Centriole duplication and maturation in animal cells

Centriole duplication: analogue control in a digital age

In preparation for mitosis, the centrosome doubles once and only once to provide the two poles of the mitotic spindle. The presence of more than two centrosomes increases the chances that mitosis will be multipolar, and chromosomes will be distributed unequally. Since the number of mother-daughter centriole pairs determines the number of centrosomes, it is important that only one daughter centriole is assembled at, but slightly separated from, the proximal end of each mother centriole. This numerical and spatial specificity has led to the belief that a 'template' on the mother centriole provides a unique site for procentriole assembly. We review observations that are leading to the demise of this intuitively attractive idea. In its place, we are left with the notion that pericentriolar material at the wall of the mother centriole provides a local environment that promotes the assembly of a macromolecular complex that seeds the daughter centriole. Even though the system normally behaves in a digital fashion to go from zero to just one daughter centriole per mother, this behaviour appears to be based in the precise analogue control of multiple proteins, their activities, and the structure provided by the mother centriole.

The G1 phase Cdks regulate the centrosome cycle and mediate oncogene-dependent centrosome amplification

Because centrosome amplification generates aneuploidy and since centrosome amplification is ubiquitous in human tumors, a strong case is made for centrosome amplification being a major force in tumor biogenesis. Various evidence showing that oncogenes and altered tumor suppressors lead to centrosome amplification and aneuploidy suggests that oncogenes and altered tumor suppressors are a major source of genomic instability in tumors, and that they generate those abnormal processes to initiate and sustain tumorigenesis. We discuss how altered tumor suppressors and oncogenes utilize the cell cycle regulatory machinery to signal centrosome amplification and aneuploidy.

Origin of the cell nucleus, mitosis and sex: roles of intracellular coevolution

BACKGROUND

The transition from prokaryotes to eukaryotes was the most radical change in cell organisation since life began, with the largest ever burst of gene duplication and novelty. According to the coevolutionary theory of eukaryote origins, the fundamental innovations were the concerted origins of the endomembrane system and cytoskeleton, subsequently recruited to form the cell nucleus and coevolving mitotic apparatus, with numerous genetic eukaryotic novelties inevitable consequences of this compartmentation and novel DNA segregation mechanism. Physical and mutational mechanisms of origin of the nucleus are seldom considered beyond the long-standing assumption that it involved wrapping pre-existing endomembranes around chromatin. Discussions on the origin of sex typically overlook its association with protozoan entry into dormant walled cysts and the likely simultaneous coevolutionary, not sequential, origin of mitosis and meiosis.

RESULTS

I elucidate nuclear and mitotic coevolution, explaining the origins of dicer and small centromeric RNAs for positionally controlling centromeric heterochromatin, and how 27 major features of the cell nucleus evolved in four logical stages, making both mechanisms and selective advantages explicit: two initial stages (origin of 30 nm chromatin fibres, enabling DNA compaction; and firmer attachment of endomembranes to heterochromatin) protected DNA and nascent RNA from shearing by novel molecular motors mediating vesicle transport, division, and cytoplasmic motility. Then octagonal nuclear pore complexes (NPCs) arguably evolved from COPII coated vesicle proteins trapped in clumps by Ran GTPase-mediated cisternal fusion that generated the fenestrated nuclear envelope, preventing lethal complete cisternal fusion, and allowing passive protein and RNA exchange. Finally, plugging NPC lumens by an FG-nucleoporin meshwork and adopting karyopherins for nucleocytoplasmic exchange conferred compartmentation advantages. These successive changes took place in naked growing cells, probably as indirect consequences of the origin of phagotrophy. The first eukaryote had 1-2 cilia and also walled resting cysts; I outline how encystation may have promoted the origin of meiotic sex. I also explain why many alternative ideas are inadequate.

CONCLUSION

Nuclear pore complexes are evolutionary chimaeras of endomembrane- and mitosis-related chromatin-associated proteins. The keys to understanding eukaryogenesis are a proper phylogenetic context and understanding organelle coevolution: how innovations in one cell component caused repercussions on others.

