tension across sister kinetochores to stabilize microtubule-kinetochore interactions and to silence the spindle checkpoint.

Dynamic recruitment of Nek2 kinase to the centrosome involves microtubules, PCM-1, and localized proteasomal degradation

Centrosomes undergo dramatic changes in composition and activity during cell cycle progression. Yet mechanisms involved in recruiting centrosomal proteins are poorly understood. Nek2 is a cell cycle-regulated protein kinase required for regulation of centrosome structure at the G2/M transition. Here, we have addressed the processes involved in trafficking of Nek2 to the centrosome of human adult cells. We find that Nek2 exists in small, highly dynamic cytoplasmic particles that move to and from the centrosome. Many of these particles align along microtubules and a motif was identified in the Nek2 C-terminal noncatalytic domain that allows both microtubule binding and centrosome localization. FRAP experiments reveal that 70% of centrosomal Nek2 is rapidly turned over (t(1/2) approximately 3 s). Microtubules facilitate Nek2 trafficking to the centrosome but only over long distances. Cytoplasmic Nek2 particles colocalize in part with PCM-1 containing centriolar satellites and depletion of PCM-1 interferes with centrosomal recruitment of Nek2 and its substrate C-Nap1. Finally, we show that proteasomal degradation is necessary to allow rapid recruitment of new Nek2 molecules to the centrosome. Together, these data highlight multiple processes involved in regulating the abundance of Nek2 kinase at the centrosome including microtubule binding, the centriolar satellite component PCM-1, and localized protein degradation.

Microtubule-associated proteins and their essential roles during mitosis

Microtubules play essential roles during mitosis, including chromosome capture, congression, and segregation. In addition, microtubules are also required for successful cytokinesis. At the heart of these processes is the ability of microtubules to do work, a property that derives from their intrinsic dynamic behavior. However, if microtubule dynamics were not properly regulated, it is certain that microtubules alone could not accomplish any of these tasks. In vivo, the regulation of microtubule dynamics is the responsibility of microtubule-associated proteins. Among these, we can distinguish several classes according to their function: (1) promotion and stabilization of microtubule polymerization, (2) destabilization or severance of microtubules, (3) functioning as linkers between various structures, or (4) motility-related functions. Here we discuss how the various properties of microtubule-associated proteins can be used to assemble an efficient mitotic apparatus capable of ensuring the bona fide transmission of the genetic information in animal cells.

The flagellum of trypanosomes

Eukaryotic cilia and flagella are cytoskeletal organelles that are remarkably conserved from protists to mammals. Their basic unit is the axoneme, a well-defined cylindrical structure composed of microtubules and up to 250 associated proteins. These complex organelles are assembled by a dynamic process called intraflagellar transport. Flagella and cilia perform diverse motility and sensitivity functions in many different organisms. Trypanosomes are flagellated protozoa, responsible for various tropical diseases such as sleeping sickness and Chagas disease. In this review, we first describe general knowledge on the flagellum: its occurrence in the living world, its molecular composition, and its mode of assembly, with special emphasis on the exciting developments that followed the discovery of intraflagellar transport. We then present recent progress regarding the characteristics of the trypanosome flagellum, highlighting the original contributions brought by this organism. The most striking phenomenon is the involvement of the flagellum in several aspects of the trypanosome cell cycle, including cell morphogenesis, basal body migration, and cytokinesis.

