Centriole duplication in lysates of Spisula solidissima oocytes

Maternal inheritance of functional centrioles in two parthenogenetic nematodes

Centrioles are the core constituent of centrosomes, microtubule-organizing centers involved in directing mitotic spindle assembly and chromosome segregation in animal cells. In sexually reproducing species, centrioles degenerate during oogenesis and female meiosis is usually acentrosomal. Centrioles are retained during male meiosis and, in most species, are reintroduced with the sperm during fertilization, restoring centriole numbers in embryos. In contrast, the presence, origin, and function of centrioles in parthenogenetic species is unknown. We found that centrioles are maternally inherited in two species of asexual parthenogenetic nematodes and identified two different strategies for maternal inheritance evolved in the two species. In Rhabditophanes diutinus, centrioles organize the poles of the meiotic spindle and are inherited by both the polar body and embryo. In Disploscapter pachys, the two pairs of centrioles remain close together and are inherited by the embryo only. Our results suggest that maternally-inherited centrioles organize the embryonic spindle poles and act as a symmetry-breaking cue to induce embryo polarization. Thus, in these parthenogenetic nematodes, centrioles are maternally-inherited and functionally replace their sperm-inherited counterparts in sexually reproducing species.

Towards understanding centriole elimination

Centrioles are microtubule-based structures crucial for forming flagella, cilia and centrosomes. Through these roles, centrioles are critical notably for proper cell motility, signalling and division. Recent years have advanced significantly our understanding of the mechanisms governing centriole assembly and architecture. Although centrioles are typically very stable organelles, persisting over many cell cycles, they can also be eliminated in some cases. Here, we review instances of centriole elimination in a range of species and cell types. Moreover, we discuss potential mechanisms that enable the switch from a stable organelle to a vanishing one. Further work is expected to provide novel insights into centriole elimination mechanisms in health and disease, thereby also enabling scientists to readily manipulate organelle fate.

Experimental and Natural Induction of de novo Centriole Formation

In cycling cells, new centrioles are assembled in the vicinity of pre-existing centrioles. Although this canonical centriole duplication is a tightly regulated process in animal cells, centrioles can also form in the absence of pre-existing centrioles; this process is termed de novo centriole formation. De novo centriole formation is triggered by the removal of all pre-existing centrioles in the cell in various manners. Moreover, overexpression of polo-like kinase 4 (Plk4), a master regulatory kinase for centriole biogenesis, can induce de novo centriole formation in some cell types. Under these conditions, structurally and functionally normal centrioles can be formed de novo. While de novo centriole formation is normally suppressed in cells with intact centrioles, depletion of certain suppressor proteins leads to the ectopic formation of centriole-related protein aggregates in the cytoplasm. It has been shown that de novo centriole formation also occurs naturally in some species. For instance, during the multiciliogenesis of vertebrate epithelial cells, massive de novo centriole amplification occurs to form numerous motile cilia. In this review, we summarize the previous findings on de novo centriole formation, particularly under experimental conditions, and discuss its regulatory mechanisms.

Kinetic and structural roles for the surface in guiding SAS-6 self-assembly to direct centriole architecture

Discovering mechanisms governing organelle assembly is a fundamental pursuit in biology. The centriole is an evolutionarily conserved organelle with a signature 9-fold symmetrical chiral arrangement of microtubules imparted onto the cilium it templates. The first structure in nascent centrioles is a cartwheel, which comprises stacked 9-fold symmetrical SAS-6 ring polymers emerging orthogonal to a surface surrounding each resident centriole. The mechanisms through which SAS-6 polymerization ensures centriole organelle architecture remain elusive. We deploy photothermally-actuated off-resonance tapping high-speed atomic force microscopy to decipher surface SAS-6 self-assembly mechanisms. We show that the surface shifts the reaction equilibrium by ~10⁴ compared to solution. Moreover, coarse-grained molecular dynamics and atomic force microscopy reveal that the surface converts the inherent helical propensity of SAS-6 polymers into 9-fold rings with residual asymmetry, which may guide ring stacking and impart chiral features to centrioles and cilia. Overall, our work reveals fundamental design principles governing centriole assembly.

The centriole exhibits an evolutionarily conserved 9-fold radial symmetry that stems from a cartwheel containing vertically stacked ring polymers that harbor 9 homodimers of the protein SAS-6. Here the authors show how dual properties inherent to surface-guided SAS-6 self-assembly possess spatial information that dictates correct scaffolding of centriole architecture.

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.

Rapid centriole assembly in Naegleria reveals conserved roles for both de novo and mentored assembly

Centrioles are eukaryotic organelles whose number and position are critical for cilia formation and mitosis. Many cell types assemble new centrioles next to existing ones ("templated" or mentored assembly). Under certain conditions, centrioles also form without pre-existing centrioles (de novo). The synchronous differentiation of Naegleria amoebae to flagellates represents a unique opportunity to study centriole assembly, as nearly 100% of the population transitions from having no centrioles to having two within minutes. Here, we find that Naegleria forms its first centriole de novo, immediately followed by mentored assembly of the second. We also find both de novo and mentored assembly distributed among all major eukaryote lineages. We therefore propose that both modes are ancestral and have been conserved because they serve complementary roles, with de novo assembly as the default when no pre-existing centriole is available, and mentored assembly allowing precise regulation of number, timing, and location of centriole assembly.

