Plk2 regulated centriole duplication is dependent on its localization to the centrosome and a functional polo-box domain

In mammalian cells, the centrosome consists of a pair of centrioles and amorphous pericentriolar material. The centrosome duplicates once per cell cycle. Polo like kinases (Plks) perform crucial functions in cell-cycle progression and during mitosis. The polo-like kinase-2, Plk2, is activated near the G1/S phase transition, and plays an important role in the reproduction of centrosomes. In this study, we show that the polo-box of Plk2 is required both for association to the centrosome and centriole duplication. Mutation of critical sites in the Plk2 polo-box prevents centrosomal localization and impairs centriole duplication. Plk2 is localized to centrosomes during early G1 phase where it only associates to the mother centriole and then distributes equally to both mother and daughter centrioles at the onset of S phase. Furthermore, our results imply that Plk2 mediated centriole duplication is dependent on Plk4 function. In addition, we find that siRNA-mediated down-regulation of Plk2 leads to the formation of abnormal mitotic spindles confirming that Plk2 may have a function in the reproduction of centrioles.     

Control of daughter centriole formation by the pericentriolar material

Controlling the number of its centrioles is vital for the cell, as supernumerary centrioles cause multipolar mitosis and genomic instability. Normally, one daughter centriole forms on each mature (mother) centriole; however, a mother centriole can produce multiple daughters within a single cell cycle. The mechanisms that prevent centriole 'overduplication' are poorly understood. Here we use laser microsurgery to test the hypothesis that attachment of the daughter centriole to the wall of the mother inhibits formation of additional daughters. We show that physical removal of the daughter induces reduplication of the mother in S-phase-arrested cells. Under conditions when multiple daughters form simultaneously on a single mother, all of these daughters must be removed to induce reduplication. The number of daughter centrioles that form during reduplication does not always match the number of ablated daughter centrioles. We also find that exaggeration of the pericentriolar material (PCM) by overexpression of the PCM protein pericentrin in S-phase-arrested CHO cells induces formation of numerous daughter centrioles. We propose that that the size of the PCM cloud associated with the mother centriole restricts the number of daughters that can form simultaneously.     

Centrioles in the cell cycle. I. Epithelial cells

A study was made of the structure of the centrosome in the cell cycle in a nonsynchronous culture of pig kidney embryo (PE) cells. In the spindle pole of the metaphase cell there are two mutually perpendicular centrioles (mother and daughter) which differ in their ultrastructure. An electron-dense halo, which surrounds only the mother centriole and is the site where spindle microtubules converge, disappears at the end of telophase. In metaphase and anaphase, the mother centriole is situated perpendicular to the spindle axis. At the beginning of the G1 period, pericentriolar satellites are formed on the mother centriole with microtubules attached to them; the two centrioles diverge. The structures of the two centrioles differ throughout interphase; the mother centriole has appendages, the daughter does not. Replication of the centrioles occurs approximately in the middle of the S period. The structure of the procentrioles differs sharply from that of the mature centriole. Elongation of procentrioles is completed in prometaphase, and their structure undergoes a number of successive changes. In the G2 period, pericentriolar satellites disappear and some time later a fibrillar halo is formed on both mother centrioles, i.e., spindle poles begin to form. In the cells that have left the mitotic cycle (G0 period), replication of centrioles does not take place; in many cells, a cilium is formed on the mother centriole. In a small number of cells a cilium is formed in the S and G2 periods, but unlike the cilium in the G0 period it does not reach the surface of the cell. In all cases, it locates on the centriole with appendages. At the beginning of the G1 period, during the G2 period, and in nonciliated cells in the G0 period, one of the centrioles is situated perpendicular to the substrate. On the whole, it takes a mature centriole a cycle and a half to form in PE cells.     

