Meiotic regulation of TPX2 protein levels governs cell cycle progression in mouse oocytes

CtIP regulates G2/M transition and bipolar spindle assembly during mouse oocyte meiosis

CtBP-interacting protein (CtIP) is known for its multifaceted roles in DNA repair and genomic stability, directing the homologous recombination-mediated DNA double-stranded break (DSB) repair pathway via DNA end resection, an essential error-free repair process vital for genome stability. Mammalian oocytes are highly prone to DNA damage accumulation due to prolonged G2/prophase arrest. Here, we explore the functions of CtIP in meiotic cell cycle regulation via a mouse oocyte model. Depletion of CtIP by siRNA injection results in delayed germinal vesicle breakdown and failed polar body extrusion. Mechanistically, CtIP deficiency increases DNA damage and decreases the expression and nuclear entry of CCNB1, resulting in marked impairment of meiotic resumption, which can be rescued by exogenous CCNB1 overexpression. Furthermore, depletion of CtIP disrupts MTOCs coalescence at spindle poles as indicated by failed accumulation of γ-tubulin, p-Aurora kinase A, Kif2A, and TPX2, leading to abnormal spindle assembly and prometaphase arrest. These results provide valuable insights into the important roles of CtIP in the G2/M checkpoint and spindle assembly in mouse oocyte meiotic cell cycle regulation.

Spatio-temporal requirements of Aurora kinase A in mouse oocyte meiotic spindle building

Meiotic spindles are critical to ensure chromosome segregation during gamete formation. Oocytes lack centrosomes and use alternative microtubule-nucleation mechanisms for spindle building. How these mechanisms are regulated is still unknown. Aurora kinase A (AURKA) is essential for mouse oocyte meiosis because in pro-metaphase I it triggers microtubule organizing-center fragmentation and its expression compensates for the loss of the two other Aurora kinases (AURKB/AURKC). Although knockout mouse models were useful for foundational studies, AURK spatial and temporal functions are not yet resolved. We provide high-resolution analyses of AURKA/AURKC requirements during meiotic spindle-building and identify the subcellular populations that carry out these functions: 1) AURKA is required in early spindle assembly and later for spindle stability, whereas 2) AURKC is required in late pro-metaphase, and 3) Targeted AURKA constructs expressed in triple AURK knockout oocytes reveal that spindle pole-localized AURKA is the most important population controlling spindle building and stability mechanisms.

Centrosome age breaks spindle size symmetry even in cells thought to divide symmetrically

Centrosomes are the main microtubule-organizing centers in animal cells. Due to the semiconservative nature of centrosome duplication, the two centrosomes differ in age. In asymmetric stem cell divisions, centrosome age can induce an asymmetry in half-spindle lengths. However, whether centrosome age affects the symmetry of the two half-spindles in tissue culture cells thought to divide symmetrically is unknown. Here, we show that in human epithelial and fibroblastic cell lines centrosome age imposes a mild spindle asymmetry that leads to asymmetric cell daughter sizes. At the mechanistic level, we show that this asymmetry depends on a cenexin-bound pool of the mitotic kinase Plk1, which favors the preferential accumulation on old centrosomes of the microtubule nucleation-organizing proteins pericentrin, γ-tubulin, and Cdk5Rap2, and microtubule regulators TPX2 and ch-TOG. Consistently, we find that old centrosomes have a higher microtubule nucleation capacity. We postulate that centrosome age breaks spindle size symmetry via microtubule nucleation even in cells thought to divide symmetrically.

Karyopherin α2 is a maternal effect gene required for early embryonic development and female fertility in mice

The nuclear transport of proteins plays an important role in mediating the transition from egg to embryo and distinct karyopherins have been implicated in this process. Here, we studied the impact of KPNA2 deficiency on preimplantation embryo development in mice. Loss of KPNA2 results in complete arrest at the 2cell stage and embryos exhibit the inability to activate their embryonic genome as well as a severely disturbed nuclear translocation of Nucleoplasmin 2. Our findings define KPNA2 as a new maternal effect gene.

Male meiotic spindle poles are stabilized by TACC3 and cKAP5/chTOG differently from female meiotic or somatic mitotic spindles in mice

Transforming acidic acid coiled-coil protein 3 (TACC3) and cytoskeleton associated protein 5 (cKAP5; or colonic hepatic tumor overexpressed gene, chTOG) are vital for spindle assembly and stabilization initiated through TACC3 Aurora-A kinase interaction. Here, TACC3 and cKAP5/chTOG localization with monospecific antibodies is investigated in eGFP-centrin-2- expressing mouse meiotic spermatocytes. Both proteins bind spermatocyte spindle poles but neither kinetochore nor interpolar microtubules, unlike in mitotic mouse fibroblasts or female meiotic oocyte spindles. Spermatocytes do not display a liquid-like spindle domain (LISD), although fusing them into maturing oocytes generates LISD-like TACC3 condensates around sperm chromatin but sparse microtubule assembly. Microtubule inhibitors do not reduce TACC3 and cKAP5/chTOG spindle pole binding. MLN 8237 Aurora-A kinase inhibitor removes TACC3, not cKAP5/chTOG, disrupting spindle organization, chromosome alignment, and impacting spindle pole γ-tubulin intensity. The LISD disruptor 1,6-hexanediol abolished TACC3 in spermatocytes, impacting spindle bipolarity and chromosome organization. Cold microtubule disassembly and rescue experiments in the presence of 1,6-hexanediol reinforce the concept that spermatocyte TACC3 spindle pole presence is not required for spindle pole microtubule assembly. Collectively, meiotic spermatocytes without a LISD localize TACC3 and cKAP5/chTOG exclusively at spindle poles to support meiotic spindle pole stabilization during male meiosis, different from either female meiosis or mitosis.

Ran-GTP assembles a specialized spindle structure for accurate chromosome segregation in medaka early embryos

Despite drastic cellular changes during cleavage, a mitotic spindle assembles in each blastomere to accurately segregate duplicated chromosomes. Mechanisms of mitotic spindle assembly have been extensively studied using small somatic cells. However, mechanisms of spindle assembly in large vertebrate embryos remain little understood. Here, we establish functional assay systems in medaka (Oryzias latipes) embryos by combining CRISPR knock-in with auxin-inducible degron technology. Live imaging reveals several unexpected features of microtubule organization and centrosome positioning that achieve rapid, accurate cleavage. Importantly, Ran-GTP assembles a dense microtubule network at the metaphase spindle center that is essential for chromosome segregation in early embryos. This unique spindle structure is remodeled into a typical short, somatic-like spindle after blastula stages, when Ran-GTP becomes dispensable for chromosome segregation. We propose that despite the presence of centrosomes, the chromosome-derived Ran-GTP pathway has essential roles in functional spindle assembly in large, rapidly dividing vertebrate early embryos, similar to acentrosomal spindle assembly in oocytes.

LSM14B is an Oocyte-Specific RNA-Binding Protein Indispensable for Maternal mRNA Metabolism and Oocyte Development in Mice

Mammalian oogenesis features reliance on the mRNAs produced and stored during early growth phase. These are essential for producing an oocyte competent to undergo meiotic maturation and embryogenesis later when oocytes are transcriptionally silent. The fate of maternal mRNAs hence ensures the success of oogenesis and the quality of the resulting eggs. Nevertheless, how the fate of maternal mRNAs is determined remains largely elusive. RNA-binding proteins (RBPs) are crucial regulators of oogenesis, yet the identity of the full complement of RBPs expressed in oocytes is unknown. Here, a global view of oocyte-expressed RBPs is presented: mRNA-interactome capture identifies 1396 RBPs in mouse oocytes. An analysis of one of these RBPs, LSM family member 14 (LSM14B), demonstrates that this RBP is specific to oocytes and associated with many networks essential for oogenesis. Deletion of Lsm14b results in female-specific infertility and a phenotype characterized by oocytes incompetent to complete meiosis and early embryogenesis. LSM14B serves as an interaction hub for proteins and mRNAs throughout oocyte development and regulates translation of a subset of its bound mRNAs. Therefore, RNP complexes tethered by LSM14B are found exclusively in oocytes and are essential for the control of maternal mRNA fate and oocyte development.

