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

Bisphenol M inhibits mouse oocyte maturation in vitro by disrupting cytoskeleton architecture and cell cycle processes

Bisphenol M (BPM), an alternative to bisphenol A (BPA), is commonly utilized in various industrial applications. However, BPM does not represent a safe substitute for BPA due to its detrimental effects on living beings. This research aimed to assess the influence of BPM exposure on the in vitro maturation of mouse oocytes. The findings revealed that BPM exposure had a notable impact on the germinal vesicle breakdown (GVBD) rate and polar body extrusion (PBE) rate throughout the meiotic progression of mouse oocytes, ultimately resulting in meiotic arrest. Investigations demonstrated that oocytes exposure to BPM led to continued activation of spindle assembly checkpoint. Further studies revealed that securin and cyclin B1 could not be degraded in BPM-exposed oocytes, and meiosis could not realize the transition from the MI to the AI stage. Mechanistically, BPM exposure resulted in abnormal spindle assembly and disrupted chromosome alignment of oocytes. Additionally, abnormal positioning of microtubule organizing center-associated proteins implied that MTOC may be dysfunctional. Furthermore, an elevation in the acetylation level of α-tubulin in oocytes was observed after BPM treatment, leading to decreased microtubule stability. In addition to its impact on microtubules, BPM exposure led to a reduction in the expression of the actin, signifying the disruption of actin assembly. Further research indicated a heightened incidence of DNA damage in oocytes following BPM exposure. Besides, BPM exposure induced alterations in histone modifications. The outcomes of this experiment demonstrate that BPM exposure impairs oocyte quality and inhibits meiotic maturation of mouse oocytes.

Mechanisms of minor pole-mediated spindle bipolarization in human oocytes

Spindle bipolarization, the process of a microtubule mass transforming into a bipolar spindle, is a prerequisite for accurate chromosome segregation. In contrast to mitotic cells, the process and mechanism of spindle bipolarization in human oocytes remains unclear. Using high-resolution imaging in more than 1800 human oocytes, we revealed a typical state of multipolar intermediates that form during spindle bipolarization and elucidated the mechanism underlying this process. We found that the minor poles formed in multiple kinetochore clusters contribute to the generation of multipolar intermediates. We further determined the essential roles of HAUS6, KIF11, and KIF18A in spindle bipolarization and identified mutations in these genes in infertile patients characterized by oocyte or embryo defects. These results provide insights into the physiological and pathological mechanisms of spindle bipolarization in human oocytes.

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.

Spermatocytes have the capacity to segregate chromosomes despite centriole duplication failure

Centrosomes are the canonical microtubule organizing centers (MTOCs) of most mammalian cells, including spermatocytes. Centrosomes comprise a centriole pair within a structurally ordered and dynamic pericentriolar matrix (PCM). Unlike in mitosis, where centrioles duplicate once per cycle, centrioles undergo two rounds of duplication during spermatogenesis. The first duplication is during early meiotic prophase I, and the second is during interkinesis. Using mouse mutants and chemical inhibition, we have blocked centriole duplication during spermatogenesis and determined that non-centrosomal MTOCs (ncMTOCs) can mediate chromosome segregation. This mechanism is different from the acentriolar MTOCs that form bipolar spindles in oocytes, which require PCM components, including gamma-tubulin and CEP192. From an in-depth analysis, we identified six microtubule-associated proteins, TPX2, KIF11, NuMA, and CAMSAP1-3, that localized to the non-centrosomal MTOC. These factors contribute to a mechanism that ensures bipolar MTOC formation and chromosome segregation during spermatogenesis when centriole duplication fails. However, despite the successful completion of meiosis and round spermatid formation, centriole inheritance and PLK4 function are required for normal spermiogenesis and flagella assembly, which are critical to ensure fertility.

