Ran GTPase promotes oocyte polarization by regulating ERM (Ezrin/Radixin/Moesin) inactivation

Phosphorylated ERM regulates meiotic maturation in mouse oocytes

The cytoskeleton of mammal oocytes provides structural support to the plasma membrane and contributes to critical cellular dynamic processes such as nuclear positioning, germinal vesicle breakdown, spindle orientation, chromosome segregation, polar body extrusion, and transmembrane signaling pathways. The ERM family (ezrin, radixin and moesin) well known as membrane-cytoskeletal crosslinkers play a crucial role in organizing plasma membrane domains through their capacity to interact with transmembrane proteins and the underlying cytoskeleton. Recent works mainly focused on the structural analysis of the ERM family members and their binding partners, together with multiple functions in cell mitosis, have significantly advanced our understanding of the importance of membrane-cytoskeletal interactions. In the present study, we documented that p-ERM was expressed and localized at cortical and nucleus during mouse oocyte meiosis. p-ERM and microfilaments were colocalized from GV to MII during mouse oocyte maturation. After being treated with cytochalasin B (CB), the F-actin was disassembled. Meanwhile, p-ERM exhibited a diffuse cytoplasmic distribution and no special staining was detected in either the oocyte membrane or condensed chromosomes. p-ERM depletion by trim-away caused the meiotic procedure arrest with a significantly lower polar body extrusion rate. Collectively, these data demonstrate that the subcellular distribution of p-ERM is correlated with microfilaments. Meanwhile, the p-ERM contributes to the first polar extrusion but does not regulate the microfilament assembly.

X centromeric drive may explain the prevalence of polycystic ovary syndrome and other conditions: Genomic structure of the human X chromosome pericentromeric region is consistent with meiotic drive associated with PCOS and other conditions

X chromosome centromeric drive may explain the prevalence of polycystic ovary syndrome and contribute to oocyte aneuploidy, menopause, and other conditions. The mammalian X chromosome may be vulnerable to meiotic drive because of X inactivation in the female germline. The human X pericentromeric region contains genes potentially involved in meiotic mechanisms, including multiple SPIN1 and ZXDC paralogs. This is consistent with a multigenic drive system comprising differential modification of the active and inactive X chromosome centromeres in female primordial germ cells and preferential segregation of the previously inactivated X chromosome centromere to the polar body at meiosis I. The drive mechanism may explain differences in X chromosome regulation in the female germlines of the human and mouse and, based on the functions encoded by the genes in the region, the transmission of X pericentromeric genetic or epigenetic variants to progeny could contribute to preeclampsia, autism, and differences in sexual differentiation.

MRCK activates mouse oocyte myosin II for spindle rotation and male pronucleus centration

Asymmetric meiotic divisions in oocytes rely on spindle positioning in close vicinity to the cortex. In metaphase II mouse oocytes, eccentric spindle positioning triggers cortical polarization, including the build-up of an actin cap surrounded by a ring of activated myosin II. While the role of the actin cap in promoting polar body formation is established, ring myosin II activation mechanisms and functions have remained elusive. Here, we show that ring myosin II activation requires myotonic dystrophy kinase-related Cdc42-binding kinase (MRCK), downstream of polarized Cdc42. MRCK inhibition resulted in spindle rotation defects during anaphase II, precluding polar body extrusion. Remarkably, disengagement of segregated chromatids from the anaphase spindle could rescue rotation. We further show that the MRCK/myosin II pathway is activated in the fertilization cone and is required for male pronucleus migration toward the center of the zygote. These findings provide novel insights into the mechanism of myosin II activation in oocytes and its role in orchestrating asymmetric division and pronucleus centration.

Tubulin post-translational modifications in meiosis

Haploid gametes are produced from diploid parents through meiosis, a process inherent to all sexually reproducing eukaryotes. Faithful chromosome segregation in meiosis is essential for reproductive success, although it is less clear how the meiotic spindle achieves this compared to the mitotic spindle. It is becoming increasingly clear that tubulin post-translational modifications (PTMs) play critical roles in regulating microtubule functions in many biological processes, and meiosis is no exception. Here, I review recent advances in the understanding of tubulin PTMs in meiotic spindles, especially focusing on their roles in spindle integrity, oocyte aging, and non-Mendelian transmission.