Cobalt chloride exposure disturbs spindle assembly and decreases mouse oocyte development potential

Cobalt is a kind of heavy metal which is widely used in petrochemical and biomedical industries. Animal studies have reported that cobalt would exert systemic toxicity, but its effects on the ovarian function in mammals, especially for oocyte quality remains unknown. In the present study, we report that cobalt chloride treatment affects ovary coefficient and follicular growth. Oocytes in cobalt chloride exposed mice exhibited a decreased development potential, with the evidence of decreased occurrence rate of germ vesicle breakdown and polar body extrusion. Besides, cobalt chloride disorganized meiotic spindle formation and movement, mechanically associated with affecting TACC3 and Ac-a-tubulin levels, and disturbing actin reorganization. In addition, cobalt chloride exposure result in mitochondrial cristae structures disappear, cluster distribution and potential depolarization. Altogether, these findings suggest that cobalt chloride impairs the ovarian follicle growth and affects oocyte development by disrupted spindle assembly and mitochondrial function.

Identification of GNG7 as a novel biomarker and potential therapeutic target for gastric cancer via bioinformatic analysis and in vitro experiments

Gastric cancer (GC) is one of the most common malignancies with unfavorable prognoses. The present study aimed to identify novel biomarkers or potential therapeutic targets in GC via bioinformatic analysis and in vitro experiments. The Gene Expression Omnibus and The Cancer Genome Atlas databases were used to screen the differentially expressed genes (DEGs). After protein-protein interaction network construction, both module and prognostic analyses were performed to identify prognosis-related genes in GC. The expression patterns and functions of G protein γ subunit 7 (GNG7) in GC were then visualized in multiple databases and further verified using in vitro experiments. A total of 897 overlapping DEGs were detected and 20 hub genes were identified via systematic analysis. After accessing the prognostic value of the hub genes using the online server Kaplan-Meier plotter, a six-gene prognostic signature was identified, which was also significantly correlated with the process of immune infiltration in GC. The results of open-access database analyses suggested that GNG7 is downregulated in GC; this downregulation was associated with tumor progression. Furthermore, the functional enrichment analysis unveiled that the GNG7-coexpressed genes or gene sets were closely correlated with the proliferation and cell cycle processes of GC cells. Finally, in vitro experiments further confirmed that GNG7 overexpression inhibited GC cell proliferation, colony formation, and cell cycle progression and induced apoptosis. As a tumor suppressor gene, GNG7 suppressed the growth of GC cells via cell cycle blockade and apoptosis induction and thus may be used as a potential biomarker and therapeutic target for GC.

Aneuploidy in mammalian oocytes and the impact of maternal ageing

During fertilization, the egg and the sperm are supposed to contribute precisely one copy of each chromosome to the embryo. However, human eggs frequently contain an incorrect number of chromosomes - a condition termed aneuploidy, which is much more prevalent in eggs than in either sperm or in most somatic cells. In turn, aneuploidy in eggs is a leading cause of infertility, miscarriage and congenital syndromes. Aneuploidy arises as a consequence of aberrant meiosis during egg development from its progenitor cell, the oocyte. In human oocytes, chromosomes often segregate incorrectly. Chromosome segregation errors increase in women from their mid-thirties, leading to even higher levels of aneuploidy in eggs from women of advanced maternal age, ultimately causing age-related infertility. Here, we cover the two main areas that contribute to aneuploidy: (1) factors that influence the fidelity of chromosome segregation in eggs of women from all ages and (2) factors that change in response to reproductive ageing. Recent discoveries reveal new error-causing pathways and present a framework for therapeutic strategies to extend the span of female fertility.

Phosphorylation of adducin-1 by TPX2 promotes interpolar microtubule homeostasis and precise chromosome segregation in mouse oocytes

BACKGROUND

ADD1 (adducin-1) and TPX2 (targeting protein for Xklp2) are centrosomal proteins and regulate mitotic spindle assembly. Mammalian oocytes that segregate homologous chromosomes in Meiosis I and sister chromatids in Meiosis II with a spindle lacking centrosomes are more prone to chromosome segregation errors than in mitosis. However, the regulatory mechanisms of oocyte spindle assembly and the functions of ADD1 and TPX2 in this process remain elusive.

RESULT

We found that the expression levels and localization of ADD1, S726 phosphorylated ADD1 (p-ADD1), and TPX2 proteins exhibited spindle assembly-dependent dynamic changes during mouse oocyte meiosis. Taxol treatment, which stabilizes the microtubule polymer and protects it from disassembly, made the signals of ADD1, p-ADD1, and TPX2 present in the microtubule organizing centers of small asters and spindles. Knockdown of approximately 60% of ADD1 protein levels destabilized interpolar microtubules in the meiotic spindle, resulting in aberrant chromosome alignment, reduced first polar body extrusion, and increased aneuploidy in metaphase II oocytes, but did not affect K-fiber homeostasis and the expression and localization of TPX2. Strikingly, TPX2 deficiency caused increased protein content of ADD1, but decreased expression and detachment of p-ADD1 from the spindle, thereby arresting mouse oocytes at the metaphase I stage with collapsed spindles.

CONCLUSION

Phosphorylation of ADD1 at S726 by TPX2 mediates acentriolar spindle assembly and precise chromosome segregation in mouse oocytes.

SphK-produced S1P in somatic cells is indispensable for LH-EGFR signaling-induced mouse oocyte maturation

Germ cell division and differentiation require intimate contact and interaction with the surrounding somatic cells. Luteinizing hormone (LH) triggers epidermal growth factor (EGF)-like growth factors to promote oocyte maturation and developmental competence by activating EGF receptor (EGFR) in somatic cells. Here, we showed that LH-EGFR signaling-activated sphingosine kinases (SphK) in somatic cells. The activation of EGFR by EGF increased S1P and calcium levels in cumulus-oocyte complexes (COCs), and decreased the binding affinity of natriuretic peptide receptor 2 (NPR2) for natriuretic peptide type C (NPPC) to release the cGMP-mediated meiotic arrest. These functions of EGF were blocked by the SphK inhibitor SKI-II, which could be reversed by the addition of S1P. S1P also activated the Akt/mTOR cascade reaction in oocytes and promoted targeting protein for Xklp2 (TPX2) accumulation and oocyte developmental competence. Specifically depleting Sphk1/2 in somatic cells reduced S1P levels and impaired oocyte meiotic maturation and developmental competence, resulting in complete female infertility. Collectively, SphK-produced S1P in somatic cells serves as a functional transmitter of LH-EGFR signaling from somatic cells to oocytes: acting on somatic cells to induce oocyte meiotic maturation, and acting on oocytes to improve oocyte developmental competence.

