Nonequivalence of maternal centrosomes/centrioles in starfish oocytes: selective casting-off of reproductive centrioles into polar bodies

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.

Taking Centrioles to the Elimination Round

Two recent papers published in The Journal of Cell Biology (Borrego-Pinto et al., 2016) and Science (Pimenta-Marques et al., 2016) have begun to shed light on the mechanism of centriole elimination during female oogenesis, highlighting a protective role for Polo kinase and the pericentriolar material.

Parthenogenesis in Insects: The Centriole Renaissance

Building a new organism usually requires the contribution of two differently shaped haploid cells, the male and female gametes, each providing its genetic material to restore diploidy of the new born zygote. The successful execution of this process requires defined sequential steps that must be completed in space and time. Otherwise, development fails. Relevant among the earlier steps are pronuclear migration and formation of the first mitotic spindle that promote the mixing of parental chromosomes and the formation of the zygotic nucleus. A complex microtubule network ensures the proper execution of these processes. Instrumental to microtubule organization and bipolar spindle assembly is a distinct non-membranous organelle, the centrosome. Centrosome inheritance during fertilization is biparental, since both gametes provide essential components to build a functional centrosome. This model does not explain, however, centrosome formation during parthenogenetic development, a special mode of sexual reproduction in which the unfertilized egg develops without the contribution of the male gamete. Moreover, whereas fertilization is a relevant example in which the cells actively check the presence of only one centrosome, to avoid multipolar spindle formation, the development of parthenogenetic eggs is ensured, at least in insects, by the de novo assembly of multiple centrosomes.Here, we will focus our attention on the assembly of functional centrosomes following fertilization and during parthenogenetic development in insects. Parthenogenetic development in which unfertilized eggs are naturally depleted of centrosomes would provide a useful experimental system to investigate centriole assembly and duplication together with centrosome formation and maturation.

Distinct mechanisms eliminate mother and daughter centrioles in meiosis of starfish oocytes

Centriole elimination is an essential process that occurs in female meiosis of metazoa to reset centriole number in the zygote at fertilization. How centrioles are eliminated remains poorly understood. Here we visualize the entire elimination process live in starfish oocytes. Using specific fluorescent markers, we demonstrate that the two older, mother centrioles are selectively removed from the oocyte by extrusion into polar bodies. We show that this requires specific positioning of the second meiotic spindle, achieved by dynein-driven transport, and anchorage of the mother centriole to the plasma membrane via mother-specific appendages. In contrast, the single daughter centriole remaining in the egg is eliminated before the first embryonic cleavage. We demonstrate that these distinct elimination mechanisms are necessary because if mother centrioles are artificially retained, they cannot be inactivated, resulting in multipolar zygotic spindles. Thus, our findings reveal a dual mechanism to eliminate centrioles: mothers are physically removed, whereas daughters are eliminated in the cytoplasm, preparing the egg for fertilization.

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.

Oocyte Meiotic Spindle Assembly and Function

Gametogenesis in animal oocytes reduces the diploid genome content of germline precursors to a haploid state in gametes by discarding ¾ of the duplicated chromosomes through a sequence of two meiotic cell divisions called meiosis I and II. The assembly of the microtubule-based spindle structure that mediates this reduction in genome content remains poorly understood compared to our knowledge of mitotic spindle assembly and function. In this review, we consider the diversity of oocyte meiotic spindle assembly and structure across animal phylogeny, review recent advances in our understanding of how animal oocytes assemble spindles in the absence of the centriole-based microtubule-organizing centers that dominate mitotic spindle assembly, and discuss different models for how chromosomes are captured and moved to achieve chromosome segregation during oocyte meiotic cell division.

Live Imaging of Centriole Dynamics by Fluorescently Tagged Proteins in Starfish Oocyte Meiosis

High throughput DNA sequencing, the decreasing costs of DNA synthesis, and universal techniques for genetic manipulation have made it much easier and quicker to establish molecular tools for any organism than it has been 5 years ago. This opens a great opportunity for reviving "nonconventional" model organisms, which are particularly suited to study a specific biological process and many of which have already been established before the era of molecular biology. By taking advantage of transcriptomics, in particular, these systems can now be easily turned into full fetched models for molecular cell biology.As an example, here we describe how we established molecular tools in the starfish Patiria miniata, which has been a popular model for cell and developmental biology due to the synchronous and rapid development, transparency, and easy handling of oocytes, eggs, and embryos. Here, we detail how we used a de novo assembled transcriptome to produce molecular markers and established conditions for live imaging to investigate the molecular mechanisms underlying centriole elimination-a poorly understood process essential for sexual reproduction of animal species.

