Unequal cell division regulated by the contents of germinal vesicles

Centriole foci persist in starfish oocytes despite Polo-like kinase 1 inactivation or loss of microtubule nucleation activity

Centrioles must be eliminated or inactivated from the oocyte to ensure that only the two functional centrioles contributed by the sperm are present in the zygote. Such removal can occur during oogenesis, as in Drosophila, where departure of Polo kinase from centrosomes leads to loss of microtubule nucleating activity and centriole removal. In other species, oocyte-derived centrioles are removed around the time of fertilization through incompletely understood mechanisms. Here, we use confocal imaging of live starfish oocytes and zygotes expressing markers of microtubule nucleating activity and centrioles to investigate this question. We first assay the role of Polo-like kinase 1 (Plk1) in centriole elimination. We find that although Plk1 localizes around oocyte-derived centrioles, kinase impairment with BI-2536 does not protect centrioles from removal in the bat star Patiria miniata. Moreover, we uncover that all four oocyte-derived centrioles lose microtubule nucleating activity when retained experimentally in the zygote of the radiate star Asterias forbesi. Interestingly, two such centrioles nevertheless retain the centriolar markers mEGFP::PACT and pmPoc1::mEGFP. Together, these findings indicate that centrioles can persist when Plk1 activity is impaired, as well as when microtubule nucleating activity is lacking, uncovering further diversity in the mechanisms governing centriole removal.

A disassembly-driven mechanism explains F-actin-mediated chromosome transport in starfish oocytes

While contraction of sarcomeric actomyosin assemblies is well understood, this is not the case for disordered networks of actin filaments (F-actin) driving diverse essential processes in animal cells. For example, at the onset of meiosis in starfish oocytes a contractile F-actin network forms in the nuclear region transporting embedded chromosomes to the assembling microtubule spindle. Here, we addressed the mechanism driving contraction of this 3D disordered F-actin network by comparing quantitative observations to computational models. We analyzed 3D chromosome trajectories and imaged filament dynamics to monitor network behavior under various physical and chemical perturbations. We found no evidence of myosin activity driving network contractility. Instead, our observations are well explained by models based on a disassembly-driven contractile mechanism. We reconstitute this disassembly-based contractile system in silico revealing a simple architecture that robustly drives chromosome transport to prevent aneuploidy in the large oocyte, a prerequisite for normal embryonic development.

A cdk1 gradient guides surface contraction waves in oocytes

Surface contraction waves (SCWs) in oocytes and embryos lead to large-scale shape changes coupled to cell cycle transitions and are spatially coordinated with the cell axis. Here, we show that SCWs in the starfish oocyte are generated by a traveling band of myosin II-driven cortical contractility. At the front of the band, contractility is activated by removal of cdk1 inhibition of the RhoA/RhoA kinase/myosin II signaling module, while at the rear, contractility is switched off by negative feedback originating downstream of RhoA kinase. The SCW's directionality and speed are controlled by a spatiotemporal gradient of cdk1-cyclinB. This gradient is formed by the release of cdk1-cyclinB from the asymmetrically located nucleus, and progressive degradation of cyclinB. By combining quantitative imaging, biochemical and mechanical perturbations with mathematical modeling, we demonstrate that the SCWs result from the spatiotemporal integration of two conserved regulatory modules, cdk1-cyclinB for cell cycle regulation and RhoA/Rok/NMYII for actomyosin contractility.Surface contraction waves (SCWs) are prominent shape changes coupled to cell cycle transitions in oocytes. Here the authors show that SCWs are patterned by the spatiotemporal integration of two conserved modules, cdk1-cyclinB for cell cycle regulation and RhoA/Rok/NMYII for actomyosin contractility.

Eccentric position of the germinal vesicle and cortical flow during oocyte maturation specify the animal-vegetal axis of ascidian embryos

The animal-vegetal (A-V) axis is already set in unfertilized eggs. It plays crucial roles to coordinate germ-layer formation. However, how the A-V axis is set has not been well studied. In ascidians, unfertilized eggs are already polarized along the axis in terms of cellular components. The polarization occurs during oocyte maturation. Oocytes within the gonad have the germinal vesicle (GV) close to the future animal pole. When the GVs of full-grown oocytes were experimentally translocated to the opposite pole by centrifugal force, every aspect that designates A-V polarity was reversed in the eggs and embryos. This was confirmed by examining the cortical allocation of the meiotic spindle, position of the polar body emission, polarized distribution of mitochondria and postplasmic/PEM mRNA, direction of the cortical flow during oocyte maturation, cleavage pattern, and germ-layer formation during embryogenesis. Therefore, the eccentric position of the GV triggers subsequent polarizing events and establishes the A-V axis in eggs and embryos. We emphasize important roles of the cortical flow. This is the first report in which the A-V axis was experimentally and completely reversed in animal oocytes before fertilization.

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.

The Mos-MAPK pathway regulates Diaphanous-related formin activity to drive cleavage furrow closure during polar body extrusion in starfish oocytes

Maintenance of spindle attachment to the cortex and formation of the cleavage furrow around the protruded spindle are essential for polar body extrusion (PBE) during meiotic maturation of oocytes. Although spindle movement to the cortex has been well-studied, how the spindle is maintained at the cortex during PBE is unknown. Here, we show that activation of Diaphanous-related formin mediated by mitogen-activated protein kinase (MAPK) is required for tight spindle attachment to the cortex and cleavage furrow closure during PBE in starfish (Asterina pectinifera) oocytes. A. pectinifera Diaphanous-related formin (ApDia) had a distinct localization in immature oocytes and was localized to the cleavage furrow during PBE. Inhibition of the Mos-MAPK pathway or the actin nucleating activity of formin homology 2 domain prevented cleavage furrow closure and resulted in PBE failure. In MEK/MAPK-inhibited oocytes, activation of ApDia by relief of its intramolecular inhibition restored PBE. In summary, this study elucidates a link between the Mos-MAPK pathway and Diaphanous-related formins, that is responsible for maintaining tight spindle attachment to the cortex and cleavage furrow closure during PBE.

Role of microtubules and centrosomes in the eccentric relocation of the germinal vesicle upon meiosis reinitiation in sea-cucumber oocytes

In the oocytes of many animals, the germinal vesicle (GV) relocates from the center to the periphery of the oocyte upon meiosis reinitiation, which is a prerequisite to the formation of meiotic spindles beneath the cell surface in order for meiosis to succeed. In the present study, we have investigated nuclear positioning using sea-cucumber oocytes. Upon meiosis reinitiation, the GV relocates to the cell periphery beneath a surface protuberance. After GV breakdown, polar bodies were extruded from the top of the protuberance, which we therefore called the animal pole process. The GV relocation was inhibited by nocodazole but not by cytochalasin. Immunofluorescent staining and electron microscopy of microtubular arrays revealed that: (i) in immature oocytes, two centrosomes were situated beneath the animal pole process far apart from the GV, anchoring to the cortex via astral microtubules; (ii) upon meiosis reinitiation, microtubular bundles were newly formed between the centrosomes and the GV; and (iii) the microtubular bundles became short as GV migration proceeded. These observations suggest that microtubules and centrosomes participate in GV relocation. A very large mass of annulate lamellae, having a 20-microm diameter, was found in the vegetal pole of the oocytes.