Mechanism of spindle pole organization and instability in human oocytes

Human oocytes are prone to assembling meiotic spindles with unstable poles, which can favor aneuploidy in human eggs. The underlying causes of spindle instability are unknown. We found that NUMA (nuclear mitotic apparatus protein)-mediated clustering of microtubule minus ends focused the spindle poles in human, bovine, and porcine oocytes and in mouse oocytes depleted of acentriolar microtubule-organizing centers (aMTOCs). However, unlike human oocytes, bovine, porcine, and aMTOC-free mouse oocytes have stable spindles. We identified the molecular motor KIFC1 (kinesin superfamily protein C1) as a spindle-stabilizing protein that is deficient in human oocytes. Depletion of KIFC1 recapitulated spindle instability in bovine and aMTOC-free mouse oocytes, and the introduction of exogenous KIFC1 rescued spindle instability in human oocytes. Thus, the deficiency of KIFC1 contributes to spindle instability in human oocytes.

Osteoarthritis Affects Mammalian Oogenesis: Effects of Collagenase-Induced Osteoarthritis on Oocyte Cytoskeleton in a Mouse Model

Known as a degenerative joint disorder of advanced age affecting predominantly females, osteoarthritis can develop in younger and actively working people because of activities involving loading and injuries of joints. Collagenase-induced osteoarthritis (CIOA) in a mouse model allowed us to investigate for the first time its effects on key cytoskeletal structures (meiotic spindles and actin distribution) of ovulated mouse oocytes. Their meiotic spindles, actin caps, and chromatin were analyzed by immunofluorescence. A total of 193 oocytes from mice with CIOA and 209 from control animals were obtained, almost all in metaphase I (M I) or metaphase II (MII). The maturation rate was lower in CIOA (26.42% M II) than in controls (55.50% M II). CIOA oocytes had significantly larger spindles (average 37 μm versus 25 μm in controls, p < 0.001), with a proportion of large spindles more than 64% in CIOA versus up to 15% in controls (p < 0.001). Meiotic spindles were wider in 68.35% M I and 54.90% M II of CIOA oocytes (mean 18.04 μm M I and 17.34 μm M II versus controls: 11.64 μm M I and 12.64 μm M II), and their poles were approximately two times broader (mean 6.9 μm) in CIOA than in controls (3.6 μm). CIOA oocytes often contained disoriented microtubules. Actin cap was visible in over 91% of controls and less than 20% of CIOA oocytes. Many CIOA oocytes without an actin cap had a nonpolarized thick peripheral actin ring (61.87% of M I and 52.94% of M II). Chromosome alignment was normal in more than 82% in both groups. In conclusion, CIOA affects the cytoskeleton of ovulated mouse oocytes—meiotic spindles are longer and wider, their poles are broader and with disorganized fibers, and the actin cap is replaced by a broad nonpolarized ring. Nevertheless, meiotic spindles were successfully formed in CIOA oocytes and, even when abnormal, allowed correct alignment of chromosomes.

Acentriolar spindle assembly in mammalian female meiosis and the consequences of its perturbations on human reproduction

The purpose of meiosis is to generate developmentally competent, haploid gametes with the correct number of chromosomes. For reasons not completely understood, female meiosis is more prone to chromosome segregation errors than meiosis in males, leading to an abnormal number of chromosomes, or aneuploidy, in gametes. Meiotic spindles are the cellular machinery essential for the proper segregation of chromosomes. One unique feature of spindle structures in female meiosis is spindles poles that lack centrioles. The process of building a meiotic spindle without centrioles is complex and requires precise coordination of different structural components, assembly factors, motor proteins, and signaling molecules at specific times and locations to regulate each step. In this review, we discuss the basics of spindle formation during oocyte meiotic maturation focusing on mouse and human studies. Finally, we review different factors that could alter the process of spindle formation and its stability. We conclude with a discussion of how different assisted reproductive technologies (ART) could affect spindles and the consequences these perturbations may have for subsequent embryo development.

Molecular insight into how γ-TuRC makes microtubules

As one of four filament types, microtubules are a core component of the cytoskeleton and are essential for cell function. Yet how microtubules are nucleated from their building blocks, the αβ-tubulin heterodimer, has remained a fundamental open question since the discovery of tubulin 50 years ago. Recent structural studies have shed light on how γ-tubulin and the γ-tubulin complex proteins (GCPs) GCP2 to GCP6 form the γ-tubulin ring complex (γ-TuRC). In parallel, functional and single-molecule studies have informed on how the γ-TuRC nucleates microtubules in real time, how this process is regulated in the cell and how it compares to other modes of nucleation. Another recent surprise has been the identification of a second essential nucleation factor, which turns out to be the well-characterized microtubule polymerase XMAP215 (also known as CKAP5, a homolog of chTOG, Stu2 and Alp14). This discovery helps to explain why the observed nucleation activity of the γ-TuRC in vitro is relatively low. Taken together, research in recent years has afforded important insight into how microtubules are made in the cell and provides a basis for an exciting era in the cytoskeleton field.

Loss of acentriolar MTOCs disrupts spindle pole Aurora A and assembly of the liquid-like meiotic spindle domain in oocytes

Oocyte-specific knockdown of pericentrin (PCNT) in transgenic (Tg) mice disrupts acentriolar microtubule-organizing center (aMTOC) formation, leading to spindle instability and error-prone meiotic division. Here, we show that PCNT-depleted oocytes lack phosphorylated Aurora A (pAURKA) at spindle poles, while overall levels are unaltered. To test aMTOC-associated AURKA function, metaphase II (MII) control (WT) and Tg oocytes were briefly exposed to a specific AURKA inhibitor (MLN8237). Similar defects were observed in Tg and MLN8237-treated WT oocytes, including altered spindle structure, increased chromosome misalignment and impaired microtubule regrowth. Yet, AURKA inhibition had a limited effect on Tg oocytes, revealing a critical role for aMTOC-associated AURKA in regulating spindle stability. Notably, spindle instability was associated with disrupted γ-tubulin and lack of the liquid-like meiotic spindle domain (LISD) in Tg oocytes. Analysis of this Tg model provides the first evidence that LISD assembly depends expressly on aMTOC-associated AURKA, and that Ran-mediated spindle formation ensues without the LISD. These data support that loss of aMTOC-associated AURKA and failure of LISD assembly contribute to error-prone meiotic division in PCNT-depleted oocytes, underscoring the essential role of aMTOCs for spindle stability.

Echinoderm Microtubule Associated Protein Like 1 Is Indispensable for Oocyte Spindle Assembly and Meiotic Progression in Mice

Completion of the first meiosis is an essential prerequisite for producing a functionally normal egg for fertilization and embryogenesis, but the precise mechanisms governing oocyte meiotic progression remains largely unclear. Here, we report that echinoderm microtubule associated protein (EMAP) like 1 (EML1), a member of the conserved EMAP family proteins, plays a crucial role in the control of oocyte meiotic progression in the mouse. Female mice carrying an ENU-induced nonsense mutation (c.1956T > A; p.Tyr652∗) of Eml1 are infertile, and the majority of their ovulated oocytes contain abnormal spindles and misaligned chromosomes. In accordance with the mutant oocyte phenotype, we find that EML1 is colocalized with spindle microtubules during the process of normal oocyte meiotic maturation, and knockdown (KD) of EML1 by specific morpholinos in the fully grown oocytes (FGOs) disrupts the integrity of spindles, and delays meiotic progression. Moreover, EML1-KD oocytes fail to progress to metaphase II (MII) stage after extrusion of the first polar body, but enter into interphase and form a pronucleus containing decondensed chromatins. Further analysis shows that EML1-KD impairs the recruitment of γ-tubulin and pericentrin to the spindle poles, as well as the attachment of kinetochores to microtubules and the proper inactivation of spindle assembly checkpoint at metaphase I (MI). The loss of EML1 also compromises the activation of maturation promoting factor around the time of oocyte resumption and completion of the first meiosis, which, when corrected by WEE1/2 inhibitor PD166285, efficiently rescues the phenotype of oocyte delay of meiotic resumption and inability of reaching MII. Through IP- mass spectrometry analysis, we identified that EML1 interacts with nuclear distribution gene C (NUDC), a critical mitotic regulator in somatic cells, and EML1-KD disrupts the specific localization of NUDC at oocyte spindles. Taken together, these data suggest that EML1 regulates acentrosomal spindle formation and the progression of meiosis to MII in mammalian oocytes, which is likely mediated by distinct mechanisms.

