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1 State Key Laboratory of Reproductive Biology, Institute of Zoology, Chinese Academy of Sciences, Beijing 100080, China and 2 Department of Veterinary Pathobiology, University of Missouri, Columbia, MO 65211, USA
Correspondence should be addressed to H Schatten; Email: schattenh{at}missouri.edu
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Abstract |
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 Top Abstract IntroductionActin filaments are not...Regulation of organelle movement...Actin filaments participate in...Actin filaments are involved...The roles of actin...Actin filaments are involved...Regulation of sperm...Actin filaments control the...Regulation of pronuclear...Actin filaments are required...Oocyte polarity versus embryo...Concluding remarksAcknowledgementsReferences |
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Introduction |
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TopAbstractIntroductionActin filaments are not...Regulation of organelle movement...Actin filaments participate in...Actin filaments are involved...The roles of actin...Actin filaments are involved...Regulation of sperm...Actin filaments control the...Regulation of pronuclear...Actin filaments are required...Oocyte polarity versus embryo...Concluding remarksAcknowledgementsReferences |
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It has been well established that eukaryotic cells require filamentous actin to maintain their shape and for migration, growth, polarization, organelle movement, endocytosis/exocytosis, replication and gene regulation, while relatively little is known about the roles of actin filaments in germ cells. However, new knowledge has been accumulating on the functions of the cytoskeleton in germ cell development, fertilization and early embryo development. The sperm head cortical cytoskeleton exhibits significant changes during the acrosome reaction, consistent with the concept of cytoskeletal proteins as highly dynamic structures participating actively in processes before fertilization (Dvoráková et al. 2005). The role of the actin cytoskeleton in sperm capacitation and acrosome reaction, and the related signal transduction pathways have recently been reviewed (Breitbart et al. 2005). Meiotic maturation in mammalian oocytes is a complex process that involves extensive rearrangement of microtubules and actin filaments (Roth & Hansen 2005), as well as other cytoskeleton-associated proteins providing the framework for various dynamic processes. Polymerization of nonfilamentous (G-) actin into filamentous (F-) actin is important for various aspects of oocyte development and fertilization. In this paper, we will summarize our understanding of the role of actin filaments in the control of dynamic events during oocyte meiotic maturation and fertilization.
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Actin filaments are not required for germinal vesicle breakdown and meiotic spindle formation, but control chromatin movement during oocyte development |
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TopAbstractIntroductionActin filaments are not...Regulation of organelle movement...Actin filaments participate in...Actin filaments are involved...The roles of actin...Actin filaments are involved...Regulation of sperm...Actin filaments control the...Regulation of pronuclear...Actin filaments are required...Oocyte polarity versus embryo...Concluding remarksAcknowledgementsReferences |
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It is well known that spindle microtubules force chromosome movement in opposite directions during mitosis and meiosis. In pig (Wang et al. 2000, Sun et al. 2001a) and mouse (Soewarto et al. 1995) oocytes cultured in medium containing cytochalasin B or D (5–10 µg/ml), the transition from meiosis (M)I to MII was inhibited. Cell-cycle progression was inhibited and most oocytes were arrested at the MI stage. Therefore, it appears that microfilaments are needed for microtubule functions, and the segregation of homologous chromosomes requires interaction between microtubules and microfilaments. In rat oocytes, significantly higher levels of associated actin were observed on metaphase I chromosomes (Funaki et al. 1995). However, when mouse oocytes were treated with low-dose (1 µg/ml) cytochalasin D, anaphase I entry (chromosome segregation) occurred, although cytokinesis was blocked (Kubiak et al. 1991, Verlhac et al. 2000). Therefore, the effect of high-dose cytochalasin D on chromosome separation may be aspecific. Whether the actin filaments interact with microtubules to ensure homologous chromosome segregation during first meiosis needs further clarification.