Centriole continuity: out with the new, in with the old

Centrioles are essential microtubule-based organelles, typically present in pairs, which organize cilia and centrosomes. Their mode of biogenesis is unique for a subcellular organelle since, during cell division, each pre-existing centriole guides the formation of a new one, a process that is coordinated with DNA replication. After centriole duplication, the new centrosomes migrate in opposite direction and localize at each pole of the mitotic spindle. This singular dynamics led to think that centrioles were permanent self-replicating structures coordinating cytoplasm and nuclear division. This vision then fell gradually into disuse when centrioles were shown to be capable to form de novo, in the absence of a pre-existing structure, and to be actually dispensable for cell division. However, new data, which are reviewed here, have breathed new life into the old ideas.

p53 protects against genome instability following centriole duplication failure

Centriole function has been difficult to study because of a lack of specific tools that allow persistent and reversible centriole depletion. Here we combined gene targeting with an auxin-inducible degradation system to achieve rapid, titratable, and reversible control of Polo-like kinase 4 (Plk4), a master regulator of centriole biogenesis. Depletion of Plk4 led to a failure of centriole duplication that produced an irreversible cell cycle arrest within a few divisions. This arrest was not a result of a prolonged mitosis, chromosome segregation errors, or cytokinesis failure. Depleting p53 allowed cells that fail centriole duplication to proliferate indefinitely. Washout of auxin and restoration of endogenous Plk4 levels in cells that lack centrioles led to the penetrant formation of de novo centrioles that gained the ability to organize microtubules and duplicate. In summary, we uncover a p53-dependent surveillance mechanism that protects against genome instability by preventing cell growth after centriole duplication failure.

Centrosome dysfunction contributes to chromosome instability, chromoanagenesis, and genome reprograming in cancer

The unique ability of centrosomes to nucleate and organize microtubules makes them unrivaled conductors of important interphase processes, such as intracellular payload traffic, cell polarity, cell locomotion, and organization of the immunologic synapse. But it is in mitosis that centrosomes loom large, for they orchestrate, with clockmaker's precision, the assembly and functioning of the mitotic spindle, ensuring the equal partitioning of the replicated genome into daughter cells. Centrosome dysfunction is inextricably linked to aneuploidy and chromosome instability, both hallmarks of cancer cells. Several aspects of centrosome function in normal and cancer cells have been molecularly characterized during the last two decades, greatly enhancing our mechanistic understanding of this tiny organelle. Whether centrosome defects alone can cause cancer, remains unanswered. Until recently, the aggregate of the evidence had suggested that centrosome dysfunction, by deregulating the fidelity of chromosome segregation, promotes and accelerates the characteristic Darwinian evolution of the cancer genome enabled by increased mutational load and/or decreased DNA repair. Very recent experimental work has shown that missegregated chromosomes resulting from centrosome dysfunction may experience extensive DNA damage, suggesting additional dimensions to the role of centrosomes in cancer. Centrosome dysfunction is particularly prevalent in tumors in which the genome has undergone extensive structural rearrangements and chromosome domain reshuffling. Ongoing gene reshuffling reprograms the genome for continuous growth, survival, and evasion of the immune system. Manipulation of molecular networks controlling centrosome function may soon become a viable target for specific therapeutic intervention in cancer, particularly since normal cells, which lack centrosome alterations, may be spared the toxicity of such therapies.

Localization of rRNA transcribed spacer domains in the nucleolinus and maternal procentrosomes of surf clam (Spisula) oocytes

The nucleolinus is a nuclear subcompartment long ago posited to play a role in cell division. In a recent study using surf clam oocytes, cytoplasmic foci containing a nucleolinar protein were shown to later recruit γ-tubulin, identifying them as centrosomal precursors. (1) We now demonstrate the presence of structural RNAs from the nucleolinus in these procentrosomes. They include the well-known but poorly understood rRNA-transcribed spacer regions. In situ hybridization revealed a specific and dynamic association of these structural RNAs with the cell division apparatus that extends through the early stages of meiosis. In addition to their bearing on the debate over the nature of centrosome- and spindle-associated RNAs, the observations also suggest that rRNA spacer regions are not simply waste products to be discarded immediately, but may be functional byproducts that play a role in formation of the cell division apparatus.

Induction of robust de novo centrosome amplification, high-grade spindle multipolarity and metaphase catastrophe: a novel chemotherapeutic approach

Centrosome amplification (CA) and resultant chromosomal instability have long been associated with tumorigenesis. However, exacerbation of CA and relentless centrosome declustering engender robust spindle multipolarity (SM) during mitosis and may induce cell death. Recently, we demonstrated that a noscapinoid member, reduced bromonoscapine, (S)-3-(R)-9-bromo-5-(4,5-dimethoxy-1,3-dihydroisobenzofuran-1-yl)-4-methoxy-6-methyl-5,6,7,8-tetrahydro-[1,3]dioxolo-[4,5-g]isoquinoline (Red-Br-nos), induces reactive oxygen species (ROS)-mediated autophagy and caspase-independent death in prostate cancer PC-3 cells. Herein, we show that Red-Br-nos induces ROS-dependent DNA damage that resulted in high-grade CA and SM in PC-3 cells. Unlike doxorubicin, which causes double-stranded DNA breaks and chronic G2 arrest accompanied by ‘templated' CA, Red-Br-nos-mediated DNA damage elicits de novo CA during a transient S/G2 stall, followed by checkpoint abrogation and mitotic entry to form aberrant mitotic figures with supernumerary spindle poles. Attenuation of multipolar phenotype in the presence of tiron, a ROS inhibitor, indicated that ROS-mediated DNA damage was partly responsible for driving CA and SM. Although a few cells (∼5%) yielded to aberrant cytokinesis following an ‘anaphase catastrophe', most mitotically arrested cells (∼70%) succumbed to ‘metaphase catastrophe,' which was caspase-independent. This report is the first documentation of rapid de novo centrosome formation in the presence of parent centrosome by a noscapinoid family member, which triggers death-inducing SM via a unique mechanism that distinguishes it from other ROS-inducers, conventional DNA-damaging agents, as well as other microtubule-binding drugs.