Single-cell transcriptome and translatome dual-omics reveals potential mechanisms of human oocyte maturation

The combined use of transcriptome and translatome as indicators of gene expression profiles is usually more accurate than the use of transcriptomes alone, especially in cell types governed by translational regulation, such as mammalian oocytes. Here, we developed a dual-omics methodology that includes both transcriptome and translatome sequencing (T&T-seq) of single-cell oocyte samples, and we used it to characterize the transcriptomes and translatomes during mouse and human oocyte maturation. T&T-seq analysis revealed distinct translational expression patterns between mouse and human oocytes and delineated a sequential gene expression regulation from the cytoplasm to the nucleus during human oocyte maturation. By these means, we also identified a functional role of OOSP2 inducing factor in human oocyte maturation, as human recombinant OOSP2 induced in vitro maturation of human oocytes, which was blocked by anti-OOSP2. Single-oocyte T&T-seq analyses further elucidated that OOSP2 induces specific signaling pathways, including small GTPases, through translational regulation.

Development of methods for simultaneous single cell analysis of transcription and translation is still underway. Here, Hu et al. develop single-cell transcriptome and translatome dual-omics on human oocytes, which enables them to identify OOSP2 as an induction factor during human oocyte maturation.

A biophysical perspective of the regulatory mechanisms of ezrin/radixin/moesin proteins

Many signal transductions resulting from ligand-receptor interactions occur at the cell surface. These signaling pathways play essential roles in cell polarization, membrane morphogenesis, and the modulation of membrane tension at the cell surface. However, due to the large number of membrane-binding proteins, including actin-membrane linkers, and transmembrane proteins present at the cell surface, the molecular mechanisms underlying the regulation at the cell surface are yet unclear. Here, we describe the molecular functions of one of the key players at the cell surface, ezrin/radixin/moesin (ERM) proteins from a biophysical point of view. We focus our discussion on biophysical properties of ERM proteins revealed by using biophysical tools in live cells and in vitro reconstitution systems. We first describe the structural properties of ERM proteins and then discuss the interactions of ERM proteins with PI(4,5)P₂ and the actin cytoskeleton. These properties of ERM proteins revealed by using biophysical approaches have led to a better understanding of their physiological functions in cells and tissues.

Supplementary Information

The online version contains supplementary material available at 10.1007/s12551-021-00928-0.

Cofilin regulates actin network homeostasis and microvilli length in mouse oocytes

How multiple actin networks coexist in a common cytoplasm while competing for a shared pool of monomers is still an ongoing question. This is exemplified by meiotic maturation in the mouse oocyte, which relies on the dynamic remodeling of distinct cortical and cytoplasmic F-actin networks. Here, we show that the conserved actin-depolymerizing factor cofilin is activated in a switch-like manner upon meiosis resumption from prophase arrest. Interfering with cofilin activation during maturation resulted in widespread elongation of microvilli, while cytoplasmic F-actin was depleted, leading to defects in spindle migration and polar body extrusion. In contrast, cofilin inactivation in metaphase II-arrested oocytes resulted in a shutdown of F-actin dynamics, along with a dramatic overgrowth of the polarized actin cap. However, inhibition of the Arp2/3 complex to promote actin cap disassembly elicited ectopic microvilli outgrowth in the polarized cortex. These data establish cofilin as a key player in actin network homeostasis in oocytes and reveal that microvilli can act as a sink for monomers upon disassembly of a competing network.

Advances in the study of CDC42 in the female reproductive system

CDC42 is a member of the Rho‐GTPase family and is involved in a variety of cellular functions including regulation of cell cycle progression, constitution of the actin backbone and membrane transport. In particular, CDC42 plays a key role in the establishment of polarity in female vertebrate oocytes, and essential to this major regulatory role is its local occupation of specific regions of the cell to ensure that the contractile ring is assembled at the right time and place to ensure proper gametogenesis. The multifactor controlled ‘inactivation‐activation’ process of CDC42 also allows it to play an important role in the multilevel signalling network, and the synergistic regulation of multiple genes ensures maximum precision during gametogenesis. The purpose of this paper is to review the role of CDC42 in the control of gametogenesis and to explore its related mechanisms, with the aim of further understanding the great research potential of CDC42 in female vertebrate germ cells and its future clinical translation.