Actin filaments are required for peripheral spindle anchorage in Xenopus and sheep oocytes (Ryabova et al. 1986, Le Guen et al. 1989, Gard et al. 1995). Recently, it has been reported that myosin-10 (Myo10) plays a critical role in spindle formation and anchoring in Xenopus oocytes. Myo10 proteins are phosphoinositide-binding, actin-based motors. X. laevis Myo10 associates with microtubules in vitro and in vivo, and is concentrated at the point where the meiotic spindle contacts the F-actin-rich cortex. Myo10 showed striking colocalization with meiotic spindle microtubules, and F-actin also colocalized with the spindle, both in the cortical cap and the interior of the spindle. In eggs expressing dominant negative Myo10 or injected with Myo10 antibody, spindle microtubule assembly was severely impaired. Disruption of Myo10 function also disrupted spindle-F-actin association and spindle anchoring (Weber et al. 2004). Although the disruption of microfilaments did not cause inward movement of chromosomes, the meiotic spindle was overlaid by F-actin in MII-stage pig oocytes (Sun et al. 2001a). It is possible that the actin cap anchors the spindle at the cortex by interacting with spindle microtubules.
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Regulation of organelle movement and positioning by the cytoskeleton |
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TopAbstractIntroductionActin filaments are not...Regulation of organelle movement...Actin filaments participate in...Actin filaments are involved...The roles of actin...Actin filaments are involved...Regulation of sperm...Actin filaments control the...Regulation of pronuclear...Actin filaments are required...Oocyte polarity versus embryo...Concluding remarksAcknowledgementsReferences |
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Mitochondria
Accumulation of active mitochondria in the peripheral cytoplasm and around the germinal vesicles is characteristic of fully grown pig oocytes collected from large follicles. Mitochondria accumulate in the perinuclear area during meiotic progression from GVBD to anaphase I stage. Larger mitochondrial foci are formed and moved to the inner cytoplasm in mature oocytes. After fertilization, mitochondria migrate to the peripronuclear region (Sun et al. 2001b). This mitochondrial translocation is mediated by microtubules, not by microfilaments, since disruption of microtubules, but not microfilaments, blocked mitochondria migration (Sun et al. 2001b). In mouse oocytes, a perinuclear accumulation of mitochondria characterizes the premetaphase I stage of reinitiated meiosis. A temporal, spatial and developmental relationship exists between the location of microtubule organizing centers and the progressive translocation of mitochondria to the nuclear region, indicating that mitochondrial translocations are mediated by microtubules (Van Blerkom 1991). Moreover, microfilament depolymerization by cytochalasin did not affect central migration of mitochondria (Calarco 2005). In contrast, it was found that, in larger, stage I Xenopus oocytes, a dense network of actin cables extends throughout the cytoplasm, linking the GV and mitochondrial mass to the cortical actin shell (Roeder & Gard 1994).
Endoplasmic reticulum (ER) and Golgi complex
The control of translocation of other organelles is not well documented in oocytes. In sheep oocytes, large aggregates of Golgi complexes and ER cisternae are associated with microtubules (Crozet 1988). In Xenopus egg cytosol, membrane fractions, including Golgi stacks and rough ER, construct a membrane network, and this involves the extension of membrane tubules along microtubules by the action of microtubule-based motor proteins. Indeed, a close relationship was observed between microtubules and ER redistribution during Drosophila oogenesis (Bobinnec et al. 2003). However, it has been reported that redistribution of ER in the cytoplasm to the nuclear area is dependent on microfilaments in starfish eggs (Terasaki 1994). Spir, an actin organizer and essential regulator of Drosophila oocyte polarity, is targeted to intracellular membrane structures, providing a novel link between actin organization and intracellular transport (Kerkhoff et al. 2001). Recently, it has been shown that translocations of the cortical ER network to the poles of the zygote depend on both microfilament-driven cortical contractions and sperm aster-microtubule-driven translocations (Sardet et al. 2003, Prodon et al. 2005). The regulation of ER and Golgi complex migration during oocyte development and fertilization is not known in mammalian eggs.
Centrosomes
Centrosomes, which govern the organization of microtubules, undergo cell-cycle-dependent changes in compaction/decompaction and separation that are synchronized with DNA cell cycles (Schatten et al. 2000). In sea urchins, the sperm centrosome is introduced into the egg during fertilization. Sperm centrosome expansion and separation typically occur concurrently in the fertilized egg, and centrosomes condense into the two spindle poles at prometaphase. Microtubule inhibitors prevent centrosome expansion and separation, while microfilament inhibition by cytochalasin D prevents centrosome separation, but not expansion or compaction (Schatten et al. 1988). During mouse oocyte maturation, microfilament depolymerization by cytochalasin B did not affect central migration of microtubule organizing centers (MTOCs) devoid of centrioles (Calarco 2005). Thus, microfilaments regulate centrosome separation, but not centrosome movement, in the ooplasm.