Calcium and Magnesium Binding to Human Centrin 3 and Interaction with Target Peptides

There are four isoforms of centrin in mammals, with variable sequence, tissue expression, and functional properties. We have recently characterized a number of structural, ion, and target binding properties of human centrin isoform HsCen2. This paper reports a similar characterization of HsCen3, overexpressed in Escherichia coli and purified by phase-reversed chromatography. Equilibrium and dynamic binding studies revealed that HsCen3 has one mixed Ca(2+)/Mg(2+) binding site of high affinity (K(d) = 3 and 10 microM for Ca(2+) and Mg(2+), respectively) and two Ca(2+)-specific sites of low affinity (K(d) = 140 microM). The metal-free protein is fragmented by an unidentified protease into a polypeptide segment of 11 kDa, which was purified by HPLC, and identified by mass spectrometry as the segment of residues 21-112. Similarly, controlled trypsinolysis on Ca(2+)-bound HsCen3 yielded a mixture of segments of residues 1-124 and 1-125. The Ca(2+)/Mg(2+) site could be assigned to this segment and thus resides in the N-terminal half of HsCen3. Temperature denaturation experiments, circular dichroism, and utilization of fluorescence hydrophobic probes allowed us to propose that the metal-free protein has molten globule characteristics and that the dication-bound forms are compact with a polar surface for the Mg(2+) form and a hydrophobic exposed surface for the Ca(2+) form. Thus, HsCen3 could be classified as a Ca(2+) sensor protein. In addition, it is able to bind strongly to a model target peptide (melittin), as well as to peptides derived from the protein XPC and Kar1p, with a moderate Ca(2+) dependence.

The centrosomal kinase Nek2 displays elevated levels of protein expression in human breast cancer

Aneuploidy and chromosome instability are common abnormalities in human cancer. Loss of control over mitotic progression, multipolar spindle formation, and cytokinesis defects are all likely to contribute to these phenotypes. Nek2 is a cell cycle-regulated protein kinase with maximal activity at the onset of mitosis that localizes to the centrosome. Functional studies have implicated Nek2 in regulation of centrosome separation and spindle formation. Here, we present the first study of the protein expression levels of the Nek2 kinase in human cancer cell lines and primary tumors. Nek2 protein is elevated 2- to 5-fold in cell lines derived from a range of human tumors including those of cervical, ovarian, breast, prostate, and leukemic origin. Most importantly, by immunohistochemistry, we find that Nek2 protein is significantly up-regulated in preinvasive in situ ductal carcinomas of the breast as well as in invasive breast carcinomas. Finally, by ectopic expression of Nek2A in immortalized HBL100 breast epithelial cells, we show that increased Nek2 protein leads to accumulation of multinucleated cells with supernumerary centrosomes. These data highlight the Nek2 kinase as novel potential target for chemotherapeutic intervention in breast cancer.

Centriole Assembly Requires Both Centriolar and Pericentriolar Material Proteins

Centrioles organize pericentriolar material to form centrosomes and also template the formation of cilia. Despite the importance of centrioles in dividing and differentiated cells, their assembly remains poorly understood at a molecular level. Here, we develop a fluorescence microscopy-based assay for centriole assembly in the 1-cell stage C. elegans embryo. We use this assay to characterize SAS-6, a centriolar protein that we identified based on its requirement for centrosome duplication. We show that SAS-6, a member of a conserved metazoan protein family, is specifically required for new centriole assembly, a result we confirm by electron microscopy. We further use the centriole assembly assay to examine the roles of three pericentriolar material proteins: SPD-5, the kinase aurora-A, and gamma-tubulin. Our results suggest that the pericentriolar material promotes daughter centriole formation by concentrating gamma-tubulin around the parent centriole. Thus, both centriolar and pericentriolar material proteins contribute to centriole assembly.

Interaction of Cep135 with a p50 dynactin subunit in mammalian centrosomes

Cep135 is a 135-kDa, coiled-coil centrosome protein important for microtubule organization in mammalian cells [Ohta et al., 2002: J. Cell Biol. 156:87-99]. To identify Cep135-interacting molecules, we screened yeast two-hybrid libraries. One clone encoded dynamitin, a p50 dynactin subunit, which localized at the centrosome and has been shown to be involved in anchoring microtubules to centrosomes. The central domain of p50 binds to the C-terminal sequence of Cep135; this was further confirmed by immunoprecipitation and immunostaining of CHO cells co-expressing the binding domains for Cep135 and p50. Exogenous p50 lacking the Cep 135-binding domain failed to locate at the centrosome, suggesting that Cep135 is required for initial targeting of the centrosome. Altered levels of Cep135 and p50 by RNAi and protein overexpression caused the release of endogenous partner molecules from centrosomes. This also resulted in dislocation of other centrosomal molecules, such as gamma-tubulin and pericentrin, ultimately leading to disorganization of microtubule patterns. These results suggest that Cep135 and p50 play an important role in assembly and maintenance of functional microtubule-organizing centers.