Spindle morphology changes between meiosis and mitosis driven by CK2 regulation of the Ran pathway

The transition from meiotic divisions in the oocyte to embryonic mitoses is a critical step in animal development. Despite negligible changes to cell size and shape, following fertilization the small, barrel-shaped meiotic spindle is replaced by a large zygotic spindle that nucleates abundant astral microtubules at spindle poles. To probe underlying mechanisms, we applied a drug screening approach using Ciona eggs and found that inhibition of Casein Kinase 2 (CK2) caused a shift from meiotic to mitotic-like spindle morphology with nucleation of robust astral microtubules, an effect reproduced in cytoplasmic extracts prepared from Xenopus eggs. In both species, CK2 activity decreased at fertilization. Phosphoproteomic differences between Xenopus meiotic and mitotic extracts that also accompanied CK2 inhibition pointed to RanGTP-regulated factors as potential targets. Interfering with RanGTP-driven microtubule formation suppressed astral microtubule growth caused by CK2 inhibition. These data support a model in which CK2 activity attenuation at fertilization leads to activation of RanGTP-regulated microtubule effectors that induce mitotic spindle morphology.

Liquid-liquid phase separation of microtubule-binding proteins in the regulation of spindle assembly

Cell division is a highly regulated process essential for the accurate segregation of chromosomes. Central to this process is the assembly of a bipolar mitotic spindle, a highly dynamic microtubule (MT)-based structure responsible for chromosome movement. The nucleation and dynamics of MTs are intricately regulated by MT-binding proteins. Over the recent years, various MT-binding proteins have been reported to undergo liquid-liquid phase separation, forming either single- or multi-component condensates on MTs. Herein, we provide a comprehensive summary of the phase separation characteristics of these proteins. We underscore their critical roles in MT nucleation, spindle assembly and kinetochore-MT attachment during the cell division process. Furthermore, we discuss the current challenges and various remaining unsolved problems, highlights the ongoing research efforts aimed at a deeper understanding of the role of the phase separation process during spindle assembly and orientation. Our review aims to contribute to the collective knowledge in this area and stimulate further investigations that will enhance our comprehension of the intricate mechanisms governing cell division.

Physiological and pathological effects of phase separation in the central nervous system

Phase separation, also known as biomolecule condensate, participates in physiological processes such as transcriptional regulation, signal transduction, gene expression, and DNA damage repair by creating a membrane-free compartment. Phase separation is primarily caused by the interaction of multivalent non-covalent bonds between proteins and/or nucleic acids. The strength of molecular multivalent interaction can be modified by component concentration, the potential of hydrogen, posttranslational modification, and other factors. Notably, phase separation occurs frequently in the cytoplasm of mitochondria, the nucleus, and synapses. Phase separation in vivo is dynamic or stable in the normal physiological state, while abnormal phase separation will lead to the formation of biomolecule condensates, speeding up the disease progression. To provide candidate suggestions for the clinical treatment of nervous system diseases, this review, based on existing studies, carefully and systematically represents the physiological roles of phase separation in the central nervous system and its pathological mechanism in neurodegenerative diseases.

Mouse oocytes sequester aggregated proteins in degradative super-organelles

Oocytes are among the longest-lived cells in the body and need to preserve their cytoplasm to support proper embryonic development. Protein aggregation is a major threat for intracellular homeostasis in long-lived cells. How oocytes cope with protein aggregation during their extended life is unknown. Here, we find that mouse oocytes accumulate protein aggregates in specialized compartments that we named endolysosomal vesicular assemblies (ELVAs). Combining live-cell imaging, electron microscopy, and proteomics, we found that ELVAs are non-membrane-bound compartments composed of endolysosomes, autophagosomes, and proteasomes held together by a protein matrix formed by RUFY1. Functional assays revealed that in immature oocytes, ELVAs sequester aggregated proteins, including TDP-43, and degrade them upon oocyte maturation. Inhibiting degradative activity in ELVAs leads to the accumulation of protein aggregates in the embryo and is detrimental for embryo survival. Thus, ELVAs represent a strategy to safeguard protein homeostasis in long-lived cells.

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.

Identification of a motif in TPX2 that regulates spindle architecture in Xenopus egg extracts

A bipolar spindle composed of microtubules and many associated proteins functions to segregate chromosomes during cell division in all eukaryotes, yet spindle size and architecture varies dramatically across different species and cell types. Targeting protein for Xklp2 (TPX2) is one candidate factor for modulating spindle microtubule organization through its roles in branching microtubule nucleation, activation of the mitotic kinase Aurora A, and association with the kinesin-5 (Eg5) motor. Here we identify a conserved nuclear localization sequence (NLS) motif, 123 KKLK 126 in X. laevis TPX2, which regulates astral microtubule formation and spindle pole morphology in Xenopus egg extracts. Addition of recombinant TPX2 with this sequence mutated to AALA dramatically increased spontaneous formation of microtubule asters and recruitment of phosphorylated Aurora A, pericentrin, and Eg5 to meiotic spindle poles. We propose that TPX2 is a linchpin spindle assembly factor whose regulation contributes to the recruitment and activation of multiple microtubule polymerizing and organizing proteins, generating distinct spindle architectures.