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Actin filaments participate in oocyte cortex formation |
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TopAbstractIntroductionActin filaments are not...Regulation of organelle movement...Actin filaments participate in...Actin filaments are involved...The roles of actin...Actin filaments are involved...Regulation of sperm...Actin filaments control the...Regulation of pronuclear...Actin filaments are required...Oocyte polarity versus embryo...Concluding remarksAcknowledgementsReferences |
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The mouse oocyte cortex possesses numerous actin filaments that emanate from the plasma membrane, forming a uniform layer of F-actin (Fig. 1A
). The membrane anchorage sites of actin filaments are marked by electron dense material on the inner leaflet of the plasma membrane. The free ends of filaments emanating from the plasma membrane of oocytes intermesh to form a dense, cortical layer. With meiotic maturation, the distribution of cortical actin is changed (Longo 1987). Relatively dense uniform layers of F-actin are also observed in porcine, bovine and human oocyte cortices (Kim et al. 1998, 2000, Pickering et al. 1998, Sun et al. 2001c). The cortices of a number of mammalian eggs are not structurally homogeneous but are polarized to form cell polarity (Longo 1985). Little is known about the molecules interacting with actin filaments in the mammalian egg cortex or proteins that connect actin to the plasma membrane. Recently, immunostaining of Rho proteins showed that Rac1 and RhoB are present in the cortical ooplasm, but Cdc42 is absent in the mouse (Kumakiri et al. 2003).
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Actin filaments are involved in oocyte polarity establishment |
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TopAbstractIntroductionActin filaments are not...Regulation of organelle movement...Actin filaments participate in...Actin filaments are involved...The roles of actin...Actin filaments are involved...Regulation of sperm...Actin filaments control the...Regulation of pronuclear...Actin filaments are required...Oocyte polarity versus embryo...Concluding remarksAcknowledgementsReferences |
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Longo and Chen (1985) first described the polarity of mouse oocytes. They found that the cortex of eggs is not structurally homogeneous but is polarized. When induced to undergo maturation, the actin cytoskeleton undergoes rearrangement to bring about polarization, which is required for asymmetric division. The meiotic spindles form in the center of immature oocytes and then move peripherally. Coincident with the cortical localization of the meiotic spindle is the formation of an area devoid of microvilli, that is, there are loss of microvilli and thickening of the actin layer associated with this region of the egg cortex. The MII mouse oocyte is transformed into a highly polarized egg, characterized by an actin cap and cortical granule-free domain (CGFD) overlying the meiotic spindle that is in close proximity to the cortex (Deng et al. 2003) (Fig. 1A
), and ER clusters restricted to the cortex of the vegetal hemisphere (FitzHarris et al. 2003). Spindle movements are often related to interactions between the cell cortex and the spindle asters. The spindles of mammalian oocytes are, however, typically devoid of astral microtubules, which normally connect the spindle to the cortex. In mouse oocytes, the spindle migrates along its long axis, but the choice of its direction of migration is the consequence of a slightly off-center positioning of the GV rather than the consequence of a predefined cortical site that could influence its migration. In mos–/– mouse oocytes, the spindle forms centrally but does not migrate. In these oocytes, a compensation mechanism exists: the spindle elongates during anaphase, and the pole closest to the cortex moves while the other remains in place. Thus, polarity and asymmetric division are established either after migration of the spindle to the cortex in wild-type oocytes, or after elongation, without migration, of the first meiotic spindle in mos–/– oocytes (Verlhac et al. 2000).