Cyclin-dependent kinase inhibitor indirubin-3′-oxime selectively inhibits human papillomavirus type 16 E7-induced numerical centrosome anomalies

Dysregulation of the centrosome duplication cycle has been implicated in tumorigenesis. Our previous work has shown that the human papillomavirus type 16 (HPV-16) E7 oncoprotein rapidly induces aberrant centrosome and centriole duplication in normal human cells. We report here that HPV E7-induced abnormal centriole duplication is specifically abrogated by a small molecule CDK inhibitor, indirubin-3'-oxime (IO), but not a kinase-inactive derivative. Importantly, normal centriole duplication was not markedly affected by IO, and the inhibitory effects were observed at concentrations that did not affect the G1/S transition of the cell division cycle. Depletion of CDK2 by siRNA similarly abrogated HPV E7-induced abnormal centrosome duplication and ectopic expression of CDK2 in combination with cyclin E or cyclin A could rescue the inhibitory effect of IO. IO treatment also reduced the steady-state level of aneuploid cells in HPV-16 E7-expressing cell populations. Our results suggest that cyclin/CDK2 activity is critically involved in abnormal centrosome duplication induced by HPV-16 E7 oncoprotein expression, but may be dispensable for normal centrosome duplication and cell cycle progression.

The Drosophila pericentrin-like protein is essential for cilia/flagella function, but appears to be dispensable for mitosis

Centrosomes consist of a pair of centrioles surrounded by an amorphous pericentriolar material (PCM). Proteins that contain a Pericentrin/AKAP450 centrosomal targeting (PACT) domain have been implicated in recruiting several proteins to the PCM. We show that the only PACT domain protein in Drosophila (the Drosophila pericentrin-like protein [D-PLP]) is associated with both the centrioles and the PCM, and is essential for the efficient centrosomal recruitment of all six PCM components that we tested. Surprisingly, however, all six PCM components are eventually recruited to centrosomes during mitosis in d-plp mutant cells, and mitosis is not dramatically perturbed. Although viable, d-plp mutant flies are severely uncoordinated, a phenotype usually associated with defects in mechanosensory neuron function. We show that the sensory cilia of these neurons are malformed and the neurons are nonfunctional in d-plp mutants. Moreover, the flagella in mutant sperm are nonmotile. Thus, D-PLP is essential for the formation of functional cilia and flagella in flies.

Accelerated alpha-synuclein aggregation after differentiation of SH-SY5Y neuroblastoma cells

Alpha-synuclein (alpha-syn) is a major component of inclusion bodies in Parkinson's disease (PD) and other synucleinopathies. To clarify the possible roles of alpha-syn in the molecular pathogenesis of neurodegenerative diseases, we have established a novel cellular model based on the differentiation of SH-SY5Y cells that overexpress alpha-syn. In the presence of ferrous iron, differentiation of the cells led to the formation of large perinuclear inclusion bodies, which developed from scattered small aggregates seen in undifferentiated cells. The iron-induced alpha-syn-positive inclusions co-localized largely with ubiquitin, and some of them were positive for nitrotyrosine, lipid, gamma-tubulin and dynein. Notably, treatment with nocodazole, a microtubule depolymerizing agent, interrupted the aggregate formation but led to a concomitant increase of apoptotic cells. Therefore, it appears that an intracellular retrograde transport system via microtubules plays a crucial role in the aggregate formation and also that the aggregates may represent a cytoprotective response against noxious stimuli. This cellular model will enable better understanding of the molecular pathomechanisms of synucleinopathy.