Inhibition of centriole duplication by centrobin depletion leads to p38-p53 mediated cell-cycle arrest

Previously, we have identified a novel centrosomal protein centrobin that asymmetrically localizes to the daughter centriole. We found that depletion of centrobin expression inhibited the centriole duplication and impaired cytokinesis. However, the biological significance of centrobin in the cell cycle remains unknown. In the current study, we observed that silencing centrobin significantly inhibited the proliferation of lung cancer cell A549 and prevented the cells from G1 to S transition, whereas the growth rate of lung cancer cell line H1299, a p53-null cell line, was not affected. Furthermore, we demonstrated that the G1-S-phase arrest induced by centrobin knockdown in A549 cells is mediated by the upregulation of cell-cycle regulator p53, which is associated with the activation of cellular stress induced p38 pathway instead of DNA damage induced ATM pathway. Inhibition of p38 activity or downregulation of p38 expression could overcome the cell-cycle arrest caused by centrobin depletion. Taken together, our current findings demonstrated that centrobin plays an important role in the progression of cell cycle, and a tight association between the cell-cycle progression and defective centrosomes caused by depletion of centrobin.

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.

Induction of centrosome amplification in p53 siRNA-treated human fibroblast cells by radiation exposure

Centrosome amplification can be detected in the tissues of p53(-/-) mice. In contrast, loss of p53 does not induce centrosome amplification in cultured human cells. However, examination of human cancer tissues and cultured cells has revealed a significant correlation between loss or mutational inactivation of p53 and occurrence of centrosome amplification, supporting the notion that p53 mutation alone is insufficient to induce centrosome amplification in human cells, and that additional regulatory mechanisms are involved. It has recently been shown that gamma irradiation of tumor cells induces centrosome amplification. However, the precise mechanism of radiation-induced centrosome amplification is not fully understood. In the present study, CCD32SK diploid normal human fibroblasts were transfected transiently with short interfering RNA (siRNA) specific for human p53 (CCD/p53i). There was a small increase in the frequency of centrosome amplification in CCD/p53i cells (4.0%) without irradiation. In contrast, CCD/p53i cells after 5-Gy irradiation showed a marked increase in abnormal nuclear shapes and pronounced amplification of centrosomes (46.0%). At 12 h after irradiation, irradiated CCD/p53i cells were arrested in G(2) phase. By laser scanning cytometry, abnormal mitosis with amplified centrosomes was observed frequently in the accumulating G(2)/M population at 48 h after irradiation. In the present study, we found that siRNA-mediated silencing of p53 in normal human fibroblasts, together with DNA damage by irradiation, efficiently induced centrosome amplification and nuclear fragmentation, but these phenomena were not observed with either siRNA-mediated silencing of p53 or irradiation alone.

Adult ependymal cells are postmitotic and are derived from radial glial cells during embryogenesis

Ependymal cells on the walls of brain ventricles play essential roles in the transport of CSF and in brain homeostasis. It has been suggested that ependymal cells also function as stem cells. However, the proliferative capacity of mature ependymal cells remains controversial, and the developmental origin of these cells is not known. Using confocal or electron microscopy (EM) of adult mice that received bromodeoxyuridine (BrdU) or [3H]thymidine for several weeks, we found no evidence that ependymal cells proliferate. In contrast, ependymal cells were labeled by BrdU administration during embryonic development. The majority of them are born between embryonic day 14 (E14) and E16. Interestingly, we found that the maturation of ependymal cells and the formation of cilia occur significantly later, during the first postnatal week. We analyzed the early postnatal ventricular zone at the EM and found a subpopulation of radial glia in various stages of transformation into ependymal cells. These cells often had deuterosomes. To directly test whether radial glia give rise to ependymal cells, we used a Cre-lox recombination strategy to genetically tag radial glia in the neonatal brain and follow their progeny. We found that some radial glia in the lateral ventricular wall transform to give rise to mature ependymal cells. This work identifies the time of birth and early stages in the maturation of ependymal cells and demonstrates that these cells are derived from radial glia. Our results indicate that ependymal cells are born in the embryonic and early postnatal brain and that they do not divide after differentiation. The postmitotic nature of ependymal cells strongly suggests that these cells do not function as neural stem cells in the adult.