In pig oocytes, as in mouse oocytes, a cortical granule-free domain (CGFD) (not as evident as in mouse oocytes) and an actin-thickening area were observed over the MII spindle of a mature oocyte, and actin filaments retain the chromatin at the proper position at the oocyte cortex (Kim et al. 1996a, Sun et al. 2001c). In MII-stage horse oocytes, the bulk of the microfilaments remain within the oocyte cortex, and an actin-rich plaque was found to overlie the spindle (Tremoleda et al. 2001). Oocyte polarity was also observed in bovine and human oocytes (Kim et al. 1998, 2000). Carroll et al.(2004) found that the mammalian unfertilized egg is polarized, with the meiotic spindle located in the cortex of the animal pole and clusters of endoplasmic reticulum in the vegetal hemisphere. Functionally, this cortical polarity may be related to the restriction of sperm–egg interaction and fusion. Sperm does not penetrate the oocytes in the microvilli-free area of the animal pole. Polarity is also involved in the dynamic changes of the egg cortex during fertilization, including polar body formation and fertilization cone development.
We are now beginning to understand the polarization mechanisms. The chromosomes, which are located beneath the cortex, play an important role in the formation of the cortical actin domain. If metaphase II chromosomes are artificially dispersed, by nocodazole treatment, each group of chromosomes will induce the formation of a domain rich in microfilaments and poor in microvilli in mouse oocytes (Verlhac et al. 2000). The CGFD and actin-rich domains are also observed over chromosome masses of nocodazole-treated pig oocytes (Sun et al. 2001c). It is now well known that the presence of chromosomes, but not the integrated meiotic spindle, initiates this cortical reorganization. Cortical polarization is altered after spindle disassembly by colcemid: the scattered meiotic chromosomes initiate myosin IIA, microfilaments assemble in the vicinity of each chromosome mass (Simerly et al. 1998), and the chromosome mass induces a focal accumulation of subcortical actin in mouse oocytes (Maro et al. 1986). Microinjected sperm chromatin in the cortical region also induces polarity formation, and this is blocked by inhibitors of microfilament polymerization or disassembly. Active mitogen-activated protein kinase (MAPK), which becomes enriched in the region of chromatin, is required for cortical reorganization, as was shown by microinjecting sperm chromatin that failed to induce cortical reorganization in Mos–/– eggs, which lack MAPK activity. Myosin light-chain kinase (MLCK), which can be directly phosphorylated and activated by MAPK, also appears to be involved, because the MLCK inhibitors ML-7 and peptide 18 prevent sperm chromatin-induced cortical reorganization (Deng et al. 2005).
The positioning of meiotic spindles is known to involve actin filaments. The evolutionarily partitioning defective (PAR) proteins (microtubule regulatory proteins) have been demonstrated to play an important role in cell polarity in many cell types. Recent studies investigated the role of PAR proteins in establishing mouse oocyte polarity. PAR proteins are found on meiotic spindles before the MII stage, accumulate at the cortex of the animal pole during spindle migration, and are concentrated within a subdomain of the polarized actin cap (Vinot et al. 2004, Duncan et al. 2005, Moore & Zernicka-Goetz 2005) (Fig. 1). After mouse oocyte germinal vesicle breakdown, PAR-1 is localized on meiotic spindles in mouse oocytes (Moore & Zernicka-Goetz 2005), and PAR-3 surrounds the condensing chromosomes and associates with the meiotic spindles. Prior to emission of the first and second polar bodies, PAR-3 is located within a central subdomain of the polarized actin cap, which overlies the spindle. This cortical PAR-3 localization depends on intact microfilaments. These results suggest a role for PAR-3 in establishing polarity in the egg and in defining the future site of polar body emission (Duncan et al. 2005). Two PAR-6-related proteins have distinct polarized distributions in mouse oocytes. mPARD6a is first localized on the spindle and then accumulates at the pole nearest the cortex during spindle migration. In the absence of microtubules, the chromosomes still migrate to the cortex, and mPARD6a is found in association with the chromosomes, facing the cortex. The other protein, mPARD6b, is found on spindle microtubules until entry into MII and is relocalized to the cortex at the animal pole during metaphase II arrest (Vinot et al. 2004). All these results suggest that PAR proteins are involved in establishment of mouse oocyte polarity and asymmetric meiotic division.