Composition and dynamics of the nucleolinus, a link between the nucleolus and cell division apparatus in surf clam (Spisula) oocytes

The nucleolinus is a little-known cellular structure, discovered over 150 years ago (Agassiz, L. (1857) Contributions to the Natural History of the United States of America, First Monograph, Part IIL, Little, Brown and Co., Boston) and thought by some investigators in the late 19th to mid-20th century to function in the formation of the centrosomes or spindle. A role for the nucleolinus in formation of the cell division apparatus has recently been confirmed in oocytes of the surf clam, Spisula solidissima (Alliegro, M. A., Henry, J. J., and Alliegro, M. C. (2010) Proc. Natl. Acad. Sci. U.S.A. 107, 13718-13723). However, we know so little about the composition and dynamics of this compartment, it is difficult to construct mechanistic hypotheses or even to be sure that prior reports were describing analogous structures in the cells of mammals, amphibians, plants, and other organisms where it was observed. Surf clam oocytes are an attractive model to approach this problem because the nucleolinus is easily visible by light microscopy, making it accessible by laser microsurgery as well as isolation by common cell fractionation techniques. In this report, we analyze the macromolecular composition of isolated Spisula nucleolini and examine the relationship of this structure to the nucleolus and cell division apparatus. Analysis of nucleolinar RNA and protein revealed a set of molecules that overlaps with but is nevertheless distinct from the nucleolus. The proteins identified were primarily ones involved in nucleic acid metabolism and cell cycle regulation. Monoclonal antibodies generated against isolated nucleolini revealed centrosomal forerunners in the oocyte cytoplasm. Finally, induction of damage to the nucleolinus by laser microsurgery altered the trafficking of α- and γ-tubulin after fertilization. These observations strongly support a role for the nucleolinus in cell division and represent our first clues regarding mechanism.

Cep152 interacts with Plk4 and is required for centriole duplication

Cep152, the orthologue of Drosophila Asterless, is a Plk4 target that functions with Plk4 in centriole assembly.

Centrioles are microtubule-based structures that organize the centrosome and nucleate cilia. Centrioles duplicate once per cell cycle, and duplication requires Plk4, a member of the Polo-like kinase family; however, the mechanism linking Plk4 activity and centriole formation is unknown. In this study, we show in human and frog cells that Plk4 interacts with the centrosome protein Cep152, the orthologue of Drosophila melanogaster Asterless. The interaction requires the N-terminal 217 residues of Cep152 and the crypto Polo-box of Plk4. Cep152 and Plk4 colocalize at the centriole throughout the cell cycle. Overexpression of Cep152 (1–217) mislocalizes Plk4, but both Cep152 and Plk4 are able to localize to the centriole independently of the other. Depletion of Cep152 prevents both normal centriole duplication and Plk4-induced centriole amplification and results in a failure to localize Sas6 to the centriole, an early step in duplication. Cep152 can be phosphorylated by Plk4 in vitro, suggesting that Cep152 acts with Plk4 to initiate centriole formation.

Centrosome biogenesis continues in the absence of microtubules during prolonged S-phase arrest

When CHO cells are arrested in S-phase, they undergo repeated rounds of centrosome duplication without cell-cycle progression. While the increase is slow and asynchronous, the number of centrosomes in these cells does rise with time. To investigate mechanisms controlling this duplication, we have arrested CHO cells in S-phase for up to 72 h, and coordinately inhibited new centriole formation by treatment with the microtubule poison colcemid. We find that in such cells, the pre-existing centrosomes remain, and a variable number of foci--containing alpha/gamma-tubulin and centrin 2--assemble at the nuclear periphery. When the colcemid is washed out, the nuclear-associated foci disappear, and cells assemble new centriole-containing centrosomes, which accumulate the centriole scaffold protein SAS-6, nucleate microtubule asters, and form functional mitotic spindle poles. The number of centrosomes that assemble following colcemid washout increases with duration of S-phase arrest, even though the number of nuclear-associated foci or pre-existing centrosomes does not increase. This suggests that during S-phase, a cryptic generative event occurs repeatedly, even in the absence of new triplet microtubule assembly. When triplet microtubule assembly is restored, these cryptic generative events become realized, and multiple centriole-containing centrosomes assemble.

Rediscovery of the nucleolinus, a dynamic RNA-rich organelle associated with the nucleolus, spindle, and centrosomes

The nucleolinus is an RNA-rich compartment, closely apposed to or embedded within the nucleolus. Discovered over 150 y ago, fewer than two dozen articles have been published on the nucleolinus, probably because complex histochemical stains are required for its visualization in the great majority of cells. The nucleolinus has been reported in invertebrate oocytes, mammalian and amphibian epithelial cells, neurons, and several transformed cell lines. A prominent nucleolinus, clearly visible with transmitted light microscopes at 10x magnification, is present in each oocyte of the surf clam, Spisula solidissima. We observed a consistent relationship between the nucleolinus and the developing meiotic apparatus following Spisula oocyte activation. Through sonication and sucrose gradient fractionation of purified oocyte nuclei, we isolated nucleolini, extracted their RNA, and prepared an in situ riboprobe (NLi-1), which is associated specifically with the nucleolinus, confirming its unique composition. Other in situ observations revealed a NLi-1 and nucleolinar association with the developing spindle and centrosomes. Laser microsurgery that targeted the nucleolinus resulted in failed meiotic cell division in parthenogenetically activated oocytes and failed mitosis in fertilized oocytes. Although the nucleolinus may be a forgotten organelle, its demonstrated role in spindle formation suggests it deserves renewed attention.

