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.     

Drosophila SPD-2 is an essential centriole component required for PCM recruitment and astral-microtubule nucleation

SPD-2 is a C. elegans centriolar protein required for both centriole duplication and pericentriolar material (PCM) recruitment [1-4]. SPD-2 is conserved in Drosophila (DSpd-2) and is a component of the fly centriole [5-7]. The analysis of a P element-induced hypomorphic mutation has shown that DSpd-2 is primarily required for PCM recruitment at the sperm centriole but is dispensable for both centriole duplication and aster formation [5]. Here we show that null mutations carrying early stop codons in the DSpd-2 coding sequence suppress astral microtubule (MT) nucleation in both neuroblasts (NBs) and spermatocytes. These mutations also disrupt proper Miranda localization in dividing NBs, as previously observed in mutants lacking astral MTs [8-10]. Spermatocyte analysis revealed that DSpd-2 is enriched at both the centrioles and the PCM and is required for the maintenance of cohesion between the two centrioles but not for centriole duplication. We found that DSpd-2 localization at the centrosome requires the wild-type activity of Asl but is independent of the function of D-PLP, Cnn, gamma-tubulin, DGrip91, and D-TACC. Conversely, DSpd-2 mutants displayed normal centrosomal accumulations of Asl and D-PLP, strongly reduced amounts of Cnn, gamma-tubulin, and DGrip91, and diffuse localization of D-TACC. These results indicate that DSpd-2 functions in a very early step of the PCM recruitment pathway.     

Maintaining the proper connection between the centrioles and the pericentriolar matrix requires Drosophila centrosomin

Centrosomes consist of two centrioles surrounded by an amorphous pericentriolar matrix (PCM), but it is unknown how centrioles and PCM are connected. We show that the centrioles in Drosophila embryos that lack the centrosomal protein Centrosomin (Cnn) can recruit PCM components but cannot maintain a proper attachment to the PCM. As a result, the centrioles "rocket" around in the embryo and often lose their connection to the nucleus in interphase and to the spindle poles in mitosis. This leads to severe mitotic defects in embryos and to errors in centriole segregation in somatic cells. The Cnn-related protein CDK5RAP2 is linked to microcephaly in humans, but cnn mutant brains are of normal size, and we observe only subtle defects in the asymmetric divisions of mutant neuroblasts. We conclude that Cnn maintains the proper connection between the centrioles and the PCM; this connection is required for accurate centriole segregation in somatic cells but is not essential for the asymmetric division of neuroblasts.     

Structured illumination of the interface between centriole and peri-centriolar material

The increase in centrosome size in mitosis was described over a century ago, and yet it is poorly understood how centrioles, which lie at the core of centrosomes, organize the pericentriolar material (PCM) in this process. Now, structured illumination microscopy reveals in Drosophila that, before clouds of PCM appear, its proteins are closely associated with interphase centrioles in two tube-like layers: an inner layer occupied by centriolar microtubules, Sas-4, Spd-2 and Polo kinase; and an outer layer comprising Pericentrin-like protein (Dplp), Asterless (Asl) and Plk4 kinase. Centrosomin (Cnn) and γ-tubulin associate with this outer tube in G2 cells and, upon mitotic entry, Polo activity is required to recruit them together with Spd-2 into PCM clouds. Cnn is required for Spd-2 to expand into the PCM during this maturation process but can itself contribute to PCM independently of Spd-2. By contrast, the centrioles of spermatocytes elongate from a pre-existing proximal unit during the G2 preceding meiosis. Sas-4 is restricted to the microtubule-associated, inner cylinder and Dplp and Cnn to the outer cylinder of this proximal part. γ-Tubulin and Asl associate with the outer cylinder and Spd-2 with the inner cylinder throughout the entire G2 centriole. Although they occupy different spatial compartments on the G2 centriole, Cnn, Spd-2 and γ-tubulin become diminished at the centriole upon entry into meiosis to become part of PCM clouds.     