Mechanisms underlying disruption of oocyte spindle stability by bisphenol compounds

Accurate chromosome segregation relies on correct chromosome-microtubule interactions within a stable bipolar spindle apparatus. Thus, exposure to spindle disrupting compounds can impair meiotic division and genomic stability in oocytes. The endocrine disrupting activity of bisphenols such as bisphenol A (BPA) is well recognized, yet their damaging effects on spindle microtubules (MTs) is poorly understood. Here, we tested the effect(s) of acute exposure to BPA and bisphenol F (BPF) on assembled spindle stability in ovulated oocytes. Brief (4 h) exposure to increasing concentrations (5, 25, and 50 µg/mL) of BPA or BPF disrupted spindle organization in a dose-dependent manner, resulting in significantly shorter spindles with highly unfocused poles and fragmented pericentrin. The chromosomes remained congressed in an abnormally elongated metaphase-like configuration, yet normal end-on chromosome-MT attachments were reduced in BPF-treated oocytes. Live-cell imaging revealed a rapid onset of bisphenol-mediated spindle MT disruption that was reversed upon compound removal. Moreover, MT stability and regrowth were impaired in BPA-exposed oocytes, with few cold-stable MTs and formation of multipolar spindles upon MT regrowth. MT-associated kinesin-14 motor protein (HSET/KIFC1) labeling along the spindle was also lower in BPA-treated oocytes. Conversely, cold stable MTs and HSET labeling persisted after BPF exposure. Notably, inhibition of Aurora Kinase A limited bisphenol-mediated spindle pole widening, revealing a potential interaction. These results demonstrate rapid MT disrupting activity by bisphenols, which is highly detrimental to meiotic spindle stability and organization. Moreover, we identify an important link between these defects and altered distribution of key spindle associated factors as well as Aurora Kinase A activity.

Aurora kinase A is essential for meiosis in mouse oocytes

The Aurora protein kinases are well-established regulators of spindle building and chromosome segregation in mitotic and meiotic cells. In mouse oocytes, there is significant Aurora kinase A (AURKA) compensatory abilities when the other Aurora kinase homologs are deleted. Whether the other homologs, AURKB or AURKC can compensate for loss of AURKA is not known. Using a conditional mouse oocyte knockout model, we demonstrate that this compensation is not reciprocal because female oocyte-specific knockout mice are sterile, and their oocytes fail to complete meiosis I. In determining AURKA-specific functions, we demonstrate that its first meiotic requirement is to activate Polo-like kinase 1 at acentriolar microtubule organizing centers (aMTOCs; meiotic spindle poles). This activation induces fragmentation of the aMTOCs, a step essential for building a bipolar spindle. We also show that AURKA is required for regulating localization of TACC3, another protein required for spindle building. We conclude that AURKA has multiple functions essential to completing MI that are distinct from AURKB and AURKC.

Gene Activation as a Cell Protection Mechanism Against Gamma-Ray radiation

Introduction: Gamma radiation is accompanied by prominent biological effects and damages. Cell proliferation and tumorigenesis are highlighted as the main resulted effects of gamma radiation on cultured cells. This study aims to assess the dysregulated mode of gene function after gamma radiation in human Jurkat cells. Methods: Six gene expression profiles from Gene Expression Omnibus (GEO) were analyzed by GEO2R to find the significant differentially expressed genes (DEGs) via gamma radiation. Action map analysis was applied to screen the query DEGs. Results: Among 108 study genes, 20 critical DEGs including AURKA, AURKB, BORA, CCNB1, CCNB2, CCNF, CDC20, CDCA8, CENPA, CENPE, CENPF, KIF18A, KIF20A, KIF23, BUB1, DLGAP5, ECT2, PLK1, SGO2, and TPX2 were introduced as down-regulated genes by the gamma ray. Conclusion: Activators of the introduced critical genes may be the cell protector against gamma radiation.

The CRL4-DCAF13 ubiquitin E3 ligase supports oocyte meiotic resumption by targeting PTEN degradation

Cullin ring-finger ubiquitin ligase 4 (CRL4) has multiple functions in the maintenance of oocyte survival and meiotic cell cycle progression. DCAF13, a novel CRL4 adaptor, is essential for oocyte development. But the mechanisms by which CRL4-DCAF13 supports meiotic maturation remained unclear. In this study, we demonstrated that DCAF13 stimulates the meiotic resumption-coupled activation of protein synthesis in oocytes, partially by maintaining the activity of PI3K signaling pathway. CRL4-DCAF13 targets the polyubiquitination and degradation of PTEN, a lipid phosphatase that inhibits PI3K pathway as well as oocyte growth and maturation. Dcaf13 knockout in oocytes caused decreased CDK1 activity and impaired meiotic cell cycle progression and chromosome condensation defects. As a result, chromosomes fail to be aligned at the spindle equatorial plate, the spindle assembly checkpoint is activated, and most Dcaf13 null oocytes are arrested at the prometaphase I. The DCAF13-dependent PTEN degradation mechanism fits in as a missing link between CRL4 ubiquitin E3 ligase and PI3K pathway, both of which are crucial for translational activation during oocyte GV-MII transition.

Prc1-rich kinetochores are required for error-free acentrosomal spindle bipolarization during meiosis I in mouse oocytes

Acentrosomal meiosis in oocytes represents a gametogenic challenge, requiring spindle bipolarization without predefined bipolar cues. While much is known about the structures that promote acentrosomal microtubule nucleation, less is known about the structures that mediate spindle bipolarization in mammalian oocytes. Here, we show that in mouse oocytes, kinetochores are required for spindle bipolarization in meiosis I. This process is promoted by oocyte-specific, microtubule-independent enrichment of the antiparallel microtubule crosslinker Prc1 at kinetochores via the Ndc80 complex. In contrast, in meiosis II, cytoplasm that contains upregulated factors including Prc1 supports kinetochore-independent pathways for spindle bipolarization. The kinetochore-dependent mode of spindle bipolarization is required for meiosis I to prevent chromosome segregation errors. Human oocytes, where spindle bipolarization is reportedly error prone, exhibit no detectable kinetochore enrichment of Prc1. This study reveals an oocyte-specific function of kinetochores in acentrosomal spindle bipolarization in mice, and provides insights into the error-prone nature of human oocytes.

Oocyte meiosis must achieve spindle bipolarization without predefined spatial cues. Yoshida et al. demonstrate that spindle bipolarization during meiosis I in mouse oocytes requires kinetochores to prevent chromosome segregation errors, a phenomenon that does not occur in error-prone human oocytes.

A liquid-like spindle domain promotes acentrosomal spindle assembly in mammalian oocytes

Mammalian oocytes segregate chromosomes with a microtubule spindle that lacks centrosomes, but the mechanisms by which acentrosomal spindles are organized and function are largely unclear. In this study, we identify a conserved subcellular structure in mammalian oocytes that forms by phase separation. This structure, which we term the liquid-like meiotic spindle domain (LISD), permeates the spindle poles and forms dynamic protrusions that extend well beyond the spindle. The LISD selectively concentrates multiple microtubule regulatory factors and allows them to diffuse rapidly within the spindle volume. Disruption of the LISD via different means disperses these factors and leads to severe spindle assembly defects. Our data suggest a model whereby the LISD promotes meiotic spindle assembly by serving as a reservoir that sequesters and mobilizes microtubule regulatory factors in proximity to spindle microtubules.