Centrosome amplification-related signature correlated with immune microenvironment and treatment response predicts prognosis and improves diagnosis of hepatocellular carcinoma by integrating machine learning and single-cell analyses

BACKGROUND

Centrosome amplification is a well-recognized oncogenic driver of tumor initiation and progression across a variety of malignancies and has been linked with tumor aggressiveness, metastasis, and adverse prognosis. Nevertheless, the significance of centrosome amplification in HCC is not well understood.

METHODS

The TCGA dataset was downloaded for centrosome amplification-related signature construction using the LASSO-penalized Cox regression algorithm, while the ICGC dataset was obtained for signature validation. Single-cell RNA sequencing from GSE149614 was analyzed to profile gene expression and the liver tumor niche.

RESULTS

A total of 134 centrosome amplification-related prognostic genes in HCC were detected and 6 key prognostic genes (SSX2IP, SPAG4, SAC3D1, NPM1, CSNK1D, and CEP55) among them were screened out to construct a signature with both high sensitivity and specificity in diagnosis and prognosis of HCC patients. The signature, as an independent factor, was associated with frequent recurrences, high mortality rates, advanced clinicopathologic features, and high vascular invasions. Moreover, the signature was intimately associated with cell cycle-related pathways and TP53 mutation profile, suggesting its underlying role in accelerating cell cycle progression and leading to liver cancer development. Meanwhile, the signature was also closely correlated with immunosuppressive cell infiltration and immune checkpoint expression, making it a vital immunosuppressive factor in the tumor microenvironment. Upon single-cell RNA sequencing, SSX2IP and SAC3D1 were found to be specially expressed in liver cancer stem-like cells, where they promoted cell cycle progression and hypoxia.

CONCLUSIONS

This study provided a direct molecular link of centrosome amplification with clinical characteristics, tumor microenvironment, and clinical drug-response, highlighting the critical role of centrosome amplification in liver cancer development and therapy resistance, thereby providing valuable insights into prognostic prediction and therapeutic response of HCC.

Predicting Infertility: How Genetic Variants in Oocyte Spindle Genes Affect Egg Quality

Successful reproduction relies on the union of a single chromosomally normal egg and sperm. Chromosomally normal eggs develop from precursor cells, called oocytes, that have undergone accurate chromosome segregation. The process of chromosome segregation is governed by the oocyte spindle, a unique cytoskeletal machine that splits chromatin content of the meiotically dividing oocyte. The oocyte spindle develops and functions in an idiosyncratic process, which is vulnerable to genetic variation in spindle-associated proteins. Human genetic variants in several spindle-associated proteins are associated with poor clinical fertility outcomes, suggesting that heritable etiologies for oocyte dysfunction leading to infertility exist and that the spindle is a crux for female fertility. This chapter examines the mammalian oocyte spindle through the lens of human genetic variation, covering the genes TUBB8, TACC3, CEP120, AURKA, AURKC, AURKB, BUB1B, and CDC20. Specifically, it explores how patient-identified variants perturb spindle development and function, and it links these molecular changes in the oocyte to their cognate clinical consequences, such as oocyte maturation arrest, elevated egg aneuploidy, primary ovarian insufficiency, and recurrent pregnancy loss. This discussion demonstrates that small genetic errors in oocyte meiosis can result in remarkably far-ranging embryonic consequences, and thus reveals the importance of the oocyte's fine machinery in sustaining life.

Phase separation on microtubules: from droplet formation to cellular function?

Microtubules are cytoskeletal polymers that play important roles in numerous cellular processes, ranging from the control of cell shape and polarity to cell division and intracellular transport. Many of these roles rely on proteins that bind to microtubule ends and shafts, carry intrinsically disordered regions, and form complex multivalent interaction networks. A flurry of recent studies demonstrated that these properties allow diverse microtubule-binding proteins to undergo liquid-liquid phase separation (LLPS) in vitro. It is proposed that LLPS could potentially affect multiple microtubule-related processes, such as microtubule nucleation, control of microtubule dynamics and organization, and microtubule-based transport. Here, we discuss the evidence in favor and against the occurrence of LLPS and its functional significance for microtubule-based processes in cells.