Mechanosensory-defective, male-sterile unc mutants identify a novel basal body protein required for ciliogenesis in Drosophila

uncoordinated (unc) mutants of Drosophila, which lack transduction in ciliated mechanosensory neurons, do not produce motile sperm. Both sensory and spermatogenesis defects are associated with disrupted ciliary structures: mutant sensory neurons have truncated cilia, and sensory neurons and spermatids show defects in axoneme ultrastructure. unc encodes a novel protein with coiled-coil segments and a LisH motif, which is expressed in type I sensory neurons and in the male germline - the only ciliogenic cells in the fly. A functional UNC-GFP fusion protein specifically localizes to both basal bodies in differentiating sensory neurons. In premeiotic spermatocytes it localizes to all four centrioles in early G2, remaining associated with them through meiosis and as they become the basal bodies for the elongating spermatid flagella. UNC is thus specifically required for normal ciliogenesis. Its localization is an early marker for the centriole-basal body transition, a central but enigmatic event in eukaryotic cell differentiation.

The Caenorhabditis elegans centrosomal protein SPD-2 is required for both pericentriolar material recruitment and centriole duplication

BACKGROUND

The centrosome is composed of a centriole pair and pericentriolar material (PCM). By marking the site of PCM assembly, the centrioles define the number of centrosomes present in the cell. The PCM, in turn, is responsible for the microtubule (MT) nucleation activity of centrosomes. Therefore, in order to assemble a functional bipolar mitotic spindle, a cell needs to control both centriole duplication and PCM recruitment. To date, however, the molecular mechanisms that govern these two processes still remain poorly understood.

RESULTS

Here we show that SPD-2 is a novel component of the C. elegans centrosome. SPD-2 localizes to the centriole throughout the cell cycle and accumulates on the PCM during mitosis. We show that SPD-2 requires SPD-5 for its accumulation on the PCM and that in the absence of SPD-2, centrosome assembly fails. We further show that centriole duplication is also defective in spd-2(RNAi) embryos, but not in spd-5(RNAi) embryos, where PCM recruitment is efficiently blocked.

CONCLUSIONS

Taken together, our results suggest that SPD-2 may link PCM recruitment and centriole duplication in C. elegans. SPD-2 shares homology with a human centrosome protein, suggesting that this key component of the C. elegans centrosome is evolutionarily conserved.

A novel ninein-interaction protein, CGI-99, blocks ninein phosphorylation by GSK3beta and is highly expressed in brain tumors

To explore more hNinein interacting proteins, the yeast two-hybrid screening using ninein C-terminal domain as bait protein was performed. One novel gene, CGI-99, was demonstrated to associate with hNinein in the yeast two-hybrid method and in vitro GST pull-down assay. Molecular characterization also showed that CGI-99 possessed a transcriptional activity at the N-terminal. In addition, CGI-99 formed a dimer with the C-terminal, which overlapped with hNinein binding site. In kinase assay, CGI-99 binds to hNinein and completely blocks the phosphorylation of hNinein by GSK3beta. Moreover, CGI-99 was highly expressed in all brain tumors which is in agreement with the Northern blot analysis. Taken together, we have isolated a novel protein CGI-99, which may be involved in the functional regulation of human ninein in the centrosome structure and may also be important in brain development and tumorigenesis.