Centrioles generate a local pulse of Polo/PLK1 activity to initiate mitotic centrosome assembly

Mitotic centrosomes are formed when centrioles start to recruit large amounts of pericentriolar material (PCM) around themselves in preparation for mitosis. This centrosome “maturation” requires the centrioles and also Polo/PLK1 protein kinase. The PCM comprises several hundred proteins and, in Drosophila, Polo cooperates with the conserved centrosome proteins Spd‐2/CEP192 and Cnn/CDK5RAP2 to assemble a PCM scaffold around the mother centriole that then recruits other PCM client proteins. We show here that in Drosophila syncytial blastoderm embryos, centrosomal Polo levels rise and fall during the assembly process—peaking, and then starting to decline, even as levels of the PCM scaffold continue to rise and plateau. Experiments and mathematical modelling indicate that a centriolar pulse of Polo activity, potentially generated by the interaction between Polo and its centriole receptor Ana1 (CEP295 in humans), could explain these unexpected scaffold assembly dynamics. We propose that centrioles generate a local pulse of Polo activity prior to mitotic entry to initiate centrosome maturation, explaining why centrioles and Polo/PLK1 are normally essential for this process.     

Centrobin-Centrosomal Protein 4.1-associated Protein (CPAP) Interaction Promotes CPAP Localization to the Centrioles during Centriole Duplication

Centriole duplication is the process by which two new daughter centrioles are generated from the proximal end of preexisting mother centrioles. Accurate centriole duplication is important for many cellular and physiological events, including cell division and ciliogenesis. Centrosomal protein 4.1-associated protein (CPAP), centrosomal protein of 152 kDa (CEP152), and centrobin are known to be essential for centriole duplication. However, the precise mechanism by which they contribute to centriole duplication is not known. In this study, we show that centrobin interacts with CEP152 and CPAP, and the centrobin-CPAP interaction is critical for centriole duplication. Although depletion of centrobin from cells did not have an effect on the centriolar levels of CEP152, it caused the disappearance of CPAP from both the preexisting and newly formed centrioles. Moreover, exogenous expression of the CPAP-binding fragment of centrobin also caused the disappearance of CPAP from both the preexisting and newly synthesized centrioles, possibly in a dominant negative manner, thereby inhibiting centriole duplication and the PLK4 overexpression-mediated centrosome amplification. Interestingly, exogenous overexpression of CPAP in the centrobin-depleted cells did not restore CPAP localization to the centrioles. However, restoration of centrobin expression in the centrobin-depleted cells led to the reappearance of centriolar CPAP. Hence, we conclude that centrobin-CPAP interaction is critical for the recruitment of CPAP to procentrioles to promote the elongation of daughter centrioles and for the persistence of CPAP on preexisting mother centrioles. Our study indicates that regulation of CPAP levels on the centrioles by centrobin is critical for preserving the normal size, shape, and number of centrioles in the cell.     

A genome-wide RNAi screen to dissect centriole duplication and centrosome maturation in Drosophila

Centrosomes comprise a pair of centrioles surrounded by an amorphous pericentriolar material (PCM). Here, we have performed a microscopy-based genome-wide RNA interference (RNAi) screen in Drosophila cells to identify proteins required for centriole duplication and mitotic PCM recruitment. We analysed 92% of the Drosophila genome (13,059 genes) and identified 32 genes involved in centrosome function. An extensive series of secondary screens classified these genes into four categories: (1) nine are required for centriole duplication, (2) 11 are required for centrosome maturation, (3) nine are required for both functions, and (4) three genes regulate centrosome separation. These 32 hits include several new centrosomal components, some of which have human homologs. In addition, we find that the individual depletion of only two proteins, Polo and Centrosomin (Cnn) can completely block centrosome maturation. Cnn is phosphorylated during mitosis in a Polo-dependent manner, suggesting that the Polo-dependent phosphorylation of Cnn initiates centrosome maturation in flies.     

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.     

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.     

Centriole inheritance

Early cell biologists perceived centrosomes to be permanent cellular structures. Centrosomes were observed to reproduce once each cycle and to orchestrate assembly a transient mitotic apparatus that segregated chromosomes and a centrosome to each daughter at the completion of cell division. Centrosomes are composed of a pair of centrioles buried in a complex pericentriolar matrix. The bulk of microtubules in cells lie with one end buried in the pericentriolar matrix and the other extending outward into the cytoplasm. Centrioles recruit and organize pericentriolar material. As a result, centrioles dominate microtubule organization and spindle assembly in cells born with centrosomes. Centrioles duplicate in concert with chromosomes during the cell cycle. At the onset of mitosis, sibling centrosomes separate and establish a bipolar spindle that partitions a set of chromosomes and a centrosome to each daughter cell at the completion of mitosis and cell division. Centriole inheritance has historically been ascribed to a template mechanism in which the parental centriole contributed to, if not directed, assembly of a single new centriole once each cell cycle. It is now clear that neither centrioles nor centrosomes are essential to cell proliferation. This review examines the recent literature on inheritance of centrioles in animal cells.Key words: centrosome, centriol, spindle, mitosis, microtubule, cell cycle, checkpoints     

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.     