Targeting centrosome amplification, an Achilles' heel of cancer

Due to cell-cycle dysregulation, many cancer cells contain more than the normal compliment of centrosomes, a state referred to as centrosome amplification (CA). CA can drive oncogenic phenotypes and indeed can cause cancer in flies and mammals. However, cells have to actively manage CA, often by centrosome clustering, in order to divide. Thus, CA is also an Achilles' Heel of cancer cells. In recent years, there have been many important studies identifying proteins required for the management of CA and it has been demonstrated that disruption of some of these proteins can cause cancer-specific inhibition of cell growth. For certain targets therapeutically relevant interventions are being investigated, for example, small molecule inhibitors, although none are yet in clinical trials. As the field is now poised to move towards clinically relevant interventions, it is opportune to summarise the key work in targeting CA thus far, with particular emphasis on recent developments where small molecule or other strategies have been proposed. We also highlight the relatively unexplored paradigm of reversing CA, and thus its oncogenic effects, for therapeutic gain.

Phase separation of TPX2 enhances and spatially coordinates microtubule nucleation

Phase separation of substrates and effectors is proposed to enhance biological reaction rates and efficiency. Targeting protein for Xklp2 (TPX2) is an effector of branching microtubule nucleation in spindles and functions with the substrate tubulin by an unknown mechanism. Here we show that TPX2 phase separates into a co-condensate with tubulin, which mediates microtubule nucleation in vitro and in isolated cytosol. TPX2-tubulin co-condensation preferentially occurs on pre-existing microtubules, the site of branching microtubule nucleation, at the endogenous and physiologically relevant concentration of TPX2. Truncation and chimera versions of TPX2 suggest that TPX2-tubulin co-condensation enhances the efficiency of TPX2-mediated branching microtubule nucleation. Finally, the known inhibitor of TPX2, the importin-α/β heterodimer, regulates TPX2 condensation in vitro and, consequently, branching microtubule nucleation activity in isolated cytosol. Our study demonstrates how regulated phase separation can simultaneously enhance reaction efficiency and spatially coordinate microtubule nucleation, which may facilitate rapid and accurate spindle formation.

RanGTP and importin β regulate meiosis I spindle assembly and function in mouse oocytes

Homologous chromosome segregation during meiosis I (MI) in mammalian oocytes is carried out by the acentrosomal MI spindles. Whereas studies in human oocytes identified Ran GTPase as a crucial regulator of the MI spindle function, experiments in mouse oocytes questioned the generality of this notion. Here, we use live-cell imaging with fluorescent probes and Förster resonance energy transfer (FRET) biosensors to monitor the changes in Ran and importin β signaling induced by perturbations of Ran in mouse oocytes while examining the MI spindle dynamics. We show that unlike RanT24N employed in previous studies, a RanT24N, T42A double mutant inhibits RanGEF without perturbing cargo binding to importin β and disrupts MI spindle function in chromosome segregation. Roles of Ran and importin β in the coalescence of microtubule organizing centers (MTOCs) and MI spindle assembly are further supported by the use of the chemical inhibitor importazole, whose effects are partially rescued by the GTP hydrolysis-resistant RanQ69L mutant. These results indicate that RanGTP is essential for MI spindle assembly and function both in humans and mice.

The clinical significance and prognostic value of Xenopus kinesin-like protein 2 expressions in human tumors: A systematic review and meta-analysis

Targeting protein for Xenopus kinesin-like protein 2 (TPX2) is a microtubule-associated protein that plays a pivotal part in the formation of spindles. There is accumulating evidence that the expression of TPX2 is upregulated in many kinds of human cancers and that this protein is involved in the occurrence and progression of tumors. The purpose of this meta-analysis was to investigate the relationship between the overexpression of TPX2 and poor prognosis in cancer patients. A total of 18 eligible studies encompassing 3115 patients were included by searching relevant databases. Hazard ratios (HRs) or odds ratios (ORs) with 95% confidence intervals (CIs) were pooled under random-/fixed-effect models. After calculation, the results showed that patients with increased TPX2 expression had a significantly shorter overall survival (HR = 2.21; 95% CI: 1.70-2.86), and disease-free survival (HR = 2.10; 95% CI: 1.67-2.64). In addition, it was found that increased TPX2 expression was significantly associated with TNM stage (OR = 2.17; 95% CI:1.42-3.32), lymph node metastasis (OR = 2.98; 95% CI: 2.28-3.89), distant metastasis (OR = 2.25; 95% CI:1.03-4.92), and vascular invasion (OR = 2.22; 95% CI:1.26-3.91). Nevertheless, there was no significant correlation between increased expression of TPX2 and either gender, tumor differentiation, or tumor size. Thus, we can come to the conclusion that the overexpression of TPX2 is related to poor clinical outcomes and can be used as a biomarker for the prognosis of patients with cancer.

Meiotic spindle formation in mammalian oocytes: implications for human infertility

In the final stage of oogenesis, mammalian oocytes generate a meiotic spindle and undergo chromosome segregation to yield an egg that is ready for fertilization. Herein, we describe the recent advances in understanding the mechanisms controlling formation of the meiotic spindle in metaphase I (MI) and metaphase II (MII) in mammalian oocytes, and focus on the differences between mouse and human oocytes. Unlike mitotic cells, mammalian oocytes lack typical centrosomes that consist of two centrioles and the surrounding pericentriolar matrix proteins, which serve as microtubule-organizing centers (MTOCs) in most somatic cells. Instead, oocytes rely on different mechanisms for the formation of microtubules in MI spindles. Two different mechanisms have been described for MI spindle formation in mammalian oocytes. Chromosome-mediated microtubule formation, including RAN-mediated spindle formation and chromosomal passenger complex-mediated spindle elongation, controls the growth of microtubules from chromatin, while acentriolar MTOC-mediated microtubule formation contributes to spindle formation. Mouse oocytes utilize both chromatin- and MTOC-mediated pathways for microtubule formation. The existence of both pathways may provide a fail-safe mechanism to ensure high fidelity of chromosome segregation during meiosis. Unlike mouse oocytes, human oocytes considered unsuitable for clinical in vitro fertilization procedures, lack MTOCs; this may explain why meiosis in human oocytes is often error-prone. Understanding the mechanisms of MI/MII spindle formation, spindle assembly checkpoint, and chromosome segregation, in mammalian oocytes, will provide valuable insights into the molecular mechanisms of human infertility.

A computational model of the early stages of acentriolar meiotic spindle assembly

The mitotic spindle is an ensemble of microtubules responsible for the repartition of the chromosomal content between the two daughter cells during division. In metazoans, spindle assembly is a gradual process involving dynamic microtubules and recruitment of numerous associated proteins and motors. During mitosis, centrosomes organize and nucleate the majority of spindle microtubules. In contrast, oocytes lack canonical centrosomes but are still able to form bipolar spindles, starting from an initial ball that self-organizes in several hours. Interfering with early steps of meiotic spindle assembly can lead to erroneous chromosome segregation. Although not fully elucidated, this process is known to rely on antagonistic activities of plus end- and minus end-directed motors. We developed a model of early meiotic spindle assembly in mouse oocytes, including key factors such as microtubule dynamics and chromosome movement. We explored how the balance between plus end- and minus end-directed motors, as well as the influence of microtubule nucleation, impacts spindle morphology. In a refined model, we added spatial regulation of microtubule stability and minus-end clustering. We could reproduce the features of early stages of spindle assembly from 12 different experimental perturbations and predict eight additional perturbations. With its ability to characterize and predict chromosome individualization, this model can help deepen our understanding of spindle assembly.