Atypical centrioles during sexual reproduction

Centrioles are conserved, self-replicating, microtubule-based, 9-fold symmetric subcellular organelles that are essential for proper cell division and function. Most cells have two centrioles and maintaining this number of centrioles is important for animal development and physiology. However, how animals gain their first two centrioles during reproduction is only partially understood. It is well established that in most animals, the centrioles are contributed to the zygote by the sperm. However, in humans and many animals, the sperm centrioles are modified in their structure and protein composition, or they appear to be missing altogether. In these animals, the origin of the first centrioles is not clear. Here, we review various hypotheses on how centrioles are gained during reproduction and describe specialized functions of the zygotic centrioles. In particular, we discuss a new and atypical centriole found in sperm and zygote, called the proximal centriole-like structure (PCL). We also discuss another type of atypical centriole, the "zombie" centriole, which is degenerated but functional. Together, the presence of centrioles, PCL, and zombie centrioles suggests a universal mechanism of centriole inheritance among animals and new causes of infertility. Since the atypical centrioles of sperm and zygote share similar functions with typical centrioles in somatic cells, they can provide unmatched insight into centriole biology.

Analysis of centriole elimination during C. elegans oogenesis

Centrosomes are the principal microtubule organizing centers (MTOCs) of animal cells and comprise a pair of centrioles surrounded by pericentriolar material (PCM). Centriole number must be carefully regulated, notably to ensure bipolar spindle formation and thus faithful chromosome segregation. In the germ line of most metazoan species, centrioles are maintained during spermatogenesis, but eliminated during oogenesis. Such differential behavior ensures that the appropriate number of centrioles is present in the newly fertilized zygote. Despite being a fundamental feature of sexual reproduction in metazoans, the mechanisms governing centriole elimination during oogenesis are poorly understood. Here, we investigate this question in C. elegans. Using antibodies directed against centriolar components and serial-section electron microscopy, we establish that centrioles are eliminated during the diplotene stage of the meiotic cell cycle. Moreover, we show that centriole elimination is delayed upon depletion of the helicase CGH-1. We also find that somatic cells make a minor contribution to this process, and demonstrate that the germ cell karyotype is important for timely centriole elimination. These findings set the stage for a mechanistic dissection of centriole elimination in a metazoan organism.

The effect of taxol microinjection on the microtubular structure in polar body formation of starfish oocytes

In starfish oocytes, microtubules (MTs) form a spindle, which plays an important role in contributing to the selective loss of chromosomes and centrosomes to the polar bodies (PBs) during meiosis. When Taxol was locally injected near the germinal vesicle (GV) or the mitotic apparatus during meiosis I, PB formation was inhibited as mentioned below. In the oocytes, which were injected with Taxol after spindle formation, the spindle became large, and then the volume of the first PB also increased more than that of the control. In contrast, in the oocytes injected with Taxol before the spindle formation, chromosome capture and alignment were inhibited. These oocytes did not form PB, but only a bulge at the cell cortex was occasionally observed. Moreover, in the oocytes injected with Taxol before GV breakdown, the chromosomes did not gather in one place, and then two asters were observed at distant positions from the cell cortex. These results suggested that MTs lost not only the ability to obtain the bipolar attachment of chromosomes by Taxol injection but also the aster closer to the cell cortex lost its interaction with the cell cortex of the animal pole.

From zero to many: control of centriole number in development and disease

Centrioles are essential for the formation of microtubule-derived structures, including cilia, flagella and centrosomes. These structures are involved in a variety of functions, from cell motility to division. In most dividing animal cells, centriole formation is coupled to the chromosome cycle. However, this is not the case in certain specialized divisions, such as meiosis, and in some differentiating cells. For example, oocytes loose their centrioles upon differentiation, whereas multiciliated epithelial cells make several of those structures after they exit the cell cycle. Aberrations of centriole number are seen in many cancer cells. Recent studies began to shed light on the molecular control of centriole number, its variations in development, and how centriole number changes in human disease. Here we review the recent developments in this field.