Overly long centrioles and defective cell division upon excess of the SAS-4-related protein CPAP

The centrosome is the principal microtubule organizing center (MTOC) of animal cells. Accurate centrosome duplication is fundamental for genome integrity and entails the formation of one procentriole next to each existing centriole, once per cell cycle. The procentriole then elongates to eventually reach the same size as the centriole. The mechanisms that govern elongation of the centriolar cylinder and their potential relevance for cell division are not known. Here, we show that the SAS-4-related protein CPAP is required for centrosome duplication in cycling human cells. Furthermore, we demonstrate that CPAP overexpression results in the formation of abnormally long centrioles. This also promotes formation of more than one procentriole in the vicinity of such overly long centrioles, eventually resulting in the presence of supernumerary MTOCs. This in turn leads to multipolar spindle assembly and cytokinesis defects. Overall, our findings suggest that centriole length must be carefully regulated to restrict procentriole number and thus ensure accurate cell division.

The small organic compound HMN-176 delays satisfaction of the spindle assembly checkpoint by inhibiting centrosome-dependent microtubule nucleation

HMN-176 is a potential new cancer therapeutic known to retard the proliferation of tumor cell lines. Here, we show that this compound inhibits meiotic spindle assembly in surf clam oocytes and delays satisfaction of the spindle assembly checkpoint in human somatic cells by inducing the formation of short and/or multipolar spindles. HMN-176 does not affect centrosome assembly, nuclear envelope breakdown, or other aspects of meiotic or mitotic progression, nor does it affect the kinetics of Spisula or mammalian microtubule (MT) assembly in vitro. Notably, HMN-176 inhibits the formation of centrosome-nucleated MTs (i.e., asters) in Spisula oocytes and oocyte extracts, as well as from isolated Spisula or mammalian centrosomes in vitro. Together, these results reveal that HMN-176 is a first-in-class anticentrosome drug that inhibits proliferation, at least in part, by disrupting centrosome-mediated MT assembly during mitosis.

Centrioles: some self-assembly required

Centrioles play an important role in organizing microtubules and are precisely duplicated once per cell cycle. New (daughter) centrioles typically arise in association with existing (mother) centrioles (canonical assembly), suggesting that mother centrioles direct the formation of daughter centrioles. However, under certain circumstances, centrioles can also selfassemble free of an existing centriole (de novo assembly). Recent work indicates that the canonical and de novo pathways utilize a common mechanism and that a mother centriole spatially constrains the self-assembly process to occur within its immediate vicinity. Other recently identified mechanisms further regulate canonical assembly so that during each cell cycle, one and only one daughter centriole is assembled per mother centriole.

Bisphenol A directly targets tubulin to disrupt spindle organization in embryonic and somatic cells

There is increasing concern that animal and human reproduction may be adversely affected by exposure to xenoestrogens that activate estrogen receptors. There is evidence that one such compound, Bisphenol A (BPA), also induces meiotic and mitotic aneuploidy, suggesting that these kinds of molecules may also have effects on cell division. In an effort to understand how Bisphenol A might disrupt cell division, a phenotypic analysis was carried out using sea urchin eggs, whose early embryonic divisions are independent of zygotic transcription. Fertilized Lytechinus pictus eggs exposed to BPA formed multipolar spindles resulting in failed cytokinesis in a dose-dependent, transcriptionally independent manner. By use of novel biotinylated BPA affinity probes to fractionate cell-free extracts, tubulin was identified as a candidate binding protein by mass spectrometry, and BPA promoted microtubule polymerization and centrosome-based microtubule nucleation in vitro but did not appear to display microtubule-stabilizing activity. Treatment of mammalian cells demonstrated that BPA as well as a series of Bisphenol A derivatives induced ectopic spindle pole formation in the absence of centrosome overduplication. Together, these results suggest a novel mechanism by which Bisphenol A affects the nucleation of microtubules, disrupting the tight spatial control associated with normal chromosome segregation, resulting in aneuploidy.

Membrane associated nonmuscle myosin II functions as a motor for actin-based vesicle transport in clam oocyte extracts

Nonmuscle myosin II (Myo2) has been shown to associate with membranes of the trans-Golgi network and to be involved in Golgi to ER retrograde protein transport. Here, we provide evidence that Myo2 not only associates with membranes but functions to transport vesicles on actin filaments (AFs). We used extracts from unactivated clam oocytes for these studies. AFs assembled spontaneously in these extracts and myosin-dependent vesicle transport was observed upon activation. In addition, actin bundles formed and moved relative to each other at an average speed of 0.30 microm/s. Motion analysis revealed that vesicles moved on the spontaneously assembled AFs at speeds greater than 1 microm/s. The motor on these vesicles was identified as a member of the nonmuscle Myo2 family based on sequence determination by Edman chemistry. Vesicles in these extracts were purified by sucrose gradient centrifugation and movement was reconstituted in vitro using skeletal muscle actin coated coverslips. When peripheral membrane proteins of vesicles including Myo2 were removed by salt stripping or when extracts were treated with an antibody specific to clam oocyte nonmuscle Myo2, vesicle movement was inhibited. Blebbistatin, a Myo2 specific inhibitor, also blocked vesicle movement. Myo2 light chain kinase activity was found to be essential for vesicle movement and sliding of actin bundles. Together, our data provide direct evidence that nonmuscle Myo2 is involved in actin-dependent vesicle transport in clam oocytes.

Cyclin E/Cdk2 is required for sperm maturation, but not DNA replication, in early sea urchin embryos

The cell cycle is driven by the activity of cyclin/cdk complexes. In somatic cells, cyclin E/cdk2 oscillates throughout the cell cycle and has been shown to promote S-phase entry and initiation of DNA replication. In contrast, cyclin E/cdk2 activity remains constant throughout the early embryonic development of the sea urchin and localizes to the sperm nucleus following fertilization. We now show that cyclin E localization to the sperm nucleus following fertilization is not unique to the sea urchin, but also occurs in the surf clam, and inhibition of cyclin E/cdk2 activity by roscovitine inhibits the morphological changes indicative of male pronuclear maturation in sea urchin zygotes. Finally, we show that inhibition of cyclin E/cdk2 activity does not block DNA replication in the early cleavage cycles of the sea urchin. We conclude that cyclin E/cdk2 activity is required for male pronuclear maturation, but not for initiation of DNA replication in early sea urchin development.