Biomarker identification of thyroid associated ophthalmopathy using microarray data

AIM

To uncover the underlying pathogenesis of thyroid associated ophthalmopathy (TAO) and explore potential biomarkers of this disease.

METHODS

The expression profile GSE9340, which was downloaded from Gene Expression Omnibus database, included 18 specimens from 10 TAO patients and 8 hyperthyroidism patients without ophthalmopathy. The platform was HumanRef-8 v2 Expression BeadChip. Raw data were normalized using preprocess. Core package and the differentially expressed genes (DEGs) were identified based on t-test with limma package of R. Functional enrichment analyses were performed recruiting the DAVID tool. Based on STRING database, a protein-protein interaction (PPI) network was constructed, from which a module was extracted. The functional enrichment for genes in the module was performed by the BinGO plugin.

RESULTS

In total, 861 DEGs (433 up-regulated and 428 down-regulated) between TAO patients and hyperthyroidism patients without ophthalmopathy were identified. Crucial nodes in the PPI network included TPX2, CDCA5, PRC1, KIF23 and MKI67, which were also remarkable in the module and all enriched in cell cycle process. Additionally, MKI67 was highly correlated with TAO. Besides, the DEGs of GTF2F1, SMC3, USF1 and ZNF263 were predicted as transcription factors (TFs).

CONCLUSION

Several crucial genes are identified such as TPX2, CDCA5, PRC1 and KIF23, which all might play significant roles in TAO via the regulation of cell cycle process. Regulatory relationships between TPX2 and CDCA5 as well as between PRC1 and KIF23 may exist. Additionally, MKI67 may be a potent biomarker of TAO, and SMC3 and ZNF263 may exert their roles as TFs in TAO progression.

Loss and Rebirth of the Animal Microtubule Organizing Center: How Maternal Expression of Centrosomal Proteins Cooperates with the Sperm Centriole in Zygotic Centrosome Reformation

Centrosomes are the main microtubule organizing centers in animal cells. In particular during embryogenesis, they ensure faithful spindle formation and proper cell divisions. As metazoan centrosomes are eliminated during oogenesis, they have to be reassembled upon fertilization. Most metazoans use the sperm centrioles as templates for new centrosome biogenesis while the egg's cytoplasm re-prepares all components for on-going centrosome duplication in rapidly dividing embryonic cells. We discuss our knowledge and the experimental challenges to analyze zygotic centrosome reformation, which requires genetic experiments to enable scrutinizing respective male and female contributions. Male and female knockout animals and mRNA injection to mimic maternal expression of centrosomal proteins could point a way to the systematic molecular dissection of the process. The most recent data suggest that timely expression of centrosome components in oocytes is the key to zygotic centrosome reformation that uses male sperm as coordinators for de novo centrosome production.

Importance of ERK1/2 in Regulation of Protein Translation during Oocyte Meiosis

Although the involvement of the extracellular signal-regulated kinases 1 and 2 (ERK1/2) pathway in the regulation of cytostatic factor (CSF) activity; as well as in microtubules organization during meiotic maturation of oocytes; has already been described in detail; rather less attention has been paid to the role of ERK1/2 in the regulation of mRNA translation. However; important data on the role of ERK1/2 in translation during oocyte meiosis have been documented. This review focuses on recent findings regarding the regulation of translation and the role of ERK1/2 in this process in the meiotic cycle of mammalian oocytes. The specific role of ERK1/2 in the regulation of mammalian target of rapamycin (mTOR); eukaryotic translation initiation factor 4E (eIF4E) and cytoplasmic polyadenylation element binding protein 1 (CPEB1) activity is addressed along with additional focus on the other key players involved in protein translation.

Shifting meiotic to mitotic spindle assembly in oocytes disrupts chromosome alignment

Mitotic spindles assemble from two centrosomes, which are major microtubule-organizing centers (MTOCs) that contain centrioles. Meiotic spindles in oocytes, however, lack centrioles. In mouse oocytes, spindle microtubules are nucleated from multiple acentriolar MTOCs that are sorted and clustered prior to completion of spindle assembly in an "inside-out" mechanism, ending with establishment of the poles. We used HSET (kinesin-14) as a tool to shift meiotic spindle assembly toward a mitotic "outside-in" mode and analyzed the consequences on the fidelity of the division. We show that HSET levels must be tightly gated in meiosis I and that even slight overexpression of HSET forces spindle morphogenesis to become more mitotic-like: rapid spindle bipolarization and pole assembly coupled with focused poles. The unusual length of meiosis I is not sufficient to correct these early spindle morphogenesis defects, resulting in severe chromosome alignment abnormalities. Thus, the unique "inside-out" mechanism of meiotic spindle assembly is essential to prevent chromosomal misalignment and production of aneuploidy gametes.

Quantitative Microinjection of Morpholino Antisense Oligonucleotides into Mouse Oocytes to Examine Gene Function in Meiosis-I

Specific protein depletion is a powerful approach for assessing individual gene function in cellular processes, and has been extensively employed in recent years in mammalian oocyte meiosis-I. Conditional knockout mice and RNA interference (RNAi) methods such as siRNA or dsRNA microinjection are among several approaches to have been applied in this system over the past decade. RNAi by microinjection of Morpholino antisense Oligonucleotides (MO), in particular, has proven highly popular and tractable in many studies, since MOs have high specificity of interaction, low cell toxicity, and are more stable than other microinjected RNAi molecules. Here, we describe a method of MO microinjection into the mouse germinal vesicle-stage (GV) oocyte followed by a simple immunofluorescence approach for examination of gene function in meiosis-I.

Plk4 and Aurora A cooperate in the initiation of acentriolar spindle assembly in mammalian oocytes

Establishing the bipolar spindle in mammalian oocytes after their prolonged arrest is crucial for meiotic fidelity and subsequent development. In contrast to somatic cells, the first meiotic spindle assembles in the absence of centriole-containing centrosomes. Ran-GTP can promote microtubule nucleation near chromatin, but additional unidentified factors are postulated for the activity of multiple acentriolar microtubule organizing centers in the oocyte. We now demonstrate that partially overlapping, nonredundant functions of Aurora A and Plk4 kinases contribute to initiate acentriolar meiosis I spindle formation. Loss of microtubule nucleation after simultaneous chemical inhibition of both kinases can be significantly rescued by drug-resistant Aurora A alone. Drug-resistant Plk4 can enhance Aurora A-mediated rescue, and, accordingly, Plk4 can phosphorylate and potentiate the activity of Aurora A in vitro. Both kinases function distinctly from Ran, which amplifies microtubule growth. We conclude that Aurora A and Plk4 are rate-limiting factors contributing to microtubule growth as the acentriolar oocyte resumes meiosis.