The oocyte: the key player in the success of assisted reproduction technologies

The ovulation of a mature oocyte at metaphase II of meiosis, with optimal potential to undergo fertilisation by a sperm cell, complete meiosis and sustain the switch to mitotic division, and support early embryo development, involves a protracted and disrupted/delayed series of processes. Many of these are targeted for exploitation in vivo , or recapitulation in vitro , by the livestock industry. Reproductive technologies, including AI, multiple ovulation embryo transfer, ovum pick-up, in vitro embryo production, and oestrus and ovulation synchronisation, offer practitioners and producers the opportunity to produce offspring from genetically valuable dams in much greater numbers than they would normally have in their lifetime, while in vitro oocyte and follicle culture are important platforms for researchers to interrogate the physiological mechanisms driving fertility. The majority of these technologies target the ovarian follicle and the oocyte within; thus, the quality and capability of the recovered oocyte determine the success of the reproductive intervention. Molecular and microscopical technologies have grown exponentially, providing powerful platforms to interrogate the molecular mechanisms which are integral to or affected by ART. The development of the bovine oocyte from its differentiation in the ovary to ovulation is described in the light of its relevance to key aspects of individual interventions, while highlighting the historical timeline.

Knockdown of TACC3 inhibits tumor cell proliferation and increases chemosensitivity in pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) is a highly malignant digestive tract tumor with limited clinical treatments. Transforming acidic coiled-coil-containing protein 3 (TACC3) is a component of the centrosome axis and a member of the TACC family, which affect mitosis and regulate chromosome stability and are involved in tumor development and progression. However, the role of TACC3 in PDAC remains elusive. In this study, by exploiting the TCGA database, we found that high TACC3 expression in PDAC is associated with poor prognosis. shRNA-mediated TACC3 knockdown caused S phase arrest of the cell cycle and inhibited proliferation in PDAC cell lines. Through RNA sequencing and protein co-immunoprecipitation combined with mass spectrometry, KIF11 was identified as a protein that interacts with TACC3. TACC3 stabilizes and regulates KIF11 protein expression levels in PDAC cells through physical interaction. Knockdown of TACC3 or KIF11 resulted in abnormal spindle formation during cell division both in vitro and in vivo. Pharmacological inhibition of TACC3 or KIF11 can suppress tumor cell proliferation and promote apoptosis. Our studies further demonstrated that high expression of TACC3 and KIF11 mediated the resistance of PDAC to gemcitabine, and deficiency of TACC3 or KIF11 increased the sensitivity of PDAC cells to chemotherapy. In conclusion, our study reveals the fundamental role of TACC3 expression in PDAC cell proliferation and chemoresistance, suggesting that TACC3 can be used as a molecular marker to evaluate the prognosis of PDAC.

Homozygous variants in CDC23 cause female infertility characterized by oocyte maturation defects

Oocyte maturation defects are major phenotypes resulting in female infertility. Although many genetic factors have been found to be responsible for these phenotypes, the underlying pathogenic genes and variants remain to be identified. The anaphase promoting complex or cyclosome (APC/C) is known to be essential in the metaphase-to-anaphase transition. In this study, we identified two homozygous missense variants (c.986A > G, p.Y329C and c.988C > T, p.R330C) in CDC23 that are responsible for female infertility characterized by oocyte maturation defects in three infertile individuals. CDC23 (cell division cycle 23) is one of the core subunits of the APC/C. In vitro experiments showed that the variant c.986A > G (p.Y329C) led to a decrease in CDC23 protein level and the variant c.988C > T (p.R330C) changed the localization of CDC23 in HeLa cells and mouse oocytes. In vivo studies showed that Cdc23Y329C/Y329C mice successfully mimicked the patients' phenotype by causing low expression of CDC23 and APC4 and the accumulation of securin and cyclin B1 in oocytes. AZ3146 treatment was able to rescue the phenotype. Taken together, our findings reveal the important roles of CDC23 in human oocyte maturation and provide a new genetic marker for female infertility.