RanGTP and the actin cytoskeleton keep paternal and maternal chromosomes apart during fertilization

Zygotes require two accurate sets of parental chromosomes, one each from the mother and the father, to undergo normal embryogenesis. However, upon egg-sperm fusion in vertebrates, the zygote has three sets of chromosomes, one from the sperm and two from the egg. The zygote therefore eliminates one set of maternal chromosomes (but not the paternal chromosomes) into the polar body through meiosis, but how the paternal chromosomes are protected from maternal meiosis has been unclear. Here we report that RanGTP and F-actin dynamics prevent egg-sperm fusion in proximity to maternal chromosomes. RanGTP prevents the localization of Juno and CD9, egg membrane proteins that mediate sperm fusion, at the cell surface in proximity to maternal chromosomes. Following egg-sperm fusion, F-actin keeps paternal chromosomes away from maternal chromosomes. Disruption of these mechanisms causes the elimination of paternal chromosomes during maternal meiosis. This study reveals a novel critical mechanism that prevents aneuploidy in zygotes.

RhoA- and Cdc42-induced antagonistic forces underlie symmetry breaking and spindle rotation in mouse oocytes

Mammalian oocyte meiotic divisions are highly asymmetric and produce a large haploid gamete and 2 small polar bodies. This relies on the ability of the cell to break symmetry and position its spindle close to the cortex before anaphase occurs. In metaphase II–arrested mouse oocytes, the spindle is actively maintained close and parallel to the cortex, until fertilization triggers sister chromatid segregation and the rotation of the spindle. The latter must indeed reorient perpendicular to the cortex to enable cytokinesis ring closure at the base of the polar body. However, the mechanisms underlying symmetry breaking and spindle rotation have remained elusive. In this study, we show that spindle rotation results from 2 antagonistic forces. First, an inward contraction of the cytokinesis furrow dependent on RhoA signaling, and second, an outward attraction exerted on both sets of chromatids by a Ran/Cdc42-dependent polarization of the actomyosin cortex. By combining live segmentation and tracking with numerical modeling, we demonstrate that this configuration becomes unstable as the ingression progresses. This leads to spontaneous symmetry breaking, which implies that neither the rotation direction nor the set of chromatids that eventually gets discarded are biologically predetermined.

Mammalian oocyte meiotic divisions are highly asymmetric and produce a large haploid gamete and two small polar bodies, but the mechanisms underlying the required symmetry breaking and spindle rotation have remained elusive. This study shows that spindle rotation in activated mouse oocytes relies on spontaneous symmetry breaking resulting from an unstable configuration generated by cleavage furrow ingression and cortical chromosome attraction.

Unravelling the mystery of female meiotic drive: where we are

Female meiotic drive is the phenomenon where a selfish genetic element alters chromosome segregation during female meiosis to segregate to the egg and transmit to the next generation more frequently than Mendelian expectation. While several examples of female meiotic drive have been known for many decades, a molecular understanding of the underlying mechanisms has been elusive. Recent advances in this area in several model species prompts a comparative re-examination of these drive systems. In this review, we compare female meiotic drive of several animal and plant species, highlighting pertinent similarities.

RAB14 GTPase is essential for actin-based asymmetric division during mouse oocyte maturation

Objectives

RAB14 is a member of small GTPase RAB family which localizes at the endoplasmic reticulum (ER), Golgi apparatus and endosomal compartments. RAB14 acts as molecular switches that shift between a GDP‐bound inactive state and a GTP‐bound active state and regulates circulation of vesicles between the Golgi and endosomal compartments. In present study, we investigated the roles of RAB14 during oocyte meiotic maturation.

Materials and methods

Microinjection with siRNA and exogenous mRNA for knock down and rescue, and immunofluorescence staining, Western blot and real‐time RT‐PCR were utilized for the study.