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The roles of actin filaments in cortical granule movement, anchoring and exocytosis |
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TopAbstractIntroductionActin filaments are not...Regulation of organelle movement...Actin filaments participate in...Actin filaments are involved...The roles of actin...Actin filaments are involved...Regulation of sperm...Actin filaments control the...Regulation of pronuclear...Actin filaments are required...Oocyte polarity versus embryo...Concluding remarksAcknowledgementsReferences |
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CG migration
In sea urchin oocytes, CG translocation requires association with microfilaments, but not microtubules. Shortly after GVBD, CGs attach to microfilaments and translocate to the cell surface. Maturation-promoting factor (MPF) activation stimulates vesicle association with microfilaments, and is a key regulatory step in the coordinated translocation of CGs to the egg cortex (Wessel et al. 2002). Rho association with the CGs is a critical regulatory step in their translocation to the egg cortex. Inhibition of Rho blocks CG translocation, and microfilaments undergo a significant disorganization. In Xenopus, cortical flow of CGs is directed toward areas of localized contraction of the cortical F-actin cytoskeleton, but is suppressed by microtubules (Benink et al. 2000).
In mouse oocytes, the peripheral migration of CGs to the periphery is driven by microfilaments (Fig. 1A), and this process is blocked by treatment of oocytes with cytochalasin D, but not with nocodazole or colchicines (Connors et al. 1998). The same mechanism of CG migration also exists in pig oocytes. During maturation of pig oocytes, cortical granules and microfilaments become localized to the cell cortex (Sun et al. 2001c), and actin filaments drive CG movement to the cortex (Kim et al. 1996b, Sun et al. 2001a).
CG anchorage
Once the CGs are localized beneath the oolemma, their anchorage to the cortex is independent of either microfilaments or microtubules; they remain at the cortex after treatment of metaphase II-arrested eggs with either microfilament inhibitors or microtubule inhibitors in mouse and pig oocytes; that is, there is neither inward movement nor precocious exocytosis (Connors et al. 1998, Sun et al. 2001a). In addition, it has been shown that the anchorage of CGs at the cortex is a Rho-independent process in mouse oocytes (Covian-Nares et al. 2004).
CG exocytosis
Sperm–egg fusion induces increased intracellular free calcium concentration ([Ca2+]i) and exocytosis of CGs. Ca2+ is released from intracellular stores and activates PKC, leading to CG exocytosis (Sun 2003). New studies suggest that CG release in mature eggs is dependent on calcium-dependent proteins like those in somatic cells required to undergo calcium-regulated exocytosis (Abbott & Ducibella 2001), and calmodulin-dependent kinase II (CaMKII) may act as a switch in the transduction of the calcium signal (Sun 2003). The role of actin filaments in the exocytosis of cortical granules is unclear. In Xenopus, the actin cytoskeleton maintains exocytosing cortical granules as discrete invaginated compartments, such that when actin is disrupted, they collapse into the plasma membrane. Invaginated, exocytosing cortical granule compartments are directly retrieved from the plasma membrane by F-actin coats that assemble at their surface. These dynamic F-actin coats appear to drive closure of the exocytic fusion pores and ultimately compress the cortical granule compartments (Sokac et al. 2003). In mouse, the microfilament inhibitor cytochalasin B blocked sperm-induced exocytosis (Tahara et al. 1996), and JAS was also found to prevent cortical granule exocytosis after artificial activation (Terada et al. 2000), suggesting that actin filaments participate in CG exocytosis. In contrast, neither microfilaments nor microtubules are involved in CG exocytosis during pig oocyte activation (Sun et al. 2001a). Furthermore, the actin cytoskeleton in the cortex is thought to be a physical barrier to CG exocytosis. A decrease in F-actin was detected upon fertilization and upon parthenogenetic activation of rat eggs. Exposing the eggs to drugs that cause either polymerization or depolymerization of actin (JAS and cytochalasin D respectively) did not prevent CG exocytosis. In addition, cytochalasin D increased the percentage of eggs undergoing complete CG exocytosis induced by PKC activator (Eliyahu et al. 2005). In support of this hypothesis, the initial cortical release of Ca2+ promoted by sperm may be due to depolymerization of actin in starfish (Lim et al. 2002). The involvement of actin filaments in CG movement, anchorage and exocytosis in other mammalian species and the related mechanisms need clarification.