Vitrification induces a focused spindle pole in mouse MI oocytes

Immature oocyte (germinal vesicle stage, GV) vitrification can avoid a cycle of ovarian stimulation, which is friendly to patients with hormone-sensitive tumors. However, the in vitro maturation of vitrification-thawed GV oocyte usually results in aneuploidy, and the underlying mechanism remains unclear. Stable spindle poles are important for accurate chromosome segregation. Acentriolar microtubule-organizing centers (aMTOCs) undergo fragmentation and reaggregation to form spindle poles. Microtubule nucleation is facilitated via the perichromosome Ran after GVBD, which plays an important role in aMTOCs fragmentation. This study showed that vitrification may reduce microtubule density by decreasing perichromosomal Ran levels, which reduced the localization of pKIF11, thereby decreased the fragmentation of aMTOCs and formed a more focused spindle pole, ultimately resulted in aneuploidy. This study revealed the mechanism of abnormal spindle pole formation in vitrified oocytes and offered a theoretical support to further improve the quality of vitrified oocytes.

Microtubule nucleation for spindle assembly: one molecule at a time

The cell orchestrates the dance of chromosome segregation with remarkable speed and fidelity. The mitotic spindle is built from scratch after interphase through microtubule (MT) nucleation, which is dependent on the γ-tubulin ring complex (γ-TuRC), the universal MT template. Although several MT nucleation pathways build the spindle framework, the question of when and how γ-TuRC is targeted to these nucleation sites in the spindle and subsequently activated remains an active area of investigation. Recent advances facilitated the discovery of new MT nucleation effectors and their mechanisms of action. In this review, we illuminate each spindle assembly pathway and subsequently consider how the pathways are merged to build a spindle.

Dynamic role of Scd1 gene during mouse oocyte growth and maturation

Mammalian reproductive ability is regulated by many factors, among which the fatty acid metabolism network provides energy for oocyte growth and primordial follicle formation during early mouse oogenesis. But the mechanism behind that is still unknown. Stearoyl-CoA desaturase 1 (Scd1) gene expression is increased during the oogenesis process, supporting the oocyte's healthy growth. Taking advantage of gene-edited mice lacking stearoyl-Coenzyme A desaturase 1 gene (Scd1-/-), we analyzed relative gene expression in perinatal ovaries from wildtype, and Scd1-/- mice. Scd1 deficiency dysregulates expression of meiosis-related genes (e.g., Sycp1, Sycp2, Sycp3, Rad51, Ddx4) and a variety of genes (e.g., Nobox, Lhx8, Bmp15, Ybx2, Dppa3, Oct4, Sohlh1, Zp3) associated with oocyte growth and differentiation, leading to a lower oocyte maturation rate. The absence of Scd1 significantly impedes meiotic progression, causes DNA damage, and inhibits damage repair in Scd1-/- ovaries. Moreover, we find that Scd1 absense dramatically disrupts the abundance of fatty acid metabolism genes (e.g., Fasn, Srebp1, Acaca) and the lipid droplet content. Thus, our findings substantiate a major role for Scd1 as a multifunctional regulator of fatty acid networks necessary for oocyte maintenance and differentiation during early follicular genesis.

Mechanisms underlying spindle assembly and robustness

The microtubule-based spindle orchestrates chromosome segregation during cell division. Following more than a century of study, many components and pathways contributing to spindle assembly have been described, but how the spindle robustly assembles remains incompletely understood. This process involves the self-organization of a large number of molecular parts - up to hundreds of thousands in vertebrate cells - whose local interactions give rise to a cellular-scale structure with emergent architecture, mechanics and function. In this Review, we discuss key concepts in our understanding of spindle assembly, focusing on recent advances and the new approaches that enabled them. We describe the pathways that generate the microtubule framework of the spindle by driving microtubule nucleation in a spatially controlled fashion and present recent insights regarding the organization of individual microtubules into structural modules. Finally, we discuss the emergent properties of the spindle that enable robust chromosome segregation.