Cep57 and Cep57L1 maintain centriole engagement in interphase to ensure centriole duplication cycle

Centrioles duplicate in interphase only once per cell cycle. Newly formed centrioles remain associated with their mother centrioles. The two centrioles disengage at the end of mitosis, which licenses centriole duplication in the next cell cycle. Therefore, timely centriole disengagement is critical for the proper centriole duplication cycle. However, the mechanisms underlying centriole engagement during interphase are poorly understood. Here, we show that Cep57 and Cep57L1 cooperatively maintain centriole engagement during interphase. Codepletion of Cep57 and Cep57L1 induces precocious centriole disengagement in interphase without compromising cell cycle progression. The disengaged daughter centrioles convert into centrosomes during interphase in a Plk1-dependent manner. Furthermore, the centrioles reduplicate and the centriole number increases, which results in chromosome segregation errors. Overall, these findings demonstrate that the maintenance of centriole engagement by Cep57 and Cep57L1 during interphase is crucial for the tight control of centriole copy number and thus for proper chromosome segregation.     

Cnn dynamics drive centrosome size asymmetry to ensure daughter centriole retention in Drosophila neuroblasts

Centrosomes comprise a pair of centrioles surrounded by an amorphous network of pericentriolar material (PCM). In certain stem cells, the two centrosomes differ in size, and this appears to be important for asymmetric cell division [1, 2]. In some cases, centrosome asymmetry is linked to centriole age because the older, mother centriole always organizes more PCM than the daughter centriole, thus ensuring that the mother centriole is always retained in the stem cell after cell division [3]. This has raised the possibility that an "immortal" mother centriole may help maintain stem cell fate [4, 5]. It is unclear, however, how centrosome size asymmetry is generated in stem cells. Here we provide compelling evidence that centrosome size asymmetry in Drosophila neuroblasts is generated by the differential regulation of Cnn incorporation into the PCM at mother and daughter centrioles. Shortly after centriole separation, mother and daughter centrioles organize similar amounts of PCM, but Cnn incorporation is then rapidly downregulated at the mother centriole, while it is maintained at the daughter centriole. This ensures that the daughter centriole maintains its PCM and so its position at the apical cortex. Thus, the?daughter centriole, rather than an "immortal" mother centriole, is ultimately retained in these stem cells.     

Centrioles in the cell cycle of L cells

The structure of centrosome in non-synchronous L-cells culture during the cell cycle has been studied. In mitosis, mother and daughter centrioles, which differ in their ultrastructure, are located perpendicularly in the pole of the spindle. Microtubules, meeting in the pole area terminate mainly in electron-dense clottings of fibrillar matter surrounding the diplosoma. In telophase, disjunction of mother and daughter centrioles begins. At the beginning of G1-period, centrioles move off from each other for several micron, and then draw together again without forming diplosome. Pericentriolar satellites form on mother centriole of some cells at this time, they disappear at the beginning of S-period, replication of centrioles begins; daughter centrioles reach the size of mother centrioles in anaphase. During growth and maturation, centrioles in L-cells undergo structural changes similar to those described for SPEV cells (Vorob'ev, Chentsov, 1982). Several types of meeting points for microtubules exist in L-cells during the whole interphase: surface of centrioles per se, pericentriolar satellites, free foci.     

Centrobin: a novel daughter centriole-associated protein that is required for centriole duplication

In mammalian cells, the centrosome consists of a pair of centrioles and amorphous pericentriolar material. The pair of centrioles, which are the core components of the centrosome, duplicate once per cell cycle. Centrosomes play a pivotal role in orchestrating the formation of the bipolar spindle during mitosis. Recent studies have linked centrosomal activity on centrioles or centriole-associated structures to cytokinesis and cell cycle progression through G1 into the S phase. In this study, we have identified centrobin as a centriole-associated protein that asymmetrically localizes to the daughter centriole. The silencing of centrobin expression by small interfering RNA inhibited centriole duplication and resulted in centrosomes with one or no centriole, demonstrating that centrobin is required for centriole duplication. Furthermore, inhibition of centriole duplication by centrobin depletion led to impaired cytokinesis.     

Mitotic regulators govern progress through steps in the centrosome duplication cycle

Centrosome duplication is marked by discrete changes in centriole structure that occur in lockstep with cell cycle transitions. We show that mitotic regulators govern steps in centriole replication in Drosophila embryos. Cdc25(string), the expression of which initiates mitosis, is required for completion of daughter centriole assembly. Cdc20(fizzy), which is required for the metaphase-anaphase transition, is required for timely disengagement of mother and daughter centrioles. Stabilization of mitotic cyclins, which prevents exit from mitosis, blocks assembly of new daughter centrioles. Common regulation of the nuclear and centrosome cycles by mitotic regulators may ensure precise duplication of the centrosome.     