Results

Our results showed that RAB14 localized in the cytoplasm and accumulated at the cortex during mouse oocyte maturation, and it was also enriched at the spindle periphery. Depletion of RAB14 did not affect polar body extrusion but caused large polar bodies, indicating the failure of asymmetric division. We found that absence of RAB14 did not affect spindle organization but caused the spindle migration defects, and this might be due to the regulation on cytoplasmic actin assembly via the ROCK‐cofilin signalling pathway. We also found that RAB14 depletion led to aberrant Golgi apparatus distribution. Exogenous Myc‐Rab14 mRNA supplement could significantly rescue these defects caused by Rab14 siRNA injection.

Conclusions

Taken together, our results suggest that RAB14 affects ROCK‐cofilin pathway for actin‐based spindle migration and Golgi apparatus distribution during mouse oocyte meiotic maturation.

Role of PB1 Midbody Remnant Creating Tethered Polar Bodies during Meiosis II

Polar body (PB) formation is an extreme form of unequal cell division that occurs in oocytes due to the eccentric position of the small meiotic spindle near the oocyte cortex. Prior to PB formation, a chromatin-centered process causes the cortex overlying the meiotic chromosomes to become polarized. This polarized cortical subdomain marks the site where a cortical protrusion or outpocket forms at the oocyte surface creating the future PBs. Using ascidians, we observed that PB1 becomes tethered to the fertilized egg via PB2, indicating that the site of PB1 cytokinesis directed the precise site for PB2 emission. We therefore studied whether the midbody remnant left behind following PB1 emission was involved, together with the egg chromatin, in defining the precise cortical site for PB2 emission. During outpocketing of PB2 in ascidians, we discovered that a small structure around 1 µm in diameter protruded from the cortical outpocket that will form the future PB2, which we define as the “polar corps”. As emission of PB2 progressed, this small polar corps became localized between PB2 and PB1 and appeared to link PB2 to PB1. We tested the hypothesis that this small polar corps on the surface of the forming PB2 outpocket was the midbody remnant from the previous round of PB1 cytokinesis. We had previously discovered that Plk1::Ven labeled midbody remnants in ascidian embryos. We therefore used Plk1::Ven to follow the dynamics of the PB1 midbody remnant during meiosis II. Plk1::Ven strongly labeled the small polar corps that formed on the surface of the cortical outpocket that created PB2. Following emission of PB2, this polar corps was rich in Plk1::Ven and linked PB2 to PB1. By labelling actin (with TRITC-Phalloidin) we also demonstrated that actin accumulates at the midbody remnant and also forms a cortical cap around the midbody remnant in meiosis II that prefigured the precise site of cortical outpocketing during PB2 emission. Phalloidin staining of actin and immunolabelling of anti-phospho aPKC during meiosis II in fertilized eggs that had PB1 removed suggested that the midbody remnant remained within the fertilized egg following emission of PB1. Dynamic imaging of microtubules labelled with Ens::3GFP, MAP7::GFP or EB3::3GFP showed that one pole of the second meiotic spindle was located near the midbody remnant while the other pole rotated away from the cortex during outpocketing. Finally, we report that failure of the second meiotic spindle to rotate can lead to the formation of two cortical outpockets at anaphase II, one above each set of chromatids. It is not known whether the midbody remnant of PB1 is involved in directing the precise location of PB2 since our data are correlative in ascidians. However, a review of the literature indicates that PB1 is tethered to the egg surface via PB2 in several species including members of the cnidarians, lophotrochozoa and echinoids, suggesting that the midbody remnant formed during PB1 emission may be involved in directing the precise site of PB2 emission throughout the invertebrates.

Importin binding mediates the intramolecular regulation of anillin during cytokinesis

Cytokinesis occurs by the ingression of an actomyosin ring that cleaves a cell into two daughters. This process is tightly controlled to avoid aneuploidy, and we previously showed that active Ran coordinates ring positioning with chromatin. Active Ran is high around chromatin, and forms an inverse gradient to cargo-bound importins. We found that the ring component anillin contains a nuclear localization signal (NLS) that binds to importin and is required for its function during cytokinesis. Here we reveal the mechanism whereby importin binding favors a conformation required for anillin's recruitment to the equatorial cortex. Active RhoA binds to the RhoA-binding domain causing an increase in accessibility of the nearby C2 domain containing the NLS. Importin binding subsequently stabilizes a conformation that favors interactions for cortical recruitment. In addition to revealing a novel mechanism for the importin-mediated regulation of a cortical protein, we also show how importin binding positively regulates protein function.