Centrosome hyperamplification and chromosomal damage after exposure to radiation

OBJECTIVE

In order to elucidate the effects of radiation on centrosome hyperamplification (CH), we examined the centrosome duplication cycle in KK47 bladder cancer cells following irradiation.

METHODS

KK47 cells were irradiated with various doses of radiation and were examined for CH immunostaining for gamma-tubulin.

RESULTS

Nearly all control cells contained one or two centrosomes, and mitotic cells displayed typical bipolar spindles. The centrosome replication cycle is well regulated in KK47. Twenty-four hours after 5-Gy irradiation, approximately 80% of irradiated cells were arrested in G2 phase, and at 48 h after irradiation, 56.9% of cells contained more than two centrosomes. Laser scanning cytometry performed 48 h after irradiation showed the following two pathways: (1) unequal distribution of chromosomes to daughter cells, or (2) failure to undergo cytokinesis, resulting in polyploidy. With mitotic collection, M-phase cells with CH could be divided into G1 cells with micronuclei and polyploidal cells. Fluorescence in situ hybridization analysis showed clear signs of chromosomal instability (CIN) at 48 h after irradiation. The present study had two major findings: (1) continual duplication of centrosomes occurred in the cell cycle-arrested cells upon irradiation, leading to centrosome amplification; (2) cytokinesis failure was due to aberrant mitotic spindle formation caused by the presence of amplified centrosomes. Abnormal mitosis with amplified centrosomes was detected in the accumulating G2/M population after irradiation, showing that this amplification of centrosomes was not caused by failure to undergo cytokinesis, but rather that abnormal mitosis resulting from amplification of centrosomes leads to cytokinesis block.

CONCLUSION

These results suggest that CH is a critical event leading to CIN following exposure to radiation.

Loss of heterozygosity and DNA aneuploidy in colorectal adenocarcinoma

BACKGROUND

This study evaluated the relationship between DNA aneuploidy and loss of heterozygosity (LOH) at different genetic loci in colorectal adenocarcinoma.

METHODS

A total of 112 patients with surgically removed colorectal adenocarcinoma in Taipei Veterans General Hospital from January 1999 to July 2001 were included in this study. The pattern of DNA ploidy was determined with DNA flow cytometry, and the LOH of various genetic loci was determined with fluorescence polymerase chain reaction and denaturing gradient gel electrophoresis. The relationship between DNA ploidy, LOH of various genetic loci, and clinicopathologic variables was analyzed with the chi(2) test with Yates' correction as well as by multivariate binary logistic regression analysis.

RESULTS

Seventy-one (63.4%) of the 112 carcinomas had DNA aneuploidy. The DNA aneuploidy was not associated with any clinicopathologic variable. Ninety-one tumors (81.3%) exhibited LOH in at least one genetic locus. In the univariate analysis, the DNA aneuploidy was associated with LOH of Tp53-penta, D8S254, D5S346, and high-frequency LOH (P =.001, P =.016, P =.041, and P <.001, respectively). In the multivariate analysis, the most significant factor influencing DNA aneuploidy was D8S254, followed by Tp53-penta, high-frequency LOH, and D5S346.

CONCLUSIONS

DNA aneuploidy is strongly associated with LOH at specific genetic loci.