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.     

From stem cell to embryo without centrioles

Centrosome asymmetry plays a key role in ensuring the asymmetric division of Drosophila neural stem cells (neuroblasts [NBs]) and male germline stem cells (GSCs) [1-3]. In both cases, one centrosome is anchored close to a specific cortical region during interphase, thus defining the orientation of the spindle during the ensuing mitosis. To test whether asymmetric centrosome behavior is a general feature of stem cells, we have studied female GSCs, which divide asymmetrically, producing another GSC and a cystoblast. The cystoblast then divides and matures into an oocyte, a process in which centrosomes exhibit a series of complex behaviors proposed to play a crucial role in oogenesis [4-6]. We show that the interphase centrosome does not define spindle orientation in female GSCs and that DSas-4 mutant GSCs [7], lacking centrioles and centrosomes, invariably divide asymmetrically to produce cystoblasts that proceed normally through oogenesis-remarkably, oocyte specification, microtubule organization, and mRNA localization are all unperturbed. Mature oocytes can be fertilized, but embryos that cannot support centriole replication arrest very early in development. Thus, centrosomes are dispensable for oogenesis but essential for early embryogenesis. These results reveal that asymmetric centrosome behavior is not an essential feature of stem cell divisions.     

A centriole's subdistal appendages: contributions to cell division,ciliogenesis and differentiation

The centrosome is a highly conserved structure composed of two centrioles surrounded by pericentriolar material. The mother, and inherently older, centriole has distal and subdistal appendages, whereas the daughter centriole is devoid of these appendage structures. Both appendages have been primarily linked to functions in cilia formation. However, subdistal appendages present with a variety of potential functions that include spindle placement, chromosome alignment, the final stage of cell division (abscission) and potentially cell differentiation. Subdistal appendages are particularly interesting in that they do not always display a conserved ninefold symmetry in appendage organization on the mother centriole across eukaryotic species, unlike distal appendages. In this review, we aim to differentiate both the morphology and role of the distal and subdistal appendages, with a particular focus on subdistal appendages.     

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 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.     

The respective contributions of the mother and daughter centrioles to centrosome activity and behavior in vertebrate cells

We have generated several stable cell lines expressing GFP-labeled centrin. This fusion protein becomes concentrated in the lumen of both centrioles, making them clearly visible in the living cell. Time-lapse fluorescence microscopy reveals that the centriole pair inherited after mitosis splits during or just after telophase. At this time the mother centriole remains near the cell center while the daughter migrates extensively throughout the cytoplasm. This differential behavior is not related to the presence of a nucleus because it is also observed in enucleated cells. The characteristic motions of the daughter centriole persist in the absence of microtubules (Mts). or actin, but are arrested when both Mts and actin filaments are disrupted. As the centrioles replicate at the G1/S transition the movements exhibited by the original daughter become progressively attenuated, and by the onset of mitosis its behavior is indistinguishable from that of the mother centriole. While both centrioles possess associated gamma-tubulin, and nucleate similar number of Mts in Mt repolymerization experiments. during G1 and S only the mother centriole is located at the focus of the Mt array. A model, based on differences in Mt anchoring and release by the mother and daughter centrioles, is proposed to explain these results.     

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.     

Cep120 is asymmetrically localized to the daughter centriole and is essential for centriole assembly

Centrioles form the core of the centrosome in animal cells and function as basal bodies that nucleate and anchor cilia at the plasma membrane. In this paper, we report that Cep120 (Ccdc100), a protein previously shown to be involved in maintaining the neural progenitor pool in mouse brain, is associated with centriole structure and function. Cep120 is up-regulated sevenfold during differentiation of mouse tracheal epithelial cells (MTECs) and localizes to basal bodies. Cep120 localizes preferentially to the daughter centriole in cycling cells, and this asymmetry between mother and daughter centrioles is relieved coincident with new centriole assembly. Photobleaching recovery analysis identifies two pools of Cep120, differing in their halftime at the centriole. We find that Cep120 is required for centriole duplication in cycling cells, centriole amplification in MTECs, and centriole overduplication in S phase-arrested cells. We propose that Cep120 is required for centriole assembly and that the observed defect in neuronal migration might derive from a defect in this process.     

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.