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.     

Independence of centriole formation and initiation of DNA synthesis in Chinese hamster ovary cells

The relationship between centriole formation and DNA synthesis was investigated by examining the effect of taxol on the centriole cycle and the initiation of DNA synthesis in synchronized cells. The centriole cycle was monitored by electron microscopy of whole-mount preparations [Kuriyama and Borisy, J. Cell Biol., 1981, 91:814-821]. A short daughter centriole appeared in perpendicular orientation to each parent during late G1 or early S and elongated slowly during S to G2. Addition of 5-20 micrograms/ml taxol to a synchronous population of cells in S phase did not inhibit centriole elongation; rather, elongation was accelerated. In contrast, when taxol was added to M phase or early G1 cells, centriole duplication was completely inhibited. The taxol block was reversible since nucleation and elongation of centrioles resumed as soon as the drug was removed. Cells exposed to taxol progressed through the cell cycle and became blocked in mitosis, as indicated by an increase in the mitotic index, but eventually the mitotic arrest was overcome, resulting in formation of multinucleated cells. A peak in mitotic index was seen in the following generation, indicating that chromosomes duplicated in the presence of taxol. Incorporation of 3H-thymidine followed by autoradiography confirmed that DNA synthesis was initiated in the presence of taxol even though formation of daughter centrioles was inhibited. It seems, therefore, that centriole duplication is not a prerequisite for entry into S phase. Since DNA synthesis has already been demonstrated not to be necessary for centriole duplication, these two events, normally coordinated in time, appear to be independent of each other.     

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.     

PLK4-phosphorylated NEDD1 facilitates cartwheel assembly and centriole biogenesis initiations

Centrosome duplication occurs under strict spatiotemporal regulation once per cell cycle, and it begins with cartwheel assembly and daughter centriole biogenesis at the lateral sites of the mother centrioles. However, although much of this process is understood, how centrosome duplication is initiated remains unclear. Here, we show that cartwheel assembly followed by daughter centriole biogenesis is initiated on the NEDD1-containing layer of the pericentriolar material (PCM) by the recruitment of SAS-6 to the mother centriole under the regulation of PLK4. We found that PLK4-mediated phosphorylation of NEDD1 at its S325 amino acid residue directly promotes both NEDD1 binding to SAS-6 and recruiting SAS-6 to the centrosome. Overexpression of phosphomimicking NEDD1 mutant S325E promoted cartwheel assembly and daughter centriole biogenesis initiations, whereas overexpression of nonphosphorylatable NEDD1 mutant S325A abolished the initiations. Collectively, our results demonstrate that PLK4-regulated NEDD1 facilitates initiation of the cartwheel assembly and of daughter centriole biogenesis in mammals.     

The centriole cycle in the amoebae of the myxomycetePhysarum polycephalum

Summary In the amoebae of the myxomycetePhysarum polycephalum, procentrioles are formed on the anterior and posterior centrioles in early prophase. Although the relative position of the parental and procentrioles is fixed, all relative positions of the daughter and parental centrioles were observed. During the different stages of mitosis daughter centrioles elongate and acquire anterior satellites, one of the characteristic features of the anterior centrioles. All other anterior morphological characteristics appear only in telophase and early reconstruction stages. In contrast to the parental posterior centrioles, which do not change morphologically during the successive mitotic stages, the parental anterior centrioles lose their morphological characteristics in late prophase and early prometaphase and then acquire the morphological features characteristic of the posterior centrioles. Thus, the following maturation scheme is suggested: a procentriole becomes an anterior centriole during the first mitosis and a posterior centriole during the second mitosis. Since posterior features are maintained during mitosis, the posterior centriole corresponds to the final state of centriole maturation.     