Drosophila parthenogenesis: a model for de novo centrosome assembly

The Drosophila egg contains all the components required to properly execute the early mitotic divisions but is unable to assemble a functional centrosome without a sperm-provided basal body. We show that 65% of unfertilized eggs obtained from a laboratory strain of Drosophila mercatorum can spontaneously assemble a number of cytoplasmic asters after activation, most of them duplicating in a cell cycle-dependent manner. Such asters are formed by a polarized array of microtubules that have their Asp-associated minus-ends converging at a main focus, where centrioles and typical centrosomal antigens are found. Aster assembly is spatially restricted to the anterior region of the oocyte. When fertilized, the parthenogenetic egg forms the poles of the gonomeric spindle by using the sperm-provided basal body, despite the presence within the same cytoplasm of maternal centrosomes. Thirty-five percent of parthenogenetic eggs and all unfertilized and fertilized eggs from the sibling bisexually reproducing D. mercatorum strain do not contain cytoplasmic asters. Thus, the Drosophila eggs have the potential for de novo formation of functional centrosomes independent of preexisting centrioles, but some control mechanisms preventing their spontaneous assembly must exist. We speculate that the release of the block preventing centrosome self-assembly could be a landmark for ensuring parthenogenetic reproduction.

Centrosome hyperamplification and chromosomal instability in bladder cancer

OBJECTIVE

Chromosomal instability (CIN) is a common feature of malignant tumors. Centrosome hyperamplification (CH) occurs frequently in human cancers, and may be a contributing factor in CIN. In this study, we investigated the relationship between CH and CIN in bladder cancer.

METHODS

Clinical samples obtained by transurethral resection from 22 patients with bladder cancer were examined (histological grade G1, 5 cases; G2, 6 cases; G3, 11 cases). CH was evaluated by immunohistochemistry using anti-pericentrin antibody. CIN was evaluated by fluorescence in situ hybridization (FISH). FISH probes for pericentromeric regions of chromosomes 3, 7, and 17 were hybridized to touch preparations of nuclei from frozen tissues. We also analyzed the centrosome replication cycle of bladder cancer by laser scanning cytometry (LSC).

RESULTS

Of the 22 cases examined, 18 (81.8%) had centrosome hyperamplification: CH 0, 4 cases (18.1%); CH I, 5 cases (22.7%); CH II, 5 cases (22.7%); CH III, 8 cases (36.4%). The grade of CH was directly proportional to the histological grade (p=0.03, chi(2) test). LSC analysis showed that the centrosome replication cycle was well regulated in pathologically low-grade bladder cancer, which did not have chromosomal instability. In contrast, we found marked variability of centrosomes in pathologically high-grade bladder cancer, which had chromosomal instability. CH and CIN were both detected in pathologically high-grade tumors. The grade of CH was directly proportional to the CIN grade (p=0.0079, chi(2) test).

CONCLUSION

The results of the present study suggest that CH may be involved in CIN in bladder cancer.

SAS-4 is a C. elegans centriolar protein that controls centrosome size

Centrosomes consist of a centriole pair surrounded by pericentriolar material (PCM). Previous work suggested that centrioles are required to organize PCM to form a structurally stable organelle. Here, we characterize SAS-4, a centriole component in Caenorhabditis elegans. Like tubulin, SAS-4 is incorporated into centrioles during their duplication and remains stably associated thereafter. In the absence of SAS-4, centriole duplication fails. Partial depletion of SAS-4 results in structurally defective centrioles that contain reduced levels of SAS-4 and organize proportionally less PCM. Thus, SAS-4 is a centriole-associated component whose amount dictates centrosome size. These results provide novel insight into the poorly understood role of centrioles as centrosomal organizers.

Basal body/centriole assembly and continuity

The long-standing interest in centrioles and basal bodies stems from the evolutionary conservation of their structural design and from their dual mode of assembly (templated versus de novo), revealed by electron microscopic studies nearly four decades ago and unique for a subcellular organelle. Molecular dissection of the assembly pathway during the past few years has recently progressed, essentially through direct and reverse genetic approaches. These studies revealed essential roles for centrins and the gamma-, delta-, epsilon - and eta-tubulins in assembly or as specific signals for centriole duplication. Identification of further components of basal bodies and centrioles might help to unravel the two assembly pathways and their regulation.