Eml5, a novel WD40 domain protein expressed in rat brain

We have isolated a novel transcript with homology to the major microtubule-associated protein in dividing sea urchin embryos, EMAP. The protein has a predicted MW of approximately 180 kDa and we have named it Eml5 (EMAP-like protein 5). Eml5 contains 11 putative WD40 domains and 3 hydrophobic stretches of 43 aa, HELP domains, which have been suggested to be involved in microtubule binding. Eml5 appears to consist of two tandem repeats of the complete EMAP protein separated by a putative dimerization domain. Eml5 mRNA and protein is expressed at high levels in the hippocampus, cerebellum and olfactory bulb, as determined by in situ hybridization and immunocytochemistry. Eml5 transcripts can be detected in fore- and hindbrain structures from embryonic day 13 onwards. Because other EMAP-like proteins are involved in regulating microtubule dynamics, it is likely that Eml5 plays a role in the regulation of cytoskeletal rearrangements during neuronal development and in adult brain

Centrosome amplification and the origin of chromosomal instability in breast cancer

The development and progression of aggressive breast cancer is characterized by genomic instability leading to multiple genetic defects, phenotypic diversity, chemoresistance, and poor outcome. Centrosome abnormalities have been implicated in the origin of chromosomal instability through the development of multipolar mitotic spindles. Breast tumor centrosomes display characteristic structural abnormalities, termed centrosome amplification , including: increase in centrosome number and volume, accumulation of excess pericentriolar material, supernumerary centrioles, and inappropriate phosphorylation of centrosome proteins. In addition, breast tumor centrosomes also show functional abnormalities characterized by inappropriate centrosome duplication during the cell cycle and nucleation of unusually large microtubule arrays. These observations have important implications for understanding the mechanisms underlying genomic instability and loss of cell polarity in cancer. This review focuses on the coordination of the centrosome, DNA, and cell cycles in normal cells and their deregulation resulting in centrosome amplification and chromosomal instability in the development and progression of breast cancer.

Suppression of p160ROCK bypasses cell cycle arrest after Aurora-A/STK15 depletion

Alterations in the expression and activity of the centrosomal kinase, Aurora-A/serine/threonine kinase 15 (STK15), affect genomic stability, disrupt the fidelity of centrosome duplication, and induce cellular transformation. Here, we provide evidence that p160ROCK, a Rho-associate serine/threonine kinase, associates with Aurora-A in a protein complex with other STK15-associated factors. Suppression of Aurora-A by small interfering RNA in HeLa cells blocks the ability of centrosomes to organize normal mitotic spindles, induces G(2)/M cell cycle arrest, and promotes accumulation of tetraploid cells. In many cases, one outcome of such abnormalities is apoptosis. Introduction of a second genetic lesion, suppression of p160ROCK by RNA interference, can rescue abnormal mitotic spindle formation, release the G(2)/M cell cycle arrest, and alleviate apoptosis, leading to a greater accumulation of aneuploid cells. These results suggest that Aurora-A and p160ROCK act in a common genetic pathway that promotes and monitors progression through G(2)/M.

DAP-like kinase, a member of the death-associated protein kinase family, associates with centrosomes, centromers, and the contractile ring during mitosis

DAP-like kinase (Dlk) is a nuclear serine/threonine-specific kinase which has been implicated in apoptosis. However, induction of apoptosis by Dlk requires its relocation to the cytoplasm, particularly association with the actin cytoskeleton, which is achieved through interaction with pro-apoptotic protein Par-4. On the other hand, nuclear Dlk does not induce apoptosis and has rather been implicated in transcription. To further explore the biological functions of Dlk, we established a cell clone of MCF-7 cells stably expressing a GFP-Dlk fusion protein at low level. Ectopic expression of GFP-Dlk did not affect the growth properties of the cells. During interphase, GFP-Dlk showed a diffuse nuclear distribution with punctate staining in a subpopulation of cells. During mitosis, however, Dlk was associated with centrosomes, centromeres, and the contractile ring, but not with the mitotic spindle. Association with centrosomes, as confirmed by colocalization with gamma-tubulin and pericentrin persisted throughout mitosis but was also seen in interphase cells. Interestingly, GFP-Dlk and gamma-tubulin could be co-immunoprecipitated indicating that they are present in the same protein complex. Association of Dlk with centromeres, as verified by confocal fluorescence microscopy with centromere-specific antibodies was more restricted and discernable from prophase to early anaphase. Centromere association of Dlk coincides with H3 phosphorylation at Thr11 that is specifically phosphorylated by Dlk in vitro (U. Preuss, G. Landsberg, K. H. Scheidtmann, Nucleic Acids Res. 31, 878-885, 2003). During cytokinesis, Dlk was enriched in the contractile acto-myosin ring and colocalized with Ser19-phosphorylated myosin light chain, which is an in vitro substrate of Dlk. Strikingly, a C-terminal truncation mutant of Dlk generated multi-nucleated cells. Together, these data suggest that Dlk participates in regulation and, perhaps, coordination of mitotis and cytokinesis.