Mitochondrial dysfunction in the gastrointestinal mucosa of children with autism: A blinded case-control study

Gastrointestinal (GI) symptoms are prevalent in autism spectrum disorder (ASD) but the pathophysiology is poorly understood. Imbalances in the enteric microbiome have been associated with ASD and can cause GI dysfunction potentially through disruption of mitochondrial function as microbiome metabolites modulate mitochondrial function and mitochondrial dysfunction is highly associated with GI symptoms. In this study, we compared mitochondrial function in rectal and cecum biopsies under the assumption that certain microbiome metabolites, such as butyrate and propionic acid, are more abundant in the cecum as compared to the rectum. Rectal and cecum mucosal biopsies were collected during elective diagnostic colonoscopy. Using a single-blind case-control design, complex I and IV and citrate synthase activities and complex I-V protein quantity from 10 children with ASD, 10 children with Crohn’s disease and 10 neurotypical children with nonspecific GI complaints were measured. The protein for all complexes, except complex II, in the cecum as compared to the rectum was significantly higher in ASD samples as compared to other groups. For both rectal and cecum biopsies, ASD samples demonstrated higher complex I activity, but not complex IV or citrate synthase activity, compared to other groups. Mitochondrial function in the gut mucosa from children with ASD was found to be significantly different than other groups who manifested similar GI symptomatology suggesting a unique pathophysiology for GI symptoms in children with ASD. Abnormalities localized to the cecum suggest a role for imbalances in the microbiome, potentially in the production of butyrate, in children with ASD.

The role of TACC3 in mitotic spindle organization

TACC3 regulates spindle organization during mitosis and also regulates centrosome-mediated microtubule nucleation by affecting γ-Tubulin ring complexes. In addition, it interacts with different proteins (such as ch-TOG, clathrin and Aurora-A) to function in mitotic spindle assembly and stability. By forming the TACC3/ch-TOG complex, TACC3 acts as a plus end-tracking protein to promote microtubule elongation. The TACC3/ch-TOG/clathrin complex is formed to stabilize kinetochore fibers by crosslinking adjacent microtubules. Furthermore, the phosphorylation of TACC3 by Aurora-A is important for the formation of TACC3/ch-TOG/clathrin and its recruitment to kinetochore fibers. Recently, the aberrant expression of TACC3 in a variety of human cancers has been linked with mitotic defects. Thus, in this review, we will discuss our current understanding of the biological roles of TACC3 in mitotic spindle organization.

Microtubule nucleation: beyond the template

Microtubules are cytoskeletal filaments central to a wide range of essential cellular functions in eukaryotic cells. Consequently, cells need to exert tight control over when, where and how many microtubules are being made. Whereas the regulation of microtubule dynamics is well studied, the molecular mechanisms of microtubule nucleation are still poorly understood. Next to the established master template of nucleation, the γ-tubulin ring complex, other microtubule-associated proteins that affect microtubule dynamic properties have recently been found to contribute to nucleation. It has begun to emerge that the nucleation efficiency is controlled not only by template activity but also by, either additionally or alternatively, the stabilization of the nascent microtubule 'nucleus'. This suggests a simple conceptual framework for the mechanisms regulating microtubule nucleation in cells.

Targeted TPX2 increases chromosome missegregation and suppresses tumor cell growth in human prostate cancer

Prostate cancer is a complex disease that can be relatively harmless or extremely aggressive. Although androgen-deprivation therapy is a commonly used treatment for men with prostate cancer, the adverse effects can be detrimental to patient health and quality of life. Therefore, identifying new target genes for tumor growth will enable the development of novel therapeutic intervention. TPX2 plays a critical role in chromosome segregation machinery during mitosis. Low rates of chromosome missegregation can promote tumor development, whereas higher levels might promote cell death and suppress tumorigenesis. Hence, the strategy of promoting cell death by inducing massive chromosome missegregation has been a therapeutic application for selectively eliminating highly proliferating tumor cells. RNAi was used for TPX2 protein expression knockdown, and a clonogenic assay, immunostaining, double thymidine block, image-cytometry analysis, and tumor spheroid assay were used to analyze the role of TPX2 in tumor cell growth, cell cycle progression, multinuclearity, ploidy, and tumorigenicity, respectively; finally, Western blotting was used to analyze anticancer mechanisms in TPX2 targeting. We demonstrated that targeting TPX2 reduced cell cycle regulators and chromosome segregation genes, resulting in increased cell micronucleation. Moreover, TPX2 depletion led to prostate cancer cell growth inhibition, increased apoptosis, and reduced tumorigenesis. These results confirmed the therapeutic potential of targeting TPX2 in prostate cancer treatment. Moreover, we found that TPX2 silencing led to deregulation of CDK1, cyclin B, securin, separase, and aurora A proteins; by contrast, p21 mRNA was upregulated. We also determined the molecular mechanisms for TPX2 targeting in prostate cancer cells. In conclusion, our study illustrates the power of TPX2 as a potential novel target gene for prostate cancer treatment.

Targeting TPX2 Suppresses the Tumorigenesis of Hepatocellular Carcinoma Cells Resulting in Arrested Mitotic Phase Progression and Increased Genomic Instability

Hepatocellular carcinoma (HCC) remains one of the most difficult cancers to treat, with chemotherapies being relatively ineffective. Therefore, a better knowledge of molecular hepatocarcinogenesis will provide opportunities for designing targeted therapies. TPX2 (targeting protein for Xklp2) is overexpressed as a consequence of oncogenic alterations and is likely to alter the proper regulation of chromosome segregation in cancer cells. Disrupting the machinery which is responsible for mitosis and chromosome instability in cancer cells can be one of the most successful strategies for cancer therapy. Therefore, we consider the targeting TPX2 could provide novel therapeutic strategies for cancer. In this study, increased TPX2 protein expression was present in 16 (42%) of 38 primary HCCs and was associated with advanced stage, distant metastatic HCCs and poor prognosis. Knockdown of TPX2 inhibited cancer cell growth and downregulation of cyclin A, cyclin E and CDK2 proteins. However, over-expressed EGFP-TPX2 protein enhanced the in vitro tumor spheroid formation and rescued the TPX2 depleted cell growth. Targeting TPX2 caused a rising impaired chromosomal instability resulting in multinuclearity, cell cycle progression arrest, apotosis, senescence and an increased polyploidy in cells. An image-cytometry analysis revealed cell cycle progression arrest after TPX2 inhibition. A correlation was observed between the downregulation of the protein levels of genes related to chromosomal segregation and spindle assembly checkpoint (securin, seprase, Aurora A, Aurora B, Cyclin B1, Cyclin B2, MPS1, BUB1, BUB3, MAD1 and MAD2) and increased cell ploidy, indicating mitotic progression failure and the loss of the balance of genomic instability. In vitro tumor spheroid assay and in vivo xenografts mouse model showed a therapeutic opportunity. Our findings indicate that targeting TPX2 lead to suppress tumorigenicity in liver cancer cells, suggesting that TPX2 is a potential target for anticancer therapy in HCC.

LIM kinase activity is required for microtubule organising centre positioning in mouse oocyte meiosis

LIM kinase 1 (LIMK1) activity is essential for cell migration and cell cycle progression. Little is known about LIMK1 expression and function in mammalian oocytes. In the present study we assessed LIMK1 protein expression, subcellular distribution and function during mouse oocyte meiosis. Western blot analysis revealed high and stable expression of LIMK1 from the germinal vesicle (GV) to MII stage. In contrast, activated LIMK1 (i.e. LIMK1 phosphorylated at threonine 508 (pLIMK1Thr508)) was only detected after GV breakdown, with levels increasing gradually to peak at MI and MII. Immunofluorescence showed pLIMK1Thr508 was colocalised with the microtubule organising centre (MTOC) components pericentrin and γ-tubulin at the spindle poles. A direct interaction between γ-tubulin and pLIMK1Thr508 was confirmed by co-immunoprecipitation. LIMK inhibition with 1 μM BMS3 damaged MTOC protein localisation to spindle poles, undermined the formation and positioning of functional MTOC and thus disrupted spindle formation and chromosome alignment. These effects were phenocopied by microinjection of LIMK1 antibody into mouse oocytes. In summary, the data demonstrate that LIMK activity is essential for MTOC organisation and distribution and so bipolar spindle formation and maintenance in mouse oocytes.