Functional role of epsilon-tubulin in the assembly of the centriolar microtubule scaffold

Centrioles and basal bodies fascinate by their spectacular architecture, featuring an arrangement of nine microtubule triplets into an axial symmetry, whose biogenesis relies on yet elusive mechanisms. However, the recent discovery of new tubulins, such as delta-, epsilon-, or eta-tubulin, could constitute a breakthrough for deciphering the assembly steps of this unconventional microtubule scaffold. Here, we report the functional analysis in vivo of epsilon-tubulin, based on gene silencing in Paramecium, which demonstrates that this protein, which localizes at the basal bodies, is essential for the assembly and anchorage of the centriolar microtubules.

Active RB elicits late G1/S inhibition

The retinoblastoma tumor suppressor protein (RB) is activated/dephosphorylated to mediate cell cycle inhibition in response to antimitogenic signals. To elucidate the mode of RB action at this critical transition, we utilized cell lines that can be induced to express a constitutively active allele of RB (PSM-RB). As expected, induction of PSM-RB, but not wild-type protein (WT), inhibited progression into S phase. It has been well documented that active RB inhibits E2F reporter activity, and this observation was confirmed upon induction of PSM-RB. Additionally, active RB inhibited E2F-2-mediated stimulation of cyclin E. By contrast, PSM-RB did not affect the mRNA or protein levels of endogenous cyclin E when mediating cell cycle inhibition. Similarly, there was no observable effect on cyclin E protein levels when p16ink4a was utilized to activate endogenous RB. CDK2/cyclin E complex formation was not disrupted and cyclin E-associated kinase activity was retained in the presence of PSM-RB. Additionally, centrosome duplication, a CDK2/cyclin E-dependent event, was not altered in the presence of active RB. Together, these data indicate that active RB does not block the G1/S transition through inhibition of cyclin E expression or activity. In contrast, PSM-RB leads to a dramatic reduction in cyclin A protein levels by coordinate transcriptional repression and degradation. This attenuation of cyclin A protein correlates with cell cycle inhibition. These studies indicate that RB inhibits cell cycle progression by targeting CDK2/cyclin A-dependent events at the G1/S transition to inhibit cell cycle progression.

Difference in the centrosome duplication regulatory activity among p53 'hot spot' mutants: potential role of Ser 315 phosphorylation-dependent centrosome binding of p53

The p53 tumor suppressor protein regulates centrosome duplication through multiple pathways, and p21(Waf1/Cip1) (Waf1), a major target of p53's transactivation function, has been shown to be one of the effectors. However, it had been unclear whether the p53's Waf1-independent centrosome duplication regulatory pathways require its transactivation function. In human cancers, specific residues of p53 are mutated at a high frequency. These 'hot spot' mutations abrogate p53's transactivation function. If p53 regulates centrosome duplication in a transactivation-independent manner, different 'hot spot' mutants may regulate centrosome duplication differently. To test this, we examined the effect of two 'hot spot' mutants (R175H and R249S) for their centrosome duplication regulatory activities. We found that R175H lost the ability to regulate centrosome duplication, while R249S partially retained it. Moreover, R249S associates with both unduplicated and duplicated centrosomes similar to wild-type p53, while R175H only associates with duplicated, but not unduplicated centrosomes. Since cyclin-dependent kinase 2 (CDK2) triggers initiation of centrosome duplication, and p53 is phosphorylated on Ser 315 by CDK2, we examined the p53 mutants with a replacement of Ser 315 to Ala (A) and Asp (D), both of which retain the transactivation function. We found that S315D retained a complete centrosome duplication activity, while S315A only partially retained it. Moreover, S315D associates with both unduplicated and duplicated centrosomes, while S315A associates with only duplicated, but not unduplicated centrosomes. Thus, p53 controls the centrosome duplication cycle both in transactivation-dependent and transactivation-independent manners, and the ability to bind to unduplicated centrosomes, which is controlled by phosphorylation on Ser 315, may be important for the overall p53-mediated regulation of centrosome duplication.