Centrosomal protein centrin is not detectable during early pre-implantation development but reappears during late blastocyst stage in porcine embryos

Centrin is an evolutionarily conserved 20 kDa, Ca+2-binding, calmodulin-related protein associated with centrioles and basal bodies of phylogenetically diverse eukaryotic cells. Earlier studies have shown that residual centrosomes of non-rodent mammalian spermatozoa retain centrin and, in theory, could contribute this protein for the reconstruction of the zygotic centrosome after fertilization. The present work shows that CEN2 and CEN3 mRNA were detected in germinal vesicle-stage (GV) oocytes, MII oocytes, and pre-implantation embryos from the two-cell through the blastocyst stage, but not in spermatozoa. Boar ejaculated spermatozoa possess centrin as revealed by immunofluorescence microscopy and western blotting. Immature, GV oocytes possess speckles of centrin particles in the perinuclear area, visualized by immunofluorescence microscopy and exhibit a 19 kDa band revealed by western blotting. Mature MII stage oocytes lacked centrin that could be detected by immunofluorescence or western blotting. The sperm centrin was lost in zygotes after in vitro fertilization. It was not detectable in embryos by immunofluorescence microscopy until the late blastocyst stage. Embryonic centrin first appeared as fine speckles in the perinuclear area of some interphase blastocyst cells and as putative centrosomes of the spindle poles of dividing cells. The cells of the hatched blastocysts developed centrin spots comparable with those of the cultured cells. Some blastomeres displayed undefined curved plate-like centrin-labeled structures. Anti-centrin antibody labeled interphase centrosomes of cultured pig embryonic fibroblast cells as distinct spots in the juxtanuclear area. Enucleated pig oocytes reconstructed by electrofusion with pig fibroblasts displayed centrin of the donor cell during the early stages of nuclear decondensation but became undetectable in the late pronuclear or cleavage stages. These observations suggest that porcine zygotes and pre-blastocyst embryonic cells lack centrin and do not retain exogenously incorporated centrin. The early embryonic centrosomes function without centrin. Centrin in the blastocyst stage embryos is likely a result of de novo synthesis at the onset of differentiation of the pluripotent blastomeres.

Functionally unequal centrosomes drive spindle orientation in asymmetrically dividing Drosophila neural stem cells

Stem cell asymmetric division requires tight control of spindle orientation. To study this key process, we have recorded Drosophila larval neural stem cells (NBs) engineered to express fluorescent reporters for microtubules, pericentriolar material (PCM), and centrioles. We have found that early in the cell cycle, the two centrosomes become unequal: one organizes an aster that stays near the apical cortex for most of the cell cycle, while the other loses PCM and microtubule-organizing activity, and moves extensively throughout the cell until shortly before mitosis when, located near the basal cortex, it recruits PCM and organizes the second mitotic aster. Upon division, the apical centrosome remains in the stem cell, while the other goes into the differentiating daughter. Apical aster maintenance requires the function of Pins. These results reveal that spindle orientation in Drosophila larval NBs is determined very early in the cell cycle, and is mediated by asymmetric centrosome function.

Direct patterning of centrosome arrays as templates for the assembly of microtubules

We have achieved, for the first time, the selective patterning of centrosomes onto solid substrates. The use of such patterned centrosome arrays as templates for the directed polymerization of microtubules was also demonstrated. Centrosomes are small organelles in animal cells that serve as nucleation and organization centers of microtubules. Directed assembly of microtubules on the patterned centrosome arrays provides a new route to control the positions and directions of microtubules on surfaces. Combining the patterning of the isolated centrosomes and the directed growth of microtubules may lead to the generation of desired microtubule networks for bio-based nanodevices.

Controlling centrosome number: licenses and blocks

Centrosomes organize microtubule structures in animal cells. The centrosome duplicates once per cell cycle in most dividing cells via a pathway that relies on a pre-existing centrosome. The molecular mechanism of this 'once and only once' control is not understood, and recent results show that centrosomes can also be assembled by a de novo pathway that bypasses this control. These results require a rethinking of how proper centrosome number is maintained. We propose that the engagement of centrioles with each other normally blocks centrosome re-duplication, and that disengagement of centrioles from each other at the end of mitosis licenses them for duplication in the subsequent cell cycle.

JNH Values Show that N1···N3 Hydrogen Bonds Are Stronger in dsRNA A:U than dsDNA A:T Base Pairs

Here, we show that 1JNH values are on average 0.4 Hz less negative for double-stranded RNA A:U than for DNA A:T base pairs, which, according to existing theory, suggests that RNA N1...N3 hydrogen bond distances are about 0.02 A shorter than those of DNA. Also, there is a statistically relevant trend between 1JNH and 2hDelta13C2 values, which supports the original hypothesis that 2hDelta13C2 values are also sensitive to hydrogen bond distances. Finally, a context dependence is observed for these values, which suggests that hydrogen-bonding and base-stacking interactions are coupled.

Centrosome Duplication and Nematodes: Recent Insights from an Old Relationship

Centrosome duplication is required for proper cell division, and centriole formation is a key step in this process. This review discusses recent studies in C. elegans that have identified five core proteins required for centriole formation, thus shedding light into the mechanisms underlying centrosome duplication in nematodes and beyond.

Kinesin-1 mediates translocation of the meiotic spindle to the oocyte cortex through KCA-1, a novel cargo adapter

In animals, female meiotic spindles are attached to the egg cortex in a perpendicular orientation at anaphase to allow the selective disposal of three haploid chromosome sets into polar bodies. We have identified a complex of interacting Caenorhabditis elegans proteins that are involved in the earliest step in asymmetric positioning of anastral meiotic spindles, translocation to the cortex. This complex is composed of the kinesin-1 heavy chain orthologue, UNC-116, the kinesin light chain orthologues, KLC-1 and -2, and a novel cargo adaptor, KCA-1. Depletion of any of these subunits by RNA interference resulted in meiosis I metaphase spindles that remained stationary at a position several micrometers from the cell cortex during the time when wild-type spindles translocated to the cortex. After this prolonged stationary period, unc-116(RNAi) spindles moved to the cortex through a partially redundant mechanism that is dependent on the anaphase-promoting complex. This study thus reveals two sequential mechanisms for translocating anastral spindles to the oocyte cortex.

The de novo centriole assembly pathway in HeLa cells: cell cycle progression and centriole assembly/maturation

It has been reported that nontransformed mammalian cells become arrested during G1 in the absence of centrioles (Hinchcliffe, E., F. Miller, M. Cham, A. Khodjakov, and G. Sluder. 2001. Science. 291:1547-1550). Here, we show that removal of resident centrioles (by laser ablation or needle microsurgery) does not impede cell cycle progression in HeLa cells. HeLa cells born without centrosomes, later, assemble a variable number of centrioles de novo. Centriole assembly begins with the formation of small centrin aggregates that appear during the S phase. These, initially amorphous "precentrioles" become morphologically recognizable centrioles before mitosis. De novo-assembled centrioles mature (i.e., gain abilities to organize microtubules and replicate) in the next cell cycle. This maturation is not simply a time-dependent phenomenon, because de novo-formed centrioles do not mature if they are assembled in S phase-arrested cells. By selectively ablating only one centriole at a time, we find that the presence of a single centriole inhibits the assembly of additional centrioles, indicating that centrioles have an activity that suppresses the de novo pathway.

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.

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.

Maternal gamma (gamma)-tubulin is involved in microtubule reorganization during bovine fertilization and parthenogenesis

In this study, gamma-tubulin distribution was determined chronologically in conjunction with microtubule dynamics during bovine fertilization and parthenogenesis. In unfertilized bovine oocytes, gamma-tubulin was identified in the cytoplasm, mainly in the cortex and concentrated in the meiotic spindle. Following sperm penetration, gamma-tubulin in the cytoplasm was recruited by a sperm component. During pronuclear apposition, gamma-tubulin was localized as spots at the spindle poles. gamma-tubulin spots were observed in blastomeres of embryos cleaved in vitro. Following electrical stimulation, gamma-tubulin and microtubule matrix were noted in oocyte cortex. In the late pronuclear stage, considerably less gamma-tubulin and microtubules were detected in the cytoplasm. At the mitotic metaphase of parthenotes, gamma-tubulin was recruited to the condensed chromatin and concentrated in the spindle. The gamma-tubulin spots were not detected until the 8-cell stage of parthenotes. This suggests that maternal gamma-tubulin is recruited by a sperm component to reconstitute the zygotic centrosome. In the absence of sperm components, the cell cycle-related assembly of gamma-tubulin organizes microtubule nucleation for positioning the pronucleus and spindle protein of mitotic metaphase during the first cell cycle of bovine parthenotes.

Centrosomes: coiled-coils organize the cell center

The centrosome serves as a structural context for cytoplasmic organization. Recent studies on mutants of the nematode worm Caenorhabditis elegans have provided new insight into the framework to which microtubules and key regulators of centrosome behavior are anchored.

De novo formation of centrosomes in vertebrate cells arrested during S phase

The centrosome usually replicates in a semiconservative fashion, i.e., new centrioles form in association with preexisting "maternal" centrioles. De novo formation of centrioles has been reported for a few highly specialized cell types but it has not been seen in vertebrate somatic cells. We find that when centrosomes are completely destroyed by laser microsurgery in CHO cells arrested in S phase by hydroxyurea, new centrosomes form by de novo assembly. Formation of new centrosomes occurs in two steps: approximately 5-8 h after ablation, clouds of pericentriolar material (PCM) containing gamma-tubulin and pericentrin appear in the cell. By 24 h, centrioles have formed inside of already well-developed PCM clouds. This de novo pathway leads to the formation of a random number of centrioles (2-14 per cell). Although clouds of PCM consistently form even when microtubules are completely disassembled by nocodazole, the centrioles are not assembled under these conditions.

Reconstitution of centrosome microtubule nucleation in Spisula

Evidence strongly supports the hypothesis that gamma-TuRCs are sites for the nucleation of microtubules within the centrosome PCM (Zheng et al., 1995; Moritz et al., 1995a,b). Further, these structures appear to be recruited to the centrosome from cytoplasmic pools during centrosome assembly and centrosome maturation (Moritz et al., 1998; Schnackenberg et al., 1998, 2000; Schnackenberg and Palazzo, 1999; Khodjakov and Rieder, 1999). It has also been shown that the centrosome PCM contains a network of filamentous fibers, which function as a scaffold for binding these microtubule nucleating sites (Schnackenberg et al., 1998). However, binding gamma-tubulin rings to this scaffold requires at least one additional factor (Moritz et al., 1998; Schnackenberg et al., 2000). Because extracts prepared from a variety of biological sources are capable of supporting the recovery of microtubule nucleation by Spisula KICRs (Schnackenberg et al., 2000), the methods outlined in this chapter could prove useful in the search for factors that are necessary for centrosome assembly and the increase in centrosome-dependent microtubule nucleation during centrosome maturation.

Re-evaluating centrosome function

Over the past 100 years, the centrosome has risen in status from an enigmatic organelle, located at the focus of microtubules, to a key player in cell-cycle progression and cellular control. A growing body of evidence indicates that centrosomes might not be essential for spindle assembly, whereas recent data indicate that they might be important for initiating S phase and completing cytokinesis. Molecules that regulate centrosome duplication have been identified, and the expanding list of intriguing centrosome-anchored activities, the functions of which have yet to be determined, promises continued discovery.

Centrioles take center stage

Centrioles are among the most beautiful and mysterious of all cell organelles. Although the ultrastructure of centrioles has been studied in great detail ever since the advent of electron microscopy, these studies raised as many questions as they answered, and for a long time both the function and mode of duplication of centrioles remained controversial. It is now clear that centrioles play an important role in cell division, although cells have backup mechanisms for dividing if centrioles are missing. The recent identification of proteins comprising the different ultrastructural features of centrioles has proven that these are not just figments of the imagination but distinct components of a large and complex protein machine. Finally, genetic and biochemical studies have begun to identify the signals that regulate centriole duplication and coordinate the centriole cycle with the cell cycle.

Kinetics and regulation of de novo centriole assembly. Implications for the mechanism of centriole duplication

BACKGROUND

Centriole duplication is a key step in the cell cycle whose mechanism is completely unknown. Why new centrioles always form next to preexisting ones is a fundamental question. The simplest model is that preexisting centrioles nucleate the assembly of new centrioles, and that although centrioles can in some cases form de novo without this nucleation, the de novo assembly mechanism should be too slow to compete with normal duplication in order to maintain fidelity of centriole duplication.

RESULTS

We have measured the rate of de novo centriole assembly in vegetatively dividing cells that normally always contain centrioles. By using mutants of Chlamydomonas that are defective in centriole segregation, we obtained viable centrioleless cells that continue to divide, and find that within a single generation, 50% of these cells reacquire new centrioles by de novo assembly. This suggests that the rate of de novo assembly is approximately half the rate of templated duplication. A mutation in the VFL3 gene causes a complete loss of the templated assembly pathway without eliminating de novo assembly. A mutation in the centrin gene also reduced the rate of templated assembly.

CONCLUSIONS

These results suggest that there are two pathways for centriole assembly, namely a templated pathway that requires preexisting centrioles to nucleate new centriole assembly, and a de novo assembly pathway that is normally turned off when centrioles are present.

Centrosome maturation

In the past, centrosome maturation has been described as the change in microtubule nucleation potential that occurs as cells pass through specific phases of the cell cycle. It is suggested that the idea of centrosome maturation be expanded to include gain of functions that are not necessarily related to microtubule nucleation. Some of these functions could be transient and dependent on the temporary association of molecules with the centrosome as cells progress through the cell cycle. Thus, the centrosome may best be viewed as a site for mediating macromolecular interactions, perhaps as a central processing station within the cell. The centromatrix, a relatively stable lattice of polymers within the centrosome's PCM, could serve as a scaffold for the transient binding of mediator molecules, as well as allow the dynamic exchange of centrosome constituents with a soluble cytoplasmic pool. New evidence adds support to the idea that centrioles are crucial for the maintenance of PCM structure. However, significant evidence indicates that aspects of centrosome structure and function can be maintained in the absence of centrioles. In the case of paternal centrosome maturation, sperm centrioles may not contain an associated centromatrix. It is proposed that regulation of paternal centrioles or centriole associated proteins could mediate centriole-dependent centromatrix assembly following fertilization. Thus, regulation of centromatrix-centriole interactions could be involved in maintaining the integrity of the centrosome's PCM and play an important role in centrosome disassembly during cell differentiation and morphogenesis.

Tektins as structural determinants in basal bodies

Tektins, present as three equimolar 47-55 kDa protein components, form highly insoluble protofilaments that are integral to the junctional region of outer doublet microtubules in cilia and flagella. To identify and quantify tektins in other compound microtubules such as centrioles or basal bodies, a rabbit antiserum was raised against tektin filaments isolated from Spisula solidissima (surf clam) sperm flagellar outer doublets and affinity-purified with nitrocellulose blot strips of tektins resolved by SDS- or SDS-urea-PAGE. These antibodies recognized analogous tektins in axonemes of organisms ranging from ctenophores to higher vertebrates. Quantitative immunoblotting established that outer doublet tektins occur in a 1:17 weight ratio to tubulin. Cilia and basal apparatuses were prepared from scallop gill epithelial cells; cilia and deciliated cells were prepared from rabbit trachea. Tektins were detected by immunoblotting in basal body-enriched preparations while tektins were localized to individual basal bodies by immunofluorescence. Supported by greater fluorescence in basal bodies than in adjacent axonemes in tracheal cells, analysis of basal apparatuses demonstrated both a proportionately greater ratio of tektin to tubulin (approximately 1:13) and two distinct solubility classes of tektins, consistent with tektins comprising the B-C junction of triplets in addition to the A-B junction as in doublets.

Role for microtubules in centrosome doubling in Chinese hamster ovary cells

The centrosome must be replicated once, and only once, during each cell cycle. To achieve this somatic cells need to synthesize centrosome proteins, target those centrosome proteins to the parental centrosome, and then assemble the centrosome subunits into a functional organelle. The mechanisms that underlie each of these processes are not known. Studies were performed to investigate whether cellular microtubules are involved in centrosome doubling events. For these experiments, CHO cells were arrested in either hydroxyurea (HU) alone or in HU plus a microtubule inhibitor for 3640 h. The cells then were induced to enter mitosis and the numbers of spindle poles/centrosomes were counted following processing of the cells for immunofluorescence microscopy using anticentrosome antiserum. These studies demonstrated that centrosome replication events occurred in cells arrested with either HU alone or HU and taxol while centrosome replication did not occur in cells treated with HU and either nocodazole or colcemid. Immunoblot analysis determined that centrosome proteins were synthesized in HU/nocodazole-arrested cells and demonstrated that the role of microtubules in the centrosome replication process is not to ensure the synthesis of centrosome subunits. Rather, our results suggest that microtubules may be involved in the transport/targeting of centrosome subunits to the parental centrosome during duplication events. For microtubules to contribute to the transport of centrosome subunits during centrosome doubling, centrosome subunits would need to be able to bind to microtubules. To test this, co-sedimentation studies were performed and it was determined that the centrosome proteins, though overproduced under these conditions, remained soluble in HU/nocodazole-treated cells and co-pelleted with taxol-stabilized microtubules in the presence of GTP and AMP-PNP. Moreover, co-sedimentation of one of the centrosome proteins, PCM-1, with microtubules could be inhibited by pre-incubation of extracts with antibodies against dynactin. Together, these data suggest that during centrosome replication in somatic mammalian cells, PCM-1, and perhaps other centrosome components, are targeted to the centrosome via transport along microtubules by motor complexes that include dynein/dynactin.