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Actin filaments are involved in polar body emission |
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TopAbstractIntroductionActin filaments are not...Regulation of organelle movement...Actin filaments participate in...Actin filaments are involved...The roles of actin...Actin filaments are involved...Regulation of sperm...Actin filaments control the...Regulation of pronuclear...Actin filaments are required...Oocyte polarity versus embryo...Concluding remarksAcknowledgementsReferences |
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). Human oocytes in late telophase showed the presence of a concentrated ring of actin in the cleavage furrow between the oocyte and the second polar body (Pickering et al. 1998). The RhoA kinase (ROCK) specific inhibitor Y-27632 and microinjection of RhoA antibody caused abnormal microfilament organization and blocked first and second polar body extrusion in mouse oocytes (Zhong et al. 2005). When actin filaments are disrupted, polar body emission is blocked in mouse (Maro et al. 1984), hamster (Terada et al. 1995) and sheep (Le Guen et al. 1989). During sperm incorporation, both myosin II isotypes concentrate in the second polar body cleavage furrow of mouse eggs (Simerly et al. 1998), and this may indicate its role in polar body emission through interaction with actin. P-MARCKS (myristoylated alanine-rich C-kinase substrate) is a major substrate for PKC. It is enriched in the periphery of the actin cap overlying the MI or MII spindle to form a ring-shaped subdomain. Because phosphorylation of MARCKS modulates its actin cross-linking function, this localization suggests that p-MARCKS functions as part of the contractile apparatus during polar body emission. p-MARCKS phosphorylation may be regulated by an atypical isoform of PKC (Michaut et al. 2005).
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Regulation of sperm incorporation by microfilaments |
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TopAbstractIntroductionActin filaments are not...Regulation of organelle movement...Actin filaments participate in...Actin filaments are involved...The roles of actin...Actin filaments are involved...Regulation of sperm...Actin filaments control the...Regulation of pronuclear...Actin filaments are required...Oocyte polarity versus embryo...Concluding remarksAcknowledgementsReferences |
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The need of microfilaments for sperm incorporation is somewhat inconclusive in mammals. In the mouse, it was reported that the sperm head is incorporated in the presence of the microfilament inhibitor latrunculin A, indicating an absence of microfilament activity at this stage (Schatten & Schatten 1986), while a recent report showed that Rho protein(s) regulating actin-based cytoskeletal reorganization is involved in the events leading to sperm incorporation (Kumakiri et al. 2003). Another report indicated that the microfilament modulator JAS inhibited sperm incorporation into mouse eggs (Terada et al. 2000). Recently, it was shown that the actin microfilament-disrupting drugs cytochalasin B, JAS and latrunculin B resulted in a decrease in the percentage of eggs fertilized and average number of sperm fused per mouse egg; however, the same group found that treatment with another microfilament inhibitor, cytochalasin D, resulted in an increase in the average number of sperm fused per egg and percentage of polyspermic eggs (McAvey et al. 2002). Preincubation of hamster eggs with cytochalasin D and washing prior to addition of spermatozoa had no effect on penetration of guinea pig and human sperm (Rogers et al. 1989). In pig and cattle, sperm incorporation is mediated by microfilaments, as cytochalasin B treatment inhibited fertilization (Sutovsky et al. 1996, Sun et al. 2001a), while cytochalasin D does not prevent sperm fusion and incorporation in sheep (Le Guen et al. 1989). The difference in results may be explained by the different microfilament disruptors used to treat the eggs before insemination or different manipulating procedures. It will be important to clarify this basic biologic question in future studies.