Distinct dynein complexes defined by DYNLRB1 and DYNLRB2 regulate mitotic and male meiotic spindle bipolarity

Spindle formation in male meiosis relies on the canonical centrosome system, which is distinct from acentrosomal oocyte meiosis, but its specific regulatory mechanisms remain unknown. Herein, we report that DYNLRB2 (Dynein light chain roadblock-type-2) is a male meiosis-upregulated dynein light chain that is indispensable for spindle formation in meiosis I. In Dynlrb2 KO mouse testes, meiosis progression is arrested in metaphase I due to the formation of multipolar spindles with fragmented pericentriolar material (PCM). DYNLRB2 inhibits PCM fragmentation through two distinct pathways; suppressing premature centriole disengagement and targeting NuMA (nuclear mitotic apparatus) to spindle poles. The ubiquitously expressed mitotic counterpart, DYNLRB1, has similar roles in mitotic cells and maintains spindle bipolarity by targeting NuMA and suppressing centriole overduplication. Our work demonstrates that two distinct dynein complexes containing DYNLRB1 or DYNLRB2 are separately used in mitotic and meiotic spindle formations, respectively, and that both have NuMA as a common target.

Chloroacetonitrile exposure induces endoplasmic reticulum stress and affects spindle assembly in mouse oocytes

Chloroacetonitrile (CAN) is a halogenated acetonitrile often produced while disinfecting drinking water. Previous studies have shown that maternal exposure to CAN interferes with fetal development; however, the adverse effects on maternal oocytes remain unknown. In this study, in vitro exposure of mouse oocytes to CAN reduced maturation significantly. Transcriptomics analysis showed that CAN altered the expression of multiple oocyte genes, especially those associated with the protein folding process. CAN exposure induced reactive oxygen species production, accompanied by endoplasmic reticulum (ER) stress and increased glucose regulated protein 78, C/EBP homologous protein and activating transcription factor 6 expression. Moreover, our results indicated that spindle morphology was impaired after CAN exposure. CAN disrupted polo-like kinase 1, pericentrin and p-Aurora A distribution, which may be an origin inducer that disrupts spindle assemble. Furthermore, exposure to CAN in vivo impaired follicular development. Taken together, our findings indicate that CAN exposure induces ER stress and affects spindle assembly in mouse oocytes.

High GTSE1 expression promotes cell proliferation, metastasis and cisplatin resistance in ccRCC and is associated with immune infiltrates and poor prognosis

Background: Clear cell renal cell carcinoma is the most common and fatal form of kidney cancer, accounting for 80% of new cases. Although it has been reported that GTSE1 is highly expressed in a variety of tumors and associated with malignant progression and poor clinical prognosis, its clinical significance, correlations with immune cell infiltration and biological function in ccRCC are still poorly understood.

Methods: The gene expression, clinicopathological features, and clinical significance of GTSE1 were analyzed using multiple databases, including TCGA, GEO, TIMER, and UALCAN Kaplan–Meier survival analysis, gene set enrichment analysis gene ontology enrichment Gene Ontology, and Kyoto Encyclopedia of Genes and Genomes (KEGG) were performed. Tumor-infiltrating immune cells and immunomodulators were extracted and analyzed using TCGA-KIRC profiles. Protein‒protein interactions were built using the STRING website. The protein level of GTSE1 in ccRCC patients was detected by immunohistochemistry using a ccRCC tissue chip. Finally, MTT assays, colony-formation assays, cell flow cytometry analyses, EdU-staining assays, wound-healing assays, and transwell migration and invasion assays were conducted to assess the biological function of GTSE1 in vitro.

Results: GTSE1 was overexpressed in ccRCC tissues and cells, and GTSE1 overexpression was associated with adverse clinical-pathological factors and poor clinical prognosis. Meanwhile, the functional enrichment analysis indicated that GTSE1 and its coexpressed genes were mainly related to the cell cycle, DNA replication, and immunoreaction, such as T-cell activation and innate immune response, through multiple signaling pathways, including the P53 signaling pathway and T-cell receptor signaling pathway. Furthermore, we observed a significant relationship between GTSE1 expression and the levels of infiltrating immune cells in ccRCC. Biological functional studies demonstrated that GTSE1 could promote the malignant progression of ccRCC by promoting cell proliferation, cell cycle transition, migration, and invasion capacity and decreasing the sensitivity of ccRCC cells to cisplatin.

Conclusion: Our results indicate that GTSE1, serving as a potential oncogene, can promote malignant progression and cisplatin resistance in ccRCC. Additionally, high GