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During mitosis, cells undergo symmetrical cell division, while oocyte meiotic maturation undergoes two consecutive, asymmetric divisions that generate a totipotent haploid oocyte and two small polar bodies not involved in DNA replication. This specialized division allows most maternal components to be maintained in the oocytes for early embryo development. Nuclear positioning, germinal vesicle breakdown, spindle migration, spindle rotation, chromosome segregation, and polar body extrusion are the most critical cellular processes during oocyte meiosis I and II, and a growing number of studies primarily using the mouse oocyte model revealed that actin filaments were critical for these processes, especially for spindle migration. Several important molecules have been reported to be involved in these processes. One family of molecules are the small GTPases, such as Rho GTPases, Ran GTPases, and Rab GTPases and another are the actin nucleators, such as the formin family and the Arp2/3 complex. The present review summarizes recent progress made regarding the roles of actin filaments in the asymmetric oocyte division. Summary Sentence Actin filaments widely involve into multiple cellular processes such as nuclear positioning, germinal vesicle breakdown, spindle migration, chromosome segregation, spindle rotation and polar body extrusion in oocyte mammalian meiosis.

Paxillin is a multi-domain scaffold protein that is involved in several cellular functions, including focal adhesion, cell motility and cell aging. Previous studies have shown that Paxillin affects the resumption of meiosis in Xenopus oocytes. In the present study, we report the distinct roles of Paxillin in mammalian oocytes including mice and pigs. Our results indicate that Paxillin is expressed in mouse oocytes and is primarily concentrated in the cortical area, which is closely associated with actin. Disruption of Paxillin expression or function did not influence the process of germinal vesicle breakdown in either mouse or porcine oocytes at GV stage, as confirmed by the expression levels of Mos and MPF. Furthermore, Paxillin disruption did not alter spindle morphology but did affect meiotic spindle migration and polar body extrusion, likely due to a decrease in actin filaments. Further analysis revealed that the actin nucleators Arp2/3 and N-WASP were reduced in Paxillin-knockdown oocytes at MI stage. Overexpression of Paxillin also did not impact GV breakdown, spindle formation, or polar body extrusion. However, the supplementation of exogenous Paxillin mRNA rescued the defects in spindle positioning and polar body extrusion caused by Paxillin knockdown. Taken together, our findings suggest that Paxillin plays a critical role in spindle migration during oocyte meiosis by mediating actin dynamics through the Arp2/3 complex, highlighting functional differences between species.

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.

In contrast to symmetric division in mitosis, mammalian oocyte maturation is characterized by asymmetric cell division that produces a large egg and a small polar body. The asymmetry results from oocyte polarization, which includes spindle positioning, migration, and cortical reorganization, and this process is critical for fertilization and the retention of maternal components for early embryo development. Although actin dynamics are involved in this process, the molecular mechanism underlying this remained unclear until the use of confocal microscopy and live cell imaging became widespread in recent years. Information obtained through a PubMed database search of all articles published in English between 2000 and 2012 that included the phrases “oocyte, actin, spindle migration,” “oocyte, actin, polar body,” or “oocyte, actin, asymmetric division” was reviewed. The actin nucleation factor actin-related protein 2/3 complex and its nucleation-promoting factors, formins and Spire, and regulators such as small GTPases, partitioning-defective/protein kinase C, Fyn, microRNAs, cis-Golgi apparatus components, myosin/myosin light-chain kinase, spindle stability regulators, and spindle assembly checkpoint regulators, play critical roles in asymmetric cell division in oocytes. This review summarizes recent findings on these actin-related regulators in mammalian oocyte asymmetric division and outlines a complete signaling pathway.

WASP homolog associated with actin, membranes and microtubules (WHAMM) is a newly discovered nucleation-promoting factor that links actin and microtubule cytoskeleton and regulates transport from the endoplasmic reticulum to the Golgi apparatus. However, knowledge of WHAMM is limited to interphase somatic cells. In this study, we examined its localization and function in mouse oocytes during meiosis. Immunostaining showed that in the germinal vesicle (GV) stage, there was no WHAMM signal; after meiosis resumption, WHAMM was associated with the spindle at prometaphase I (Pro MI), metaphase I (MI), telophase I (TI) and metaphase II (MII) stages. Nocodazole and taxol treatments showed that WHAMM was localized around the MI spindle. Depletion of WHAMM by microinjection of specific short interfering (si)RNA into the oocyte cytoplasm resulted in failure of spindle migration, disruption of asymmetric cytokinesis and a decrease in the first polar body extrusion rate during meiotic maturation. Moreover, actin cap formation was also disrupted after WHAMM depletion, confirming the failure of spindle migration. Taken together, our data suggest that WHAMM is required for peripheral spindle migration and asymmetric cytokinesis during mouse oocyte maturation.

Background Dimethyl sulfoxide (DMSO) is used extensively as a permeable cryoprotectant and is a common solvent utilized for several water-insoluble substances. DMSO has various biological and pharmacological activities; however, the effect of DMSO on mouse oocyte meiotic maturation remains unknown. Results In DMSO-treated oocytes, we observed abnormal MII oocytes that contained large polar bodies, including 2-cell–like MII oocytes, during in vitro maturation. Oocyte polarization did not occur, due to the absence of actin cap formation and spindle migration. These features are among the primary causes of abnormal symmetric division; however, analysis of the mRNA expression levels of genes related to asymmetric division revealed no significant difference in the expression of these factors between the 3% DMSO-treated group and the control group. After each “blastomere” of the 2-cell–like MII stage oocytes was injected by one sperm head respectively, the oocytes still possessed the ability to extrude the second polar body from each “blastomere” and to begin cleavage. However, MII oocytes with large polar bodies developed to the blastocyst stage after intracytoplasmic sperm injection (ICSI). Furthermore, other permeable cryoprotectants, such as ethylene glycol and glycerol, also caused asymmetric division failure. Conclusion Permeable cryoprotectants, such as DMSO, ethylene glycol, and glycerol, affect asymmetric division. DMSO disrupts cytokinesis completion by inhibiting cortical reorganization and polarization. Oocytes that undergo symmetric division maintain the ability to begin cleavage after ICSI.

Spindle movement, including spindle migration during first meiosis and spindle rotation during second meiosis, is essential for asymmetric divisions in mouse oocytes. Previous studies by others and us have shown that microfilaments are required for both spindle migration and rotation. In the present study, we aimed to further investigate the mechanism controlling spindle movement during mouse oocyte meiosis. By employing drug treatment and immunofluorescence microscopy, we showed that dynamic microtubule assembly was involved in both spindle migration and rotation. Furthermore, we found that the calcium/CaM/CaMKII pathway was important for regulating spindle rotation.

Spindle orientation (SO) plays a critical role in tissue morphogenesis, homeostasis, and tumorigenesis by ensuring accurate division plane positioning in response to intrinsic and extrinsic cues. While SO has been extensively linked to cell shape sensing and cortical forces, the interplay between shape- and force-sensing mechanisms remains poorly understood. Here, we reveal that SO is governed by two parallel mechanisms that ensure redundancy and adaptability in diverse cellular environments. Using live-cell imaging of cultured cells, we demonstrate that the long prometaphase axis (LPA) is a superior predictor of SO compared to the long interphase axis, reflecting adhesive geometry and force distribution efficiently at prometaphase. Importantly, we uncover a pivotal role for focal adhesion kinase (FAK) in mediating cortical mechanosensing to regulate SO in cells undergoing complete metaphase rounding. We show that in cells with complete metaphase rounding, FAK-dependent force sensing aligns the spindle with the major force vector, ensuring accurate division. Conversely, in cells retaining shape anisotropy during mitosis, a FAK-independent shape-sensing mechanism drives SO. These findings highlight a dual regulatory system for SO, where shape sensing and force sensing operate in parallel to maintain division plane fidelity, shedding light on the mechanisms that enable cells to adapt to diverse physical and mechanical environments.

The non-receptor tyrosine kinase Src plays a key role in cell division, migration, adhesion, and survival. Src is overactivated in several cancers, where it transmits signals that promote cell survival, mitosis, and other important cancer hallmarks. Src is therefore a promising target in cancer therapy, but the underlying mechanisms are still uncertain. Here we show that Src is highly conserved across different species. Src expression increases during mitosis and is localized to the chromosomal passenger complex. Knockdown or inhibition of Src induces multipolar spindle formation, resulting in abnormal expression of the Aurora B and INCENP components of the chromosomal passenger complex. Molecular mechanism studies have found that Src interacts with and phosphorylates INCENP. This then leads to incorrect chromosome arrangement and segregation, resulting in cell division failure. Herein, Src and chromosomal passenger complex co-localize and Src inhibition impedes mitotic progression by inducing multipolar spindle formation. These findings provide novel insights into the molecular basis for using Src inhibitors to treat cancer.

The orientation of mitotic spindles is tightly regulated in polarized cells, but it has been unclear whether there is a mechanism regulating spindle orientation in nonpolarized cells. Here we show that integrin‐dependent cell adhesion to the substrate orients the mitotic spindle of nonpolarized cultured cells parallel to the substrate plane. The spindle is properly oriented in cells plated on fibronectin or collagen, but misoriented in cells on poly‐ L ‐lysine or treated with the RGD peptide or anti‐β1‐integrin antibody, indicating requirement of integrin‐mediated cell adhesion for this mechanism. Remarkably, this mechanism is independent of gravitation or cell–cell adhesion, but requires actin cytoskeleton and astral microtubules. Furthermore, myosin X and the microtubule plus‐end‐tracking protein EB1 are shown to play a role in this mechanism through remodeling of actin cytoskeleton and stabilization of astral microtubules, respectively. Our results thus uncover the existence of a mechanism that orients the spindle parallel to the cell–substrate adhesion plane, and identify crucial factors involved in this novel mechanism.