Daughter centrioles assemble preferentially towards the nuclear envelope in Drosophila syncytial embryos

Centrosomes are important organizers of microtubules within animal cells. They comprise a pair of centrioles surrounded by the pericentriolar material, which nucleates and organizes the microtubules. To maintain centrosome numbers, centrioles must duplicate once and only once per cell cycle. During S-phase, a single new ‘daughter’ centriole is built orthogonally on one side of each radially symmetric ‘mother’ centriole. Mis-regulation of duplication can result in the simultaneous formation of multiple daughter centrioles around a single mother centriole, leading to centrosome amplification, a hallmark of cancer. It remains unclear how a single duplication site is established. It also remains unknown whether this site is pre-defined or randomly positioned around the mother centriole. Here, we show that within Drosophila syncytial embryos daughter centrioles preferentially assemble on the side of the mother facing the nuclear envelope, to which the centrosomes are closely attached. This positional preference is established early during duplication and remains stable throughout daughter centriole assembly, but is lost in centrosomes forced to lose their connection to the nuclear envelope. This shows that non-centrosomal cues influence centriole duplication and raises the possibility that these external cues could help establish a single duplication site.     

Mother centrioles are kicked out so that starfish zygote can grow

Most oocytes eliminate their centrioles during meiotic divisions through unclear mechanisms. In this issue, Borrego-Pinto et al. (2016. J Cell. Biol. http://dx.doi.org/10.1083/jcb.201510083) show that mother centrioles need to be eliminated from starfish oocytes by extrusion into the polar bodies for successful embryo development.Canonical centrosomes contain a pair of centrioles, often made of nine triplets of microtubules and surrounded by the pericentriolar material (PCM). They are the major microtubule organizing centers in most cells, which organize the microtubule spindle required to segregate chromosomes during cell division. Yet, most oocytes get rid of their centrioles. The biological significance of oocyte centriole riddance remains a mystery. Removing centrioles in oocytes could prevent some species, like Xenopus, from undergoing parthenogenetic development (Tournier et al., 1991). Also, eliminating the maternal centrioles is required to prevent the zygote from having an abnormal number of centrioles after fertilization, as sperm contribute two centrioles (motile sperm cells require centriole-based flagellar assembly and must retain their centrioles until fertilization [Manandhar et al., 2005]). In Drosophila, Xenopus, nematode, mouse, and human oocytes, egg centrioles are eliminated during meiotic prophase before oocyte asymmetric divisions (Szollosi et al., 1972; Manandhar et al., 2005; Januschke et al., 2006). Apart from the involvement of a helicase of undefined substrates, the pathway leading to centriole elimination has not been identified (Mikeladze-Dvali et al., 2012).In contrast, starfish oocytes, like sea urchin or mollusk, eliminate their centrioles later in meiotic divisions (Nakashima and Kato, 2001; Shirato et al., 2006). Centrioles are replicated in a semiconservative manner during the S phase of the cell cycle. The old centriole, named the mother, is characterized by the presence of distal and subdistal appendages and serves as a template for the assembly of a new daughter centriole, lacking appendages (Bornens and Gönczy, 2014). However, to become haploid, oocytes undergo two consecutive divisions with no intervening DNA replication. Hence, centrioles are not duplicated between the two meiotic divisions and oocytes keep their number of centrioles limited to four. This also means that starfish oocytes assemble their first meiotic spindle in the presence of a pair of centrioles at each pole (Fig. 1 A). Out of the four centrioles contained in the oocyte, two (one mother and one daughter centriole) are extruded into the first polar body during the first asymmetric division. Subsequently, the second meiotic spindle is formed with only one centriole per pole (Fig. 1 A), and one centriole is extruded in the second polar body. Previous work suggested that the poles of the second meiotic spindle in starfish are not functionally equivalent (Uetake et al., 2002). In this issue, Borrego-Pinto et al. find that the mother centriole retains the ability to nucleate asters but is specifically guided into the second polar body for extrusion, whereas the daughter centriole is inactivated and then eliminated within the oocyte.Open in a separate windowFigure 1.Centriole elimination during meiotic maturation of starfish oocytes. (A) Scheme of starfish oocyte meiotic divisions and early egg development. Oocyte divisions are asymmetric in size; meiotic spindles are off-centered in these large cells; and daughter cells are tiny, tailored to the chromatin mass, and named polar bodies. Microtubules are green, DNA is pink, maternal centrosomes are yellow, and sperm centrosomes are orange. (B) Fate of mother and daughter centrioles during meiotic divisions. Centrosomes are artificially enlarged to emphasize the centrioles. PB1 and PB2, first and second polar body, respectively. During anaphase I, the DNA and centrioles are segregated; one set of chromosomes and one pair of centrioles are extruded into PB1 during anaphase I. The remaining mother centriole separates from its paired daughter and rapidly moves toward the plasma membrane, where it is extruded in the second polar body (PB2) during anaphase II, leaving one set of oocyte chromatids to combine with the sperm chromatids. The remaining oocyte daughter centriole is inactivated and degraded after anaphase II. Therefore, only the sperm centrioles form the first mitotic spindle in the fertilized oocyte. Oocytes forced to retain a mother centriole form a tripolar aster upon fertilization, which stops development.To investigate the mechanism of centriole elimination in the starfish Patiria miniata, Borrego-Pinto et al. (2016) first isolated homologues of centrosomal proteins and constructed fluorescent protein fusions to several centriolar proteins to track centriole fate in 3D time-lapse imaging during oocyte asymmetric divisions. Using specific markers of mother versus daughter centrioles, they established that, in meiosis I, the two spindle poles are equivalent, being constituted of a pair of mother and daughter centrioles. At anaphase I, one pair of mother/daughter centrioles is extruded into the first polar body. Importantly, the authors described an asymmetry in metaphase II, with the second meiotic spindle always having the mother centriole facing the cortex and the daughter centriole deep inside the cytoplasm (Fig. 1 B).Borrego-Pinto et al. (2016) went on to identify the origin of this asymmetry. They show that the mother centriole, but not the daughter one, starts being rapidly transported toward the plasma membrane before completion of meiosis I spindle disassembly in a microtubule- and dynein-dependent manner, as its trafficking could be impaired by the dynein inhibitor ciliobrevin D (Firestone et al., 2012). In a second step, the mother centriole is anchored to the plasma membrane through the second meiotic division. Interestingly, electron microscopy of starfish oocytes revealed electron-dense material as well as vesicles between the mother centriole and the plasma membrane, suggesting that the mother centriole’s plasma membrane anchorage occurs via its appendages (Reiter et al., 2012; Stinchcombe et al., 2015). Whether the mother centriole migrates to the cortex with its appendages facing or opposite the plasma membrane has not been addressed. However, it is reasonable to assume that, in a viscous environment such as the oocyte cytoplasm, a motion with the appendages up would be favored (Fig. 1 B). Moreover, whereas the migration of the mother centriole to the plasma membrane requires microtubules, its anchoring does not depend on microtubules or microfilaments, as shown by the continued tight association between the centriole and the membrane in the presence of microtubule- and/or actin-depolymerizing agents. This close anchoring via the centriole’s appendages is reminiscent of the anchoring of centrioles forming cilia or at the immunological synapse in T cells (Stinchcombe et al., 2015). The precise mechanisms involved in mother centriole anchoring to the plasma membrane in starfish might be conserved in other systems that also require proximity between these two structures. It would be interesting to assess whether astral microtubules emanating from the mother centriole progressively depolymerize as the mother centriole approaches the plasma membrane to allow the intimate anchoring of the appendages with the plasma membrane. If so, Katanin, a microtubule-severing enzyme whose activity is regulated during meiotic divisions in the nematode oocyte, would be a good candidate to promote such a progressive destabilization (Srayko et al., 2000).Future work will tell us why the daughter centriole does not experience such a migration event. This strongly argues for a functional asymmetry between the two types of centrioles. From the work of Borrego-Pinto et al. (2016), it appears that the daughter centriole is passively pushed inside the oocyte cytoplasm as a result of meiosis II spindle assembly and elongation. Dynein, which controls the migration of the mother centriole, could specifically associate with this centriole, like it does in Saccharomyces cerevisiae, by localizing preferentially to the spindle pole body (the yeast equivalent of the centrosome) facing the bud (Grava et al., 2006). Centrosome asymmetry has been described in several stem cell types (Roubinet and Cabernard, 2014) and this asymmetry is often rooted in its activity. However, Borrego-Pinto et al. (2016) show that the microtubule nucleation capacity of the daughter and mother centrioles is equivalent up to the metaphase II stage. It is only after fertilization and anaphase II that a difference in activity is detected between the mother and daughter centrioles. Thus, what underlies the asymmetry in behavior between the mother and daughter centrioles at anaphase I remains to be discovered. One possibility is that the presence of appendages in the mother centriole allows the recruitment of specific factors, such as dynein, which in turn regulate mother centriole migration and anchoring.Borrego-Pinto et al. (2016) also discovered that specific anchoring of the mother centriole to the plasma membrane, at which the second polar body will form, is the mechanism by which oocytes get rid of the remaining mother centriole. Importantly, actively removing the mother centriole after anaphase II is essential for zygotic development. Indeed, the researchers used the actin polymerization inhibitor cytochalasin D to prevent extrusion of the second polar body and artificially retain the mother centriole in the oocyte after anaphase II. When a mother centriole is retained, it keeps its microtubule nucleation capacity and participates in the first mitotic spindle pole organization of the fertilized egg, whereas the daughter centriole is inactivated and dismantled after anaphase II. As a consequence, because of the two centrioles contributed by the sperm cell, the mitotic spindle ends up being tripolar in the presence of an additional mother centriole, precluding correct chromosome segregation and further development (Fig. 1 B).The origin of the difference in behavior between mother and daughter centrioles after anaphase II will require further investigation. To explain the loss in nucleation capacity of the daughter centriole, it will be important to check for the presence of various PCM components. Indeed, it is reasonable to assume that the daughter centriole loses its PCM association. PCM size scales with centriole size; thus, appendages of the mother centriole might possess an innate ability to maintain association with the PCM (Bobinnec et al., 1998; Delattre et al., 2004). A possible cell cycle–dependent enzymatic activity appearing after anaphase II might explain the rapid loss in microtubule nucleation capacity of the daughter centriole. It is surprising that the starfish zygote cannot cluster the mother centriole material with the centrioles from the sperm, unlike mouse oocytes, which, like cancer cells, are able to cluster PCM to regulate the total number of microtubule organizing centers (Kwon et al., 2008; Breuer et al., 2010). It will be interesting to determine whether starfish zygotes express proteins such as HURP or HSET, which are major players in extra-centrosome clustering (Kwon et al., 2008; Breuer et al., 2010).Altogether, the results from Borrego-Pinto et al. (2016) address a major unresolved question: why do oocytes lose or inactivate their canonical centrioles during female meiosis? They show for the first time that maternal centrioles must be extruded from or inactivated in the starfish egg before fertilization so that they do not perturb mitotic spindle assembly. This is a very important step in our understanding of female gamete formation. Moreover, this work establishes starfish oocyte meiosis as a novel model system to study both functional and structural centrosome asymmetry, an essential component of asymmetric divisions.     