Rsp1p, a J Domain Protein Required for Disassembly and Assembly of Microtubule Organizing Centers during the Fission Yeast Cell Cycle

Regulation of microtubule organizing centers (MTOCs) orchestrates the reorganization of the microtubule (MT) cytoskeleton. In the fission yeast Schizosaccharomyces pombe, an equatorial MTOC (eMTOC) at the cell division site disassembles after cytokinesis, and multiple interphase MTOCs (iMTOCs) appear on the nucleus. Here, we show that, upon eMTOC disassembly, small satellites carrying MTOC components such as the gamma-tubulin complex travel in both directions along interphase MTs. We identify rsp1p, an MTOC protein required for eMTOC disassembly. In rsp1 loss-of-function mutants, the eMTOC persists and organizes an abnormal microtubule aster, while iMTOCs and satellites are greatly reduced. Conversely, rsp1p overexpression inhibits eMTOC formation. Rsp1p is a J domain protein that interacts with an hsp70. Thus, our findings suggest a model in which rsp1p is part of a chaperone-based mechanism that disassembles the eMTOC into satellites, contributing to the dynamic redistribution of MTOC components for organization of interphase microtubules.

The arithmetic of centrosome biogenesis

How do cells regulate centrosome number? A canonical duplication cycle generates two centrosomes from one in most proliferating cells. Centrioles are key to this process, and molecules such as centrins, SAS-4 and ZYG-1 govern daughter centriole formation. Cdk2 activity probably couples centrosome duplication with the S phase, and a licensing mechanism appears to limit centrosome duplication to once per cell cycle. However, such mechanisms must be altered in some cells – for example, spermatocytes – in which centrosome duplication and DNA replication are uncoupled. There are also alternative pathways of centrosome biogenesis. For example, one centrosome is reconstituted from two gametes at fertilization; in this case, the most common strategy involves differential contributions of centrioles and pericentriolar material (PCM) from each gamete. Furthermore, centrioles can sometimes form de novo from no apparent template. This occurs, for instance, in the early mouse embryo and in parthenogenetic species and might rely on a pre-existing seed that resides within PCM but is not visible by ultrastructural analysis.

Centrosome Maturation and Duplication in C. elegans Require the Coiled-Coil Protein SPD-2

Centrosomes are major determinants of mitotic spindle structure, but the mechanisms regulating their behavior remain poorly understood. The spd-2 gene of C. elegans is required for centrosome assembly or "maturation." Here we show that spd-2 encodes a coiled-coil protein that localizes within pericentriolar material (PCM) and in the immediate vicinity of centrioles. During maturation, SPD-2 gradually accumulates at the centrosome in a manner that is partially dependent on Aurora-A kinase and cytoplasmic dynein. Interestingly, SPD-2 interacts genetically with dynein heavy chain and SPD-5, another coiled-coil protein required for centrosome maturation. SPD-2 and SPD-5 are codependent for localization to the PCM, but SPD-2 localizes to centrioles independently of SPD-5. Surprisingly, we also find that SPD-2 is required for centrosome duplication and genetically interacts with ZYG-1, a kinase required for duplication. Thus, we have identified SPD-2 as a factor critical for the two basic functions of the centrosome-microtubule organization and duplication.

Localized effects of cAMP mediated by distinct routes of protein kinase A

More than 20% of the human genome encodes proteins involved in transmembrane and intracellular signaling pathways. The cAMP-protein kinase A (PKA) pathway is one of the most common and versatile signal pathways in eukaryotic cells and is involved in regulation of cellular functions in almost all tissues in mammals. Various extracellular signals converge on this signal pathway through ligand binding to G protein-coupled receptors, and the cAMP-PKA pathway is therefore tightly regulated at several levels to maintain specificity in the multitude of signal inputs. Ligand-induced changes in cAMP concentration vary in duration, amplitude, and extension into the cell, and cAMP microdomains are shaped by adenylyl cyclases that form cAMP as well as phosphodiesterases that degrade cAMP. Different PKA isozymes with distinct biochemical properties and cell-specific expression contribute to cell and organ specificity. A kinase anchoring proteins (AKAPs) target PKA to specific substrates and distinct subcellular compartments providing spatial and temporal specificity for mediation of biological effects channeled through the cAMP-PKA pathway. AKAPs also serve as scaffolding proteins that assemble PKA together with signal terminators such as phosphatases and cAMP-specific phosphodiesterases as well as components of other signaling pathways into multiprotein signaling complexes that serve as crossroads for different paths of cell signaling. Targeting of PKA and integration of a wide repertoire of proteins involved in signal transduction into complex signal networks further increase the specificity required for the precise regulation of numerous cellular and physiological processes.

Distinct MDM2 and P14ARF expression and centrosome amplification in well-differentiated liposarcomas

Well-differentiated liposarcomas (WDLs) are common soft-tissue tumors in adults. They are characterized by large marker chromosomes and/or ring chromosomes containing 12q-derived sequences in which MDM2 is consistently amplified. WDLs are subdivided into two subtypes according to their karyotype. Type D cells exhibit a near-diploid karyotype, with very few or no chromosome changes. Type H cells exhibit a near-tetraploid karyotype and many structural changes. Expression of P14ARF, MDM2, and TP53 proteins was assayed in the two WDL subtypes to establish whether distinct expression profiles correlated with cell ploidy. Although a transcriptionally functional TP53 was present in most tumors independent of their karyotype, type H cells were characterized by high levels of P14ARF and MDM2 proteins. Although amplified within similar chromosome markers in type D tumors, MDM2 did not appear to be overexpressed. In addition, it was present as a C-terminal truncated protein, indicative of alternatively spliced variants of MDM2 mRNA. As the existence of karyotypically distinct tumors could result from alterations of the mitotic machinery, we investigated the centrosome behavior in the two WDL subtypes. Centrosome amplification occurred in WDL tumors types H and D independent of their ploidy status. Moreover, no functional centrosome difference was found between the two tumor subtypes.

Adenomatous polyposis coli and EB1 localize in close proximity of the mother centriole and EB1 is a functional component of centrosomes

Adenomatous polyposis coli (APC) and End-binding protein 1 (EB1) localize to centrosomes independently of cytoplasmic microtubules (MTs) and purify with centrosomes from mammalian cell lines. Localization of EB1 to centrosomes is independent of its MT binding domain and is mediated by its C-terminus. Both APC and EB1 preferentially localize to the mother centriole and EB1 forms a cap at the end of the mother centriole that contains the subdistal appendages as defined by epsilon-tubulin localization. Like endogenous APC and EB1, fluorescent protein fusions of APC and EB1 localize preferentially to the mother centriole. Depletion of EB1 by RNA interference reduces MT minus-end anchoring at centrosomes and delays MT regrowth from centrosomes. In summary, our data indicate that APC and EB1 are functional components of mammalian centrosomes and that EB1 is important for anchoring cytoplasmic MT minus ends to the subdistal appendages of the mother centriole.