WDR62 is a novel participator in spindle migration and asymmetric cytokinesis during mouse oocyte meiotic maturation

In female meiosis, oocyte meiotic maturation is a form of asymmetric cell division, producing the first polar body and a large oocyte, in which the asymmetry of oocyte meiotic division depends on spindle migration and positioning, and cortical polarization. In this study, we conclude that WDR62 (WD40-repeat protein 62) plays an important role in asymmetric meiotic division during mouse oocyte maturation. Our initial study demonstrated that WDR62 mainly co-localized with chromosomes during mouse oocyte meiotic maturation. Interference of Wdr62 by siRNA microinjection did not affect germinal vesicle breakdown (GVBD) but compromised the first polar body extrusion (PBE) with the large polar bodies generated, which is coupled with a higher incidence of spindle abnormality and chromosome misalignment. Further analysis concluded that loss of WDR62 blocked asymmetric spindle positioning and actin cap formation, which should be responsible for large polar body extrusion. Moreover, WDR62 decline intervened with the Arp2/3 complex, an upstream regulator for the cortical actin. Besides for p-MAPK, a critical regulator for the asymmetric division of oocyte, WDR62-depleted oocytes showed perturbation only in localization pattern but not expression level. In summary, our study defines WDR62 as an essential cytoskeletal regulator of spindle migration and asymmetric division during mouse oocyte meiotic maturation.

Ran promotes membrane targeting and stabilization of RhoA to orchestrate ovarian cancer cell invasion

Ran is a nucleocytoplasmic shuttle protein that is involved in cell cycle regulation, nuclear-cytoplasmic transport, and cell transformation. Ran plays an important role in cancer cell survival and cancer progression. Here, we show that, in addition to the nucleocytoplasmic localization of Ran, this GTPase is specifically associated with the plasma membrane/ruffles of ovarian cancer cells. Ran depletion has a drastic effect on RhoA stability and inhibits RhoA localization to the plasma membrane/ruffles and RhoA activity. We further demonstrate that the DEDDDL domain of Ran is required for the interaction with serine 188 of RhoA, which prevents RhoA degradation by the proteasome pathway. Moreover, the knockdown of Ran leads to a reduction of ovarian cancer cell invasion by impairing RhoA signalling. Our findings provide advanced insights into the mode of action of the Ran-RhoA signalling axis and may represent a potential therapeutic avenue for drug development to prevent ovarian tumour metastasis.

Ran, a nucleus-cytoplasm shuttle protein, is implicated in cancer development and survival. Here, the authors show that Ran binds RhoA to impair its degradation and allow its localisation to the plasma membrane of ovarian cancer cells for tumour invasion.

Assembly and Positioning of the Oocyte Meiotic Spindle

Fertilizable eggs develop from diploid precursor cells termed oocytes. Once every menstrual cycle, an oocyte matures into a fertilizable egg in the ovary. To this end, the oocyte eliminates half of its chromosomes into a small cell termed a polar body. The egg is then released into the Fallopian tube, where it can be fertilized. Upon fertilization, the egg completes the second meiotic division, and the mitotic division of the embryo starts. This review highlights recent work that has shed light on the cytoskeletal structures that drive the meiotic divisions of the oocyte in mammals. In particular, we focus on how mammalian oocytes assemble a microtubule spindle in the absence of centrosomes, how they position the spindle in preparation for polar body extrusion, and how the spindle segregates the chromosomes. We primarily focus on mouse oocytes as a model system but also highlight recent insights from human oocytes.

Spindle asymmetry drives non-Mendelian chromosome segregation

Genetic elements compete for transmission through meiosis, when haploid gametes are created from a diploid parent. Selfish elements can enhance their transmission through a process known as meiotic drive. In female meiosis, selfish elements drive by preferentially attaching to the egg side of the spindle. This implies some asymmetry between the two sides of the spindle, but the molecular mechanisms underlying spindle asymmetry are unknown. Here we found that CDC42 signaling from the cell cortex regulated microtubule tyrosination to induce spindle asymmetry and that non-Mendelian segregation depended on this asymmetry. Cortical CDC42 depends on polarization directed by chromosomes, which are positioned near the cortex to allow the asymmetric cell division. Thus, selfish meiotic drivers exploit the asymmetry inherent in female meiosis to bias their transmission.

Active Ran regulates anillin function during cytokinesis

We describe a novel mechanism by which active Ran regulates anillin during cytokinesis. Anillin is highly conserved and coordinates RhoA, actomyosin, microtubules, and the membrane for cytokinesis in mammalian cells. This study implicates Ran-GTP in influencing cortical contractility during anaphase by regulating anillin function.

Cytokinesis cleaves a cell into two daughters at the end of mitosis, and must be spatially coordinated with chromosome segregation to prevent aneuploidy. The dogma is that the mitotic spindle governs the assembly and constriction of an actomyosin ring. Here, we reveal a function for active Ran in spatially restricting the ring. Our model is that during anaphase, “free” importins, whose gradient inversely correlates with active Ran and chromatin position, function as a molecular ruler for the recruitment and localization of anillin, a contractile protein and a crucial regulator of cytokinesis. We found that decreasing Ran-GTP levels or tethering active Ran to the equatorial membrane affects anillin’s localization and causes cytokinesis phenotypes. Anillin contains a conserved nuclear localization signal (NLS) at its C-terminus that binds to importin-β and is required for cortical polarity and cytokinesis. Mutating the NLS decreases anillin’s cortical affinity, causing it to be more dominantly regulated by microtubules. Anillin contains a RhoA-GTP binding domain, which autoinhibits the NLS and the neighboring microtubule-binding domain, and RhoA-GTP binding may relieve this inhibition during mitosis. Retention of the C-terminal NLS in anillin homologues suggests that this is a conserved mechanism for controlling anillin function.

GTPase-activating protein Elmod2 is essential for meiotic progression in mouse oocytes

Meiotic failure in oocytes is the major determinant of human zygote-originated reproductive diseases, the successful accomplishment of meiosis largely relay on the normal functions of many female fertility factors. Elmod2 is a member of the Elmod family with the strongest GAP (GTPase-activating protein) activity; although it was identified as a possible maternal protein, its actual physiologic role in mammalian oocytes has not been elucidated. Herein we reported that among Elmod family proteins, Elmod2 is the most abundant in mouse oocytes, and that inhibition of Elmod2 by specific siRNA caused severe meiotic delay and abnormal chromosomal segregation during anaphase. Elmod2 knockdown also significantly decreased the rate of oocyte maturation (to MII, with first polar body extrusion), and significantly greater numbers of Elmod2-knockdown MII oocytes were aneuploid. Correspondingly, Elmod2 knockdown dramatically decreased fertilization rate. To investigate the mechanism(s) involved, we found that Elmod2 knockdown caused significantly more abnormal mitochondrial aggregation and diminished cellular ATP levels; and we also found that Elmod2 co-localized and interacted with Arl2, a GTPase that is known to maintain mitochondrial dynamics and ATP levels in oocytes. In summary, we found that Elmod2 is the GAP essential to meiosis progression of mouse oocytes, most likely by regulating mitochondrial dynamics.

Inhibition of Rac1 GTPase activity affects porcine oocyte maturation and early embryo development

Mammalian oocyte asymmetric division relies on the eccentric positioning of the spindle, resulting in the polar body formation. Small signaling G protein Rac1 is a member of GTPases, which regulates a diverse array of cellular events, including the control of cell growth, cytoskeletal reorganization, and the activation of protein kinases. However, effects of Rac1 on the porcine oocyte maturation and early embryo development are not fully understood. In present study we investigated the role of Rac1 in oocyte maturation and embryo cleavage. We first found that Rac1 localized at the cortex of the porcine oocytes, and disrupting the Rac1 activities by treating with NSC 23766 led to the failure of polar body emission. In addition, a majority of treated oocytes exhibited abnormal spindle morphology, indicating that Rac1 may involve into porcine oocyte spindle formation. This might be due to the regulation of Rac1 on MAPK, since p-MAPK expression decreased after NSC 23766 treatments. Moreover, we found that the position of most meiotic spindles in treated oocytes were away from the cortex, indicating the roles of Rac1 on meiotic spindle positioning. Our results also showed that inhibition of Rac1 activity caused the failure of early embryo development. Therefore, our study showed the critical roles of Rac1 GTPase on porcine oocyte maturation and early embryo cleavage.

Coupling changes in cell shape to chromosome segregation

Animal cells undergo dramatic changes in shape, mechanics and polarity as they progress through the different stages of cell division. These changes begin at mitotic entry, with cell-substrate adhesion remodelling, assembly of a cortical actomyosin network and osmotic swelling, which together enable cells to adopt a near spherical form even when growing in a crowded tissue environment. These shape changes, which probably aid spindle assembly and positioning, are then reversed at mitotic exit to restore the interphase cell morphology. Here, we discuss the dynamics, regulation and function of these processes, and how cell shape changes and sister chromatid segregation are coupled to ensure that the daughter cells generated through division receive their fair inheritance.

Roles of actin binding proteins in mammalian oocyte maturation and beyond

Actin nucleation factors, which promote the formation of new actin filaments, have emerged in the last decade as key regulatory factors controlling asymmetric division in mammalian oocytes. Actin nucleators such as formin-2, spire, and the ARP2/3 complex have been found to be important regulators of actin remodeling during oocyte maturation. Another class of actin-binding proteins including cofilin, tropomyosin, myosin motors, capping proteins, tropomodulin, and Ezrin-Radixin-Moesin proteins are thought to control actin cytoskeleton dynamics at various steps of oocyte maturation. In addition, actin dynamics controlling asymmetric-symmetric transitions after fertilization is a new area of investigation. Taken together, defining the mechanisms by which actin-binding proteins regulate actin cytoskeletons is crucial for understanding the basic biology of mammalian gamete formation and pre-implantation development.

TGN38 is required for the metaphase I/anaphase I transition and asymmetric cell division during mouse oocyte meiotic maturation

The cellular functions of the trans-Golgi network protein TGN38 remain unknown. In this research, we studied the expression, localization and functions of TGN38 in the meiotic maturation of mouse oocytes. TGN38 was expressed at every stage of oocyte meiotic maturation and colocalized with γ-tubulin at metaphase I and metaphase II. The spindle microtubule disturbing agents nocodazole and taxol did not affect the colocalization of TGN38 and γ-tubulin. Depletion of TGN38 with specific siRNAs resulted in increased metaphase I arrest, accompanied with spindle assembly checkpoint activation and decreased first polar extrusion (PB1). In the oocytes that had extruded the PB1 after the depletion of TGN38, symmetric division occurred, leading to the production of 2 similarly sized cells. Moreover, the peripheral migration of metaphase I spindle and actin cap formation were impaired in TGN38-depleted oocytes. Our data suggest that TGN38 may regulate the metaphase I/anaphase I transition and asymmetric cell division in mouse oocytes.

Kinetochore-localized PP1-Sds22 couples chromosome segregation to polar relaxation

Cell division requires the precise coordination of chromosome segregation and cytokinesis. This coordination is achieved by the recruitment of an actomyosin regulator, Ect2, to overlapping microtubules at the centre of the elongating anaphase spindle. Ect2 then signals to the overlying cortex to promote the assembly and constriction of an actomyosin ring between segregating chromosomes. Here, by studying division in proliferating Drosophila and human cells, we demonstrate the existence of a second, parallel signalling pathway, which triggers the relaxation of the polar cell cortex at mid anaphase. This is independent of furrow formation, centrosomes and microtubules and, instead, depends on PP1 phosphatase and its regulatory subunit Sds22 (refs 2, 3). As separating chromosomes move towards the polar cortex at mid anaphase, kinetochore-localized PP1-Sds22 helps to break cortical symmetry by inducing the dephosphorylation and inactivation of ezrin/radixin/moesin proteins at cell poles. This promotes local softening of the cortex, facilitating anaphase elongation and orderly cell division. In summary, this identifies a conserved kinetochore-based phosphatase signal and substrate, which function together to link anaphase chromosome movements to cortical polarization, thereby coupling chromosome segregation to cell division.