Inactivation of MAPK in mature oocytes triggers progression into mitosis via a Ca2+-dependent pathway but without completion of S phase

Unfertilized sea urchin eggs that are arrested at G1 phase after completion of meiosis contain a highly phosphorylated mitogen-activated protein (MAP) kinase (MAPK), the ERK-like protein (ERK-LP). Several data including our previous results show that ERK-LP is inactivated after fertilization, which agrees with results obtained in other species including Xenopus, starfish and mammals. The question is to elucidate the function of a high MAPK activity in sea urchin eggs. We report here that dephosphorylation of ERK-LP with very low concentrations of two MEK inhibitors, PD98059 or U0126, triggers entry into mitosis. Under these conditions, recurrent oscillations of the phosphorylation of ERK-LP and of a tyrosine residue in Cdc2 occur, and the intracellular Ca2+ level (Ca2+i) progressively and slowly increases. Nuclear envelope breakdown and all mitotic events initiated after dephosphorylation of ERK-LP are inhibited when changes in Ca2+i are prevented; however, they are independent of the intracellular pH. These results suggest that inactivation of a MEK-ERK pathway, normally induced after fertilization of sea urchin eggs, triggers entry into mitosis by altering Ca2+i but cannot trigger full DNA replication. We discuss the hypothesis that neither inactivation nor activation of a MEK-ERK pathway is required for S phase completion in sea urchin egg.

Nud1p, the yeast homolog of Centriolin, regulates spindle pole body inheritance in meiosis

Nud1p, a protein homologous to the mammalian centrosome and midbody component Centriolin, is a component of the budding yeast spindle pole body (SPB), with roles in anchorage of microtubules and regulation of the mitotic exit network during vegetative growth. Here we analyze the function of Nud1p during yeast meiosis. We find that a nud1-2 temperature-sensitive mutant has two meiosis-related defects that reflect genetically distinct functions of Nud1p. First, the mutation affects spore formation due to its late function during spore maturation. Second, and most important, the mutant loses its ability to distinguish between the ages of the four spindle pole bodies, which normally determine which SPB would be preferentially included in the mature spores. This affects the regulation of genome inheritance in starved meiotic cells and leads to the formation of random dyads instead of non-sister dyads under these conditions. Both functions of Nud1p are connected to the ability of Spc72p to bind to the outer plaque and half-bridge (via Kar1p) of the SPB.

Centrosome inheritance in the parthenogenetic egg of the collembolan Folsomia candida

Unfertilized eggs commonly lack centrioles, which are usually provided by the male gamete at fertilization, and are unable to assemble functional reproducing centrosomes. However, some insect species lay eggs that develop to adulthood without a contribution from sperm. We report that the oocyte of the parthenogenetic collembolan Folsomia candida is able to self-assemble microtubule-based asters in the absence of pre-existing maternal centrosomes. The asters, which develop near the innermost pole of the meiotic apparatus, interact with the female chromatin to form the first mitotic spindle. The appearance of microtubule-based asters in the cytoplasm of the activated Folsomia oocyte might represent a conserved mechanism for centrosome formation during insect parthenogenesis. We also report that the architecture of the female meiotic apparatus and the structure of the mitotic spindles during the early embryonic divisions are unusual in comparison with that of insects.

p90Rsk is required for G1 phase arrest in unfertilized starfish eggs

The cell cycle in oocytes generally arrests at a particular meiotic stage to await fertilization. This arrest occurs at metaphase of meiosis II (meta-II) in frog and mouse, and at G1 phase after completion of meiosis II in starfish. Despite this difference in the arrest phase, both arrests depend on the same Mos-MAPK (mitogen-activated protein kinase) pathway, indicating that the difference relies on particular downstream effectors. Immediately downstream of MAPK, Rsk (p90 ribosomal S6 kinase, p90(Rsk)) is required for the frog meta-II arrest. However, the mouse meta-II arrest challenges this requirement, and no downstream effector has been identified in the starfish G1 arrest. To investigate the downstream effector of MAPK in the starfish G1 arrest, we used a neutralizing antibody against Rsk and a constitutively active form of Rsk. Rsk was activated downstream of the Mos-MAPK pathway during meiosis. In G1 eggs, inhibition of Rsk activity released the arrest and initiated DNA replication without fertilization. Conversely, maintenance of Rsk activity prevented DNA replication following fertilization. In early embryos, injection of Mos activated the MAPK-Rsk pathway, resulting in G1 arrest. Moreover, inhibition of Rsk activity during meiosis I led to parthenogenetic activation without meiosis II. We conclude that immediately downstream of MAPK, Rsk is necessary and sufficient for the starfish G1 arrest. Although CSF (cytostatic factor) was originally defined for meta-II arrest in frog eggs, we propose to distinguish ;G1-CSF' for starfish from ;meta-II-CSF' for frog and mouse. The present study thus reveals a novel role of Rsk for G1-CSF.