Interaction of cell cycle kinases, microtubules, and chromatin in ascidian oocytes during meiosis

We used kinase assays and confocal microscopy to study the interaction of cell cycle proteins with microtubule organising centres (MTOC) and chromatin in ascidian oocytes during meiosis. The activity of maturation promoting factor (MPF) and mitogen activated protein kinase (MAPK) appear not to be correlated in control oocytes. MPF activity peaks during metaphase I and II of the meiotic cell cycle whereas the activity of MAPK peaks at telophase I and is subsequently degraded to remain at low levels for the remainder of meiosis. The protein synthesis inhibitor emetine induces the degradation in MPF activity in unfertilized metaphase-I (M-I) oocytes, while MAPK is unaffected. Emetine does not alter the activities of these cell cycle kinases in fertilized oocytes during meiosis I but MPF activity remains low while MAPK activity is high for an elongated time period and oocytes do not complete meiosis I. Emetine induces maternal MTOC duplication in unfertilized M-I oocytes and prevents sperm aster growth in fertilized oocytes, but it does not alter the M-I meiotic apparatus in unfertilized oocytes. These experiments suggest that neither MPF alone nor emetine-sensitive proteins are responsible for M-I arrest in ascidian oocytes, MAPK may ensure this stability. In addition, we showed that the maternal MTOC is present at M-I but suppressed from duplicating in an emetine-sensitive manner.

Reconstitution of microtubule nucleation potential in centrosomes isolated from Spisula solidissima oocytes

Treatment of isolated Spisula solidissima centrosomes with KI removes (gamma)-tubulin, 25 nm rings, and their microtubule nucleation potential, revealing the presence of a filamentous lattice, the ‘centromatrix’. Treatment of this centromatrix with Spisula oocyte extract results in the binding of (gamma)-tubulin and 25 nm rings, and the recovery of microtubule nucleation potential. Fractionation of this extract resulted in the separation of elements that are required for the recovery of microtubule nucleation potential. We show that some, but not all, of the elements needed cosediment with microtubules. Further, extracts prepared from activated (meiotic) and non-activated (interphase) Spisula oocytes, CHO cells blocked in S phase, Drosophila embryos and Xenopus oocytes all support the recovery of microtubule nucleation potential by the Spisula centromatrix. These results demonstrate that components necessary for centrosome-dependent microtubule nucleation are functionally conserved and abundant in both interphase and meiotic/mitotic cytoplasm.

How centrioles work: lessons from green yeast

Centrioles are the organizing centers around which centrosomes assemble. Despite a century of study, the molecular details of centriole function and assembly remain largely unknown. Recent work has exploited the unique advantages of unicellular algae to reveal proteins that play central roles in centriole biology.

Paternal contributions to the mammalian zygote: fertilization after sperm-egg fusion

Mammalian fertilization has traditionally been regarded as a simple blending of two gametes, during which the haploid genome of the fertilizing spermatozoon constitutes the primary paternal contribution to the resulting embryo. In contrast to this view, new research provides evidence of important cytoplasmic contributions made by the fertilizing spermatozoon to the zygotic makeup, to the organization of preimplantation development, and even reproductive success of new forms of assisted fertilization. The central role of the sperm-contributed centriole in the reconstitution of zygotic centrosome has been established in most mammalian species and is put in contrast with strictly maternal centrosomal inheritance in rodents. The complementary reduction or multiplication of sperm and oocyte organelles during gametogenesis, exemplified by the differences in the biogenesis of centrosome in sperm and oocytes, represents an intriguing mechanism for avoiding their redundancy during early embryogenesis. New studies on perinuclear theca of sperm revealed its importance for both spermatogenesis and fertilization. Remodeling of the sperm chromatin into a male pronucleus is guided by oocyte-produced, reducing peptide glutathione and a number of molecules required for the reconstitution of the functional nuclear envelope and nuclear skeleton. Although some of the sperm structures are transformed into zygotic components, the elimination of others is vital to early stages of embryonic development. Sperm mitochondria, carrying potentially harmful paternal mtDNA, appear to be eliminated by a ubiquitin-dependent mechanism. Other accessory structures of the sperm axoneme, including fibrous sheath, microtubule doublets, outer dense fibers, and the striated columns of connecting piece, are discarded in an orderly fashion. The new methods of assisted fertilization, represented by intracytoplasmic sperm injection and round spermatid injection, bypass multiple steps of natural fertilization by introducing an intact spermatozoon or spermatogenic cell into oocyte cytoplasm. Consequently, the carryover of sperm accessory structures that would normally be eliminated before or during the entry of sperm into oocyte cytoplasm persist therein and may interfere with early embryonic development, thus decreasing the success rate of assisted fertilization and possibly causing severe embryonic anomalies. Similarly, foreign organelles, proteins, messenger RNAs, and mitochondrial DNAs, which may have a profound impact on the embryonic development, are propagated by the nuclear transfer of embryonic blastomeres and somatic cell nuclei. This aspect of assisted fertilization is yet to be explored by a focused effort.