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Actin filaments control the meiotic spindle rotation after egg activation |
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TopAbstractIntroductionActin filaments are not...Regulation of organelle movement...Actin filaments participate in...Actin filaments are involved...The roles of actin...Actin filaments are involved...Regulation of sperm...Actin filaments control the...Regulation of pronuclear...Actin filaments are required...Oocyte polarity versus embryo...Concluding remarksAcknowledgementsReferences |
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). The rotation of the meiotic spindle is controlled by microfilaments. When oocytes are treated with cytochalasin B or D, spindle rotation is inhibited. Consequently, the oocyte forms an extra female pronucleus in the ooplasm instead of extruding a second polar body (Zhu et al. 2003). By using Pol-Scope (Cambridge Research & Instrumentation, Boston, MA, USA), Navarro et al.(2005) also observed the inhibition of spindle rotation by cytochalasin D in mouse oocytes. Actin filaments are also required for meiotic spindle rotation in Xenopus oocytes. Bipolar spindles are observed in cytochalasin B-treated oocytes; however, rotation of the MI and MII spindles into an orientation orthogonal to the oocyte surface is inhibited by cytochalasin B (Gard et al. 1995). When the mouse oocyte metaphase spindle is dispersed with nocodazole, small clusters of chromosomes are redistributed around the egg cortex in a microfilament-dependent process. Each cluster has the capacity, after removal from nocodazole, to organize a spindle that rotates to yield a polar body (Maro et al. 1986). A small GTPase, RhoA, which regulates microfilament organization, also regulates spindle rotation (Zhong et al. 2005). Unlike at first meiosis, inhibition of actin filaments immediately after egg activation by cytochalsin B or D or inhibition of RhoA does not affect sister chromatid separation, and two pronuclei form; however, second polar body emission is blocked in mouse and pig oocytes (Landa & Hajkova 1990, Boediono et al. 1995, Zhu et al. 2003, Zhong et al. 2005). The intriguing finding that microfilaments regulate homologous chromosome segregation, but not sister chromatid separation, needs clarification. In mitotic cells that undergo 90° spindle rotation, there is increasing evidence that close cooperation between cortical filamentous actin and astral microtubules is indispensable for successful spindle rotation. In recent years, the dynactin complex has emerged as the key agent to mediate actin/microtubule interactions at the cortex (Schaerer-Brodbeck & Riezman 2000). The mechanisms controlling meiotic spindle rotation in mouse oocytes require further investigation.
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Regulation of pronuclear apposition by cytoskeletal components |
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TopAbstractIntroductionActin filaments are not...Regulation of organelle movement...Actin filaments participate in...Actin filaments are involved...The roles of actin...Actin filaments are involved...Regulation of sperm...Actin filaments control the...Regulation of pronuclear...Actin filaments are required...Oocyte polarity versus embryo...Concluding remarksAcknowledgementsReferences |
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Mouse fertilization is a special case in which centrosomes are maternally inherited. Microtubules are organized by numerous egg cytoplasmic sites, and microtubule activity is required during pronuclear apposition in the mouse egg (Schatten & Schatten 1981, 1986). Pronuclear apposition also requires microfilaments in fertilized mouse oocytes (Schatten & Schatten 1981, Maro et al. 1984, Terada et al. 2000). Thus, both microtubules and microfilaments are required for pronuclear apposition in the mouse.
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Actin filaments are required for contractile ring formation during cleavage |
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TopAbstractIntroductionActin filaments are not...Regulation of organelle movement...Actin filaments participate in...Actin filaments are involved...The roles of actin...Actin filaments are involved...Regulation of sperm...Actin filaments control the...Regulation of pronuclear...Actin filaments are required...Oocyte polarity versus embryo...Concluding remarksAcknowledgementsReferences |
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During cytokinesis in the mouse, the cleavage furrow staining of actin is more intensive than the rest of the egg cortex, and microfilament inhibitors block cleavage (Schatten et al. 1986, Schatten & Schatten 1986). Myosin IIA isoform accumulates in the mitotic cleavage furrow (Simerly et al. 1998), and RhoA plays essential roles in the formation of the actin filaments and the cleavage furrow (Zhong et al. 2005). A microfilament-rich cleavage furrow was also observed in fertilized pig eggs, in which actin filaments are required for cleavage (Kim et al. 1997). Two-cell, bovine embryos were arrested when exposed to heat shock that caused disruption of microtubules and microfilaments (Rivera et al. 2004).
The role of actin filaments in the control of various dynamic events during oocyte meiotic maturation and fertilization in several representative animals is summarized in Tables 1 and 2.
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Oocyte polarity versus embryo polarity and embryo development in different species |
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TopAbstractIntroductionActin filaments are not...Regulation of organelle movement...Actin filaments participate in...Actin filaments are involved...The roles of actin...Actin filaments are involved...Regulation of sperm...Actin filaments control the...Regulation of pronuclear...Actin filaments are required...Oocyte polarity versus embryo...Concluding remarksAcknowledgementsReferences |
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Two mechanisms are employed to establish cell polarity: one is the cytoskeleton-oriented polar transport and the other is the capture and concentration of polar cell markers by the actin-rich cortex (Baum 2002). The establishment of oocyte polarity, especially the anchoring of GV and spindle as well as the anchoring of ‘determinants’, depends on microfilaments. The Drosophila oocyte is a highly polarized cell that contains a large number of localized mRNAs and proteins. The precocious localizations of these ‘determinants’ before maturation and fertilization determine the future antero-posterior and dorsoventral polarities of the embryo (Sardet et al. 2002). During oogenesis, microtubules direct the transport of osk mRNA to the end of the oocyte, and actin filaments play a secondary role in osk mRNA anchorage, which ensures oocyte polarity and defines the future posterior pole of the embryo (for review, see Baum 2002, Albertini & Barrett 2004). PAR proteins have been shown to be crucial for oocyte determination and polarization in Drosophila (Wodarz 2002).
The division of the Xenopus oocyte cortex into structurally and functionally distinct animal pole and vegetal pole during oogenesis involves interactions between different cytoskeletal filament systems. As mentioned above, microtubules are involved in the translocation of the message to the vegetal hemisphere and microfilaments are important in anchoring the vegetal message at the cortex (Yisraeli et al. 1989). The network of microfilaments in the animal cortex is thicker and more contractile than that in the vegetal cortex (Sardet et al. 2002). The primary A/V oocyte polarity created by anchoring mRNAs in the vegetal cortex is the basis of early embryo organization (Alarcon & Elinson 2001). These specific maternal mRNAs accumulated in the vegetal cortex specify the development of the endoderm and mesoderm, as well as the dorsal-anterior development ‘determinants’ and germ plasm. The animal cortex becomes specialized for the events associated with sperm entry, cortical granule exocytosis, and polarized cortical contraction during fertilization (for review, see Chang et al. 1999). Unlike in Drosophila, fertilization will bring about the redistribution of the vegetal cortex ‘determinants’ (the remodeling of the A/V axis) to yield the embryonic axes in X. laevis (Chang et al. 1999, Sardet et al. 2004).
Mammalian oocytes also have distinct A/V polarity, with the meiotic spindle located in the cortex of the animal pole and clusters of ER in the vegetable pole (Carroll et al. 2004). Recent evidence supports the notion that mammals retain axis-orienting mechanisms that contribute to oocyte polarity. Axis specification in relation to the oocyte cortex, eccentric positioning of GV, anchoring of GV and spindle, and patterning of follicle cell/oocyte attachment are proposed as conserved mammalian oogenesis features that may be important to the survival and development of preimplantation embryos (for review, see Albertini & Barrett 2004). However, unlike in lower animals, it is commonly accepted that there are no mosaically distributed developmental determinants in mammalian zygotes and early embryos (Louvet-Vallee et al. 2005). Whether oocyte polarity determines cleavage pattern is still controversial. Although there is evidence that the site of the second meiotic division and the sperm entry point may affect the first cleavage pattern (Piotrowska & Zernicka-Goetz 2001, Plusa et al. 2002), and that specification of embryonic axes begins before cleavage in mouse development (Gardner 2001), recent video microscopy showed that the cleavage planes are oriented randomly in two-cell mouse embryos (Louvet-Vallee et al. 2005).
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Concluding remarks |
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TopAbstractIntroductionActin filaments are not...Regulation of organelle movement...Actin filaments participate in...Actin filaments are involved...The roles of actin...Actin filaments are involved...Regulation of sperm...Actin filaments control the...Regulation of pronuclear...Actin filaments are required...Oocyte polarity versus embryo...Concluding remarksAcknowledgementsReferences |
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Acknowledgements |
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TopAbstractIntroductionActin filaments are not...Regulation of organelle movement...Actin filaments participate in...Actin filaments are involved...The roles of actin...Actin filaments are involved...Regulation of sperm...Actin filaments control the...Regulation of pronuclear...Actin filaments are required...Oocyte polarity versus embryo...Concluding remarksAcknowledgementsReferences |
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Footnotes |
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References |
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TopAbstractIntroductionActin filaments are not...Regulation of organelle movement...Actin filaments participate in...Actin filaments are involved...The roles of actin...Actin filaments are involved...Regulation of sperm...Actin filaments control the...Regulation of pronuclear...Actin filaments are required...Oocyte polarity versus embryo...Concluding remarksAcknowledgementsReferences |
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