RhoA-mediated FMNL1 regulates GM130 for actin assembly and phosphorylates MAPK for spindle formation in mouse oocyte meiosis

Formin-like 1 (FMNL1) is a member of Formin family proteins which are the actin nucleators. Although FMNL1 activities have been shown to be essential for cell adhesion, cytokinesis, cell polarization and migration in mitosis, the functional roles of mammalian FMNL1 during oocyte meiosis remain uncertain. In this study, we investigated the functions of FMNL1 in mouse oocytes using specific morpholino (MO) microinjection and live cell imaging. Immunofluorescent staining showed that in addition to its cytoplasmic distribution, FMNL1 was primarily localized at the spindle poles after germinal vesicle breakdown (GVBD). FMNL1 knockdown caused the low rate of polar body extrusion and resulted in large polar bodies. Time-lapse microscopic and immunofluorescence intensity analysis indicated that this might be due to the aberrant actin expression levels. Cortical polarity was disrupted as shown by a loss of actin cap and cortical granule free domain (CGFD) formation, which was confirmed by a failure of meiotic spindle positioning. And this might be the reason for the large polar body formation. Spindle formation was also disrupted, which might be due to the abnormal localization of p-MAPK. These results indicated that FMNL1 affected both actin dynamics and spindle formation for the oocyte polar body extrusion. Moreover, FMNL1 depletion resulted in aberrant localization and expression patterns of a cis-Golgi marker protein, GM130. Finally, we found that the small GTPase RhoA might be the upstream regulator of FMNL1. Taken together, our data indicate that FMNL1 is required for spindle organization and actin assembly through a RhoA-FMNL1-GM130 pathway during mouse oocyte meiosis.

ROCK inhibitor Y-27632 prevents porcine oocyte maturation

The inhibitor Y-27632 is a specific selective inhibitor of Rho-associated protein kinases (ROCKs), which are downstream effectors of Rho guanosine triphosphatease (GTPases) and regulate Rho-associated cellular functions, including actin cytoskeletal organization. Little is known regarding the effects of Y-27632 on mammalian oocyte maturation. In the present study, we investigated the effects of Y-27632 on porcine oocyte meiosis and possible regulatory mechanisms of ROCK during porcine oocyte maturation. We found that ROCK accumulated not only at spindles, but also at the cortex in porcine oocytes. Y-27632 treatment reduced ROCK expression, and inhibited porcine oocyte meiotic maturation, which might be because of the impairment of actin expression and actin-related spindle positioning. Y-27632 treatment also disrupted the formation of actin cap and cortical granule-free domain, which further confirmed a spindle positioning failure. Thus, Y-27632 has significant effects on the meiotic competence of mammalian oocytes by reducing ROCK expression, and the regulation is related to its effects on actin-mediated spindle positioning.

Symmetry breaking and polarity establishment during mouse oocyte maturation

Mammalian oocyte meiosis encompasses two rounds of asymmetric divisions to generate a totipotent haploid egg and, as by-products, two small polar bodies. Two intracellular events, asymmetric spindle positioning and cortical polarization, are critical to such asymmetric divisions. Actin but not microtubule cytoskeleton has been known to be directly involved in both events. Recent work has revealed a positive feedback loop between chromosome-mediated cortical activation and the Arp2/3-orchestrated cytoplasmic streaming that moves chromosomes. This feedback loop not only maintains meiotic II spindle position during metaphase II arrest, but also brings about symmetry breaking during meiosis I. Prior to an Arp2/3-dependent phase of fast movement, meiotic I spindle experiences a slow and non-directional first phase of migration driven by a pushing force from Fmn2-mediated actin polymerization. In addition to illustrating these molecular mechanisms, mathematical simulations are presented to elucidate mechanical properties of actin-dependent force generation in this system.

Restarting life: fertilization and the transition from meiosis to mitosis

Fertilization triggers a complex cellular programme that transforms two highly specialized meiotic germ cells, the oocyte and the sperm, into a totipotent mitotic embryo. Linkages between sister chromatids are remodelled to support the switch from reductional meiotic to equational mitotic divisions; the centrosome, which is absent from the egg, is reintroduced; cell division shifts from being extremely asymmetric to symmetric; genomic imprinting is selectively erased and re-established; and protein expression shifts from translational control to transcriptional control. Recent work has started to reveal how this remarkable transition from meiosis to mitosis is achieved.