Centrosome destined to decay in starfish oocytes

In contrast to the somatic cell cycle, duplication of the centrioles does not occur in the second meiotic cycle. Previous studies have revealed that in starfish each of the two centrosomes in fully-grown immature oocytes consists of two centrioles with different destinies: one survives and retains its reproductive capacity, and the other is lost after completion of meiosis. In this study, we investigated whether this heterogeneity of the meiotic centrioles is already determined before the re-initiation of meiosis. We prepared a small fragment of immature oocyte containing the four centrioles and fused it electrically with a mature egg in order to transfer two sets of the premeiotic centrioles into the mature cytoplasm. Two asters were present in this conjugate, and in each of them only a single centriole was detected by electron microscopy. In the first mitosis of the conjugate artificially activated without sperm, two division poles formed, each of which doubled in each subsequent round of mitosis. These results indicate that only two of the four premeiotic centrioles survived in the mature cytoplasm and that they retained their reproductive capacity, which suggests that the heterogeneity of the maternal centrioles is determined well before re-initiation of meiosis, and that some factor in the mature cytoplasm is responsible for suppressing the reproductive capacity of the centrioles destined to decay.

Strongylocentrotus drobachiensis oocytes maintain a microtubule organizing center throughout oogenesis: Implications for the establishment of egg polarity in sea urchins

Although it has been known for over a century that sea urchin eggs are polarized cells, very little is known about the mechanism responsible for establishing and maintaining polarity. Our previous studies of microtubule organization during sea urchin oogenesis described a cortical microtubule-organizing center (MTOC) present during germinal vesicle (GV) migration in large oocytes. This MTOC was localized within the future animal pole of the mature egg. In this study we have used electron microscopy and immunocytochemistry to characterize the structure of this MTOC and have established that this organelle appears prior to GV migration. We show that the cortical MTOC contains all the components of a centrosome, including a pair of centrioles. Although a centrosome proper was not found in small oocytes, the centriole pair in these cells was always found in association with a striated rootlet, a structural remnant of the flagellar apparatus present in precursor germinal cells (PGCs). The centrioles/striated rootlet complex was asymmetrically localized to the side of the oocyte closest to the gonadal wall. These data are consistent with the previously proposed hypothesis that in echinoderms the polarity of the PGCs in the germinal epithelium influences the final polarity of the mature egg.

Spore number control and breeding in Saccharomyces cerevisiae: a key role for a self-organizing system

Spindle pole bodies (SPBs) provide a structural basis for genome inheritance and spore formation during meiosis in yeast. Upon carbon source limitation during sporulation, the number of haploid spores formed per cell is reduced. We show that precise spore number control (SNC) fulfills two functions. SNC maximizes the production of spores (1-4) that are formed by a single cell. This is regulated by the concentration of three structural meiotic SPB components, which is dependent on available amounts of carbon source. Using experiments and computer simulation, we show that the molecular mechanism relies on a self-organizing system, which is able to generate particular patterns (different numbers of spores) in dependency on one single stimulus (gradually increasing amounts of SPB constituents). We also show that SNC enhances intratetrad mating, whereby maximal amounts of germinated spores are able to return to a diploid lifestyle without intermediary mitotic division. This is beneficial for the immediate fitness of the population of postmeiotic cells.

Aster self-organization at meiosis: a conserved mechanism in insect parthenogenesis?

Unfertilized eggs usually lack maternal centrosomes and cannot develop without sperm contribution. However, several insect species lay eggs that develop to adulthood as unfertilized in the absence of a preexisting centrosome. We report that the oocyte of the parthenogenetic viviparous pea aphid Acyrthosiphon pisum is able to self-organize microtubule-based asters, which in turn interact with the female chromatin to form the first mitotic spindle. This mode of reproduction provides a good system to investigate how the oocyte can assemble new centrosomes and how their number can be exactly monitored. We propose that the cooperative interaction of motor proteins and randomly nucleated surface microtubules could lead to the formation of aster-like structures in the absence of pre-existing centrosomes. Recruitment of material along the microtubules might contribute to the accumulation of pericentriolar material and centriole precursors at the focus of the asters, thus leading to the formation of true centrosomes. The appearance of microtubule asters at the surface of activated oocytes could represent a possible common mechanism for centrosome formation during insect parthenogenesis.

Formation of polar-body-like structures induced in polyspermic starfish oocytes that had been inseminated at the germinal vesicle stage

Immature starfish oocytes, which are arrested at the first meiotic prophase and contain a large nucleus called the germinal vesicle (GV), are known to accept multiple sperm on insemination. We found that if these polyspermic starfish oocytes are induced to mature, they often form small protrusion(s) adjacent to the first polar body emitted shortly earlier. We refer to these protrusion(s) as 'polar-body-like structures (PLS).' Fluorescent staining of PLS indicated that they were not merely cytoplasmic protrusions, but contained some chromatin. Maturing process of these polyspermic oocytes was examined by immnofluorescent staining, which showed that: (i) numerous sperm asters were observed after the onset of GV breakdown; (ii) before the first polar body (PB1) emission, a complex microtubular structure resembling a multipolar spindle was formed; and (iii) several isolated asters were observed after PB1 emission. These results indicate that PLS formation may be induced by interaction of meiosis-I spindle with paternal centrosomes incorporated at GV stage.

Unequal cell division regulated by the contents of germinal vesicles

Fertilization occurs during meiosis in many animals, when maternal centrosomes participate in the formation of spindles at the animal pole, which results in polar body formation. Paternal centrosomes do not participate in cell division during oocyte maturation. After meiosis, they form the spindles while the maternal centrosomes are discarded. It is unknown why paternal centrosomes do not form spindles during meiosis. Here, we show that the artificial incorporation of sperm at the animal pole of immature starfish oocytes causes unequal cell division and the formation of polar body-like fragments. The removal of germinal vesicles from the animal pole blocks the formation of polar body-like fragments. Furthermore, translocation of germinal vesicles to the vegetal pole by centrifugation induces the extrusion of polar body-like fragments from the vegetal pole, where sperm penetration is prerequisite. After germinal vesicle breakdown, cyclin B is localized in the maternal and paternal asters and spindles near the germinal vesicle. These results suggest that germinal vesicle components such as the cdc2-cyclin B complex interact with asters and spindles and can induce unequal cell division. During normal fertilization, paternal centrosomes are likely kept away from the germinal vesicle components, resulting in the inhibition of unequal paternal centrosome-dependent cell division.

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.

Regulation of the paternal inheritance of centrosomes in starfish zygotes

In most animals, fertilized eggs inherit one centrosome from a meiosis-II spindle of oocytes and another centrosome from the sperm. However, since first proposed by Boveri [Sitzungsber. Ges. Morph. Phys. Münch. 3 (1887) 151-164] at the turn of the last century, it has been believed that only the paternal (sperm) centrosome provides the division poles for mitosis in animal zygotes. This uniparental (paternal) inheritance of centrosomes is logically based on the premise that the maternal (egg) centrosome is lost before the onset of the first mitosis. For the processes of the selective loss of the maternal centrosome, three models have been proposed: One stresses the intrinsic factors within the centrosome itself; the other two emphasize external factors such as cytoplasmic conditions or the sperm centrosome. In the present study, we have examined the validity of one of the models in which the sperm centrosome overwhelms the maternal centrosomes. Because centrosomes cast off into both the first and the second polar bodies (PB) are known to retain the capacity for reproduction and cell-division pole formation, we observed the behavior of those PB centrosomes with reproductive capacity and the sperm centrosome in the same zygotic cytoplasm. We prepared two kinds of fertilized eggs that contain reproductive maternal centrosomes, (1) by micromanipulative transplantation of the PB centrosomes into fertilized eggs, and (2) by suppression of the PB extrusions of fertilized eggs with cytochalasin B. In both types of eggs, the PB centrosomes could double and form cell-division poles, indicating that they are not suppressed by the sperm centrosome, which in turn indicates that selective loss of the maternal centrosome is due to intrinsic factors within the centrosomes themselves.