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.     

Centriole number and the reproductive capacity of spindle poles

The reproduction of spindle poles is a key event in the cell's preparation for mitosis. To gain further insight into how this process is controlled, we systematically characterized the ultrastructure of spindle poles whose reproductive capacity had been experimentally altered. In particular, we wanted to determine if the ability of a pole to reproduce before the next division is related to the number of centrioles it contains. We used mercaptoethanol to indirectly induce the formation of monopolar spindles in sea urchin eggs. We followed individually treated eggs in vivo with a polarizing microscope during the induction and development of monopolar spindles. We then fixed each egg at one of three predetermined key stages and serially semithick sectioned it for observation in a high-voltage electron microscope. We thus know the history of each egg before fixation and, from earlier studies, what that cell would have done had it not been fixed. We found that spindle poles that would have given rise to monopolar spindles at the next mitosis have only one centriole whereas spindle poles that would have formed bipolar spindles at the next division have two centrioles. By serially sectioning each egg, we were able to count all centrioles present. In the twelve cells examined, we found no cases of acentriolar spindle poles or centriole reduplication. Thus, the reproductive capacity of a spindle pole is linked to the number of centrioles it contains. Our experimental results also show, contrary to existing reports, that the daughter centriole of a centrosome can acquire pericentriolar material without first becoming a parent. Furthermore, our results demonstrate that the splitting apart of mother and daughter centrioles is an event that is distinct from, and not dependent on, centriole duplication.     