A MAPK cascade couples maternal mRNA translation and degradation to meiotic cell cycle progression in mouse oocytes

Mammalian oocyte maturation depends on the translational activation of stored maternal mRNAs upon meiotic resumption. Cytoplasmic polyadenylation element binding protein 1 (CPEB1) is a key oocyte factor that regulates maternal mRNA translation. However, the signal that triggers CPEB1 activation at the onset of mammalian oocyte maturation is not known. We provide evidence that a mitogen-activated protein kinase (MAPK) cascade couples maternal mRNA translation to meiotic cell cycle progression in mouse oocytes by triggering CPEB1 phosphorylation and degradation. Mutations of the phosphorylation sites or ubiquitin E3 ligase binding sites in CPEB1 have a dominant-negative effect in oocytes, and mimic the phenotype of ERK1/2 knockout, by impairing spindle assembly and mRNA translation. Overexpression of the CPEB1 downstream translation activator DAZL in ERK1/2-deficient oocytes partially rescued the meiotic defects, indicating that ERK1/2 is essential for spindle assembly, metaphase II arrest and maternal-zygotic transition (MZT) primarily by triggering the translation of key maternal mRNAs. Taken together, ERK1/2-mediated CPEB1 phosphorylation/degradation is a major mechanism of maternal mRNA translational activation, and is crucial for mouse oocyte maturation and MZT.

Meiotic spindle assembly and chromosome segregation in oocytes

Centrosomes play a key role in organizing the microtubule spindle that separates chromosomes during mitosis. Bennabi et al. review how microtubule spindle formation and chromosomal segregation also occur in oocytes during cell division by meiosis despite the absence of centrosomes.

Oocytes accumulate maternal stores (proteins, mRNAs, metabolites, etc.) during their growth in the ovary to support development after fertilization. To preserve this cytoplasmic maternal inheritance, they accomplish the difficult task of partitioning their cytoplasm unequally while dividing their chromosomes equally. Added to this complexity, most oocytes, for reasons still speculative, lack the major microtubule organizing centers that most cells use to assemble and position their spindles, namely canonical centrosomes. In this review, we will address recent work on the mechanisms of meiotic spindle assembly and chromosome alignment/segregation in female gametes to try to understand the origin of errors of oocyte meiotic divisions. The challenge of oocyte divisions appears indeed not trivial because in both mice and humans oocyte meiotic divisions are prone to chromosome segregation errors, a leading cause of frequent miscarriages and congenital defects.

Bcl2l10, a new Tpx2 binding partner, is a master regulator of Aurora kinase A in mouse oocytes

Previously, we demonstrated that Bcl-2-like 10 (Bcl2l10) is associated with meiotic spindle assembly and that the gene that is most strongly down-regulated by Bcl2l10 RNAi is targeting protein for Xklp2 (Tpx2). Tpx2 is a well-known cofactor that controls the activity and localization of Aurora kinase A (Aurka) during mitotic spindle assembly. Therefore, this study was conducted (1) to identify the associations among Bcl2l10, Tpx2, and Aurka and (2) to understand how Bcl2l10 regulates meiotic spindle assembly in mouse oocytes. Bcl2l10, Tpx2, and Aurka co-localized on the meiotic spindles, and Bcl2l10 was present in the same complex with Tpx2. Tpx2 and Aurka expression decreased whereas phospho-Aurka increased in Bcl2l10 RNAi-treated oocytes. Counterbalancing changes in the levels of these 2 activators, Tpx2 and phospho-Aurka, resulted in decreased Aurka catalytic activity after Bcl2l10 RNAi treatment. Bcl2l10 RNAi decreased the expression of microtubule organizing center (MTOC)-related proteins, disturbed MTOC formation and disrupted meiotic spindle assembly. Our data demonstrate that Bcl2l10 is a binding partner of Tpx2 and a new regulator of the complex controlling the organization of microtubules and MTOC biogenesis in meiotic spindle assembly. The discovery of Bcl2l10 as a new effector of Aurka suggests that Bcl2l10 may have diverse functions in mitotic cells.

Haspin kinase regulates microtubule-organizing center clustering and stability through Aurora kinase C in mouse oocytes

Meiotic oocytes lack classic centrosomes and, therefore, bipolar spindle assembly depends on clustering of acentriolar microtubule-organizing centers (MTOCs) into two poles. However, the molecular mechanism regulating MTOC assembly into two poles is not fully understood. The kinase haspin (also known as GSG2) is required to regulate Aurora kinase C (AURKC) localization at chromosomes during meiosis I. Here, we show that inhibition of haspin perturbed MTOC clustering into two poles and the stability of the clustered MTOCs. Furthermore, we show that AURKC localizes to MTOCs in mouse oocytes. Inhibition of haspin perturbed the localization of AURKC at MTOCs, and overexpression of AURKC rescued the MTOC-clustering defects in haspin-inhibited oocytes. Taken together, our data uncover a role for haspin as a regulator of bipolar spindle assembly by regulating AURKC function at acentriolar MTOCs in oocytes.

From Meiosis to Mitosis: The Astonishing Flexibility of Cell Division Mechanisms in Early Mammalian Development

The execution of female meiosis and the establishment of the zygote is arguably the most critical stage of mammalian development. The egg can be arrested in the prophase of meiosis I for decades, and when it is activated, the spindle is assembled de novo. This spindle must function with the highest of fidelity and yet its assembly is unusually achieved in the absence of conventional centrosomes and with minimal influence of chromatin. Moreover, its dramatic asymmetric positioning is achieved through remarkable properties of the actin cytoskeleton to ensure elimination of the polar bodies. The second meiotic arrest marks a uniquely prolonged metaphase eventually interrupted by egg activation at fertilization to complete meiosis and mark a period of preparation of the male and female pronuclear genomes not only for their entry into the mitotic cleavage divisions but also for the imminent prospect of their zygotic expression.

Non-centrosomal TPX2-Dependent Regulation of the Aurora A Kinase: Functional Implications for Healthy and Pathological Cell Division

Aurora A has been extensively characterized as a centrosomal kinase with essential functions during cell division including centrosome maturation and separation and spindle assembly. However, Aurora A localization is not restricted to the centrosomes and compelling evidence support the existence of specific mechanisms of activation and functions for non-centrosomal Aurora A in the dividing cell. It has been now well established that spindle assembly involves an acentrosomal RanGTP-dependent pathway that triggers microtubule assembly and organization in the proximity of the chromosomes whether centrosomes are present or not. The mechanism involves the regulation of a number of NLS-containing proteins, generically called SAFS (Spindle Assembly Factors) that exert their functions upon release from karyopherins by RanGTP. One of them, the nuclear protein TPX2 interacts with and activates Aurora A upon release from importins by RanGTP. This basic mechanism triggers the activation of Aurora A in the proximity of the chromosomes potentially translating the RanGTP signaling gradient centered on the chromosome into an Aurora A phosphorylation network. Here, we will review our current knowledge on the RanGTP-dependent TPX2 activation of Aurora A away from centrosomes: from the mechanism of activation and its functional consequences on the kinase stability and regulation to its roles in spindle assembly and cell division. We will then focus on the substrates of the TPX2-activated Aurora A having a role in microtubule nucleation, stabilization, and organization. Finally, we will briefly discuss the implications of the use of Aurora A inhibitors in anti-tumor therapies in the light of its functional interaction with TPX2.