Integrity of the Pericentriolar Material Is Essential for Maintaining Centriole Association during M Phase

A procentriole is assembled next to the mother centriole during S phase and remains associated until M phase. After functioning as a spindle pole during mitosis, the mother centriole and procentriole are separated at the end of mitosis. A close association of the centriole pair is regarded as an intrinsic block to the centriole reduplication. Therefore, deregulation of this process may cause a problem in the centriole number control, resulting in increased genomic instability. Despite its importance for faithful centriole duplication, the mechanism of centriole separation is not fully understood yet. Here, we report that centriole pairs are prematurely separated in cells whose cell cycle is arrested at M phase by STLC. Dispersal of the pericentriolar material (PCM) was accompanied. This phenomenon was independent of the separase activity but needed the PLK1 activity. Nocodazole effectively inhibited centriole scattering in STLC-treated cells, possibly by reducing the microtubule pulling force around centrosomes. Inhibition of PLK1 also reduced the premature separation of centrioles and the PCM dispersal as well. These results revealed the importance of PCM integrity in centriole association. Therefore, we propose that PCM disassembly is one of the driving forces for centriole separation during mitotic exit.     

Centrin-2 is required for centriole duplication in mammalian cells

BACKGROUND: Centrosomes are the favored microtubule-organizing framework of eukaryotic cells. Centrosomes contain a pair of centrioles that normally duplicate once during the cell cycle to give rise to two mitotic spindle poles, each containing one old and one new centriole. However, aside from their role as an anchor point for pericentriolar material and as basal bodies of flagella and cilia, the functional attributes of centrioles remain enigmatic. RESULTS: Here, using RNA interference, we demonstrate that "knockdown" of centrin-2, a protein of centrioles, results in failure of centriole duplication during the cell cycle in HeLa cells. Following inhibition of centrin-2 synthesis, the preexisting pair of centrioles separate, and functional bipolar spindles form with only one centriole at each spindle pole. Centriole dilution results from the ensuing cell division, and daughter cells are "born" with only a single centriole. Remarkably, these unicentriolar daughter cells may complete a second and even third bipolar mitosis in which spindle microtubules converge onto unusually broad spindle poles and in which cell division results in daughter cells containing either one or no centrioles at all. Cells thus denuded of the mature or both centrioles fail to undergo cytokinesis in subsequent cell cycles, give rise to multinucleate products, and finally die. CONCLUSIONS: These results demonstrate a requirement for centrin in centriole duplication and demonstrate that centrioles play a role in organizing spindle pole morphology and in the completion of cytokinesis.     

Ciliogenesis and centriole formation in the mouse embryonic nervous system. An ultrastructural analysis

Serial ultrathin sections were used to study the formation of the primary cilium and the centriolar apparatus, basal body, and centriole in the neuroepithelial primordial cell of the embryonic nervous system in the mouse. At the end of mitosis, the centrioles seem to migrate toward the ventricular process of the neuroepithelial cell, near the ventricular surface. One of these centrioles, the nearest to the ventricular surface, begins to mature to form a basal body, since its tip is capped by a vesicle probably originating in the cytoplasm. This vesicle fuses with the plasmalemma and the cilium growth by the centrifugal extension of the 9 sets of microtubule doublets. These 9 sets invade the thick base of the cilium which is initially capped by a ball-shaped tip with the appearance of a mushroom cilium. The secondary extension of 7, then 5, and finally 2 sets of microtubule doublets contribute to form the tip of the mature cilium, which is associated with a mature centriolar apparatus formed by a basal body and a centriole. Centriologenesis occurs before mitosis and is concomitant with the progressive resorption of the cilium. The daughter centriole, or procentriole, begins to take form near the tips of fibrils that extend perpendicularly and at a short distance from the wall of the parent centriole. Osmiophilic material accumulates around these fibrils, and gives rise to the microtubules of the mature daughter centriole. These centrioles formed by a centriolar process are further engaged in mitosis, after the total resorption of the cilium. This pattern of development suggests that in the primordial cells of the embryonic nervous system, centriologenesis and ciliogenesis are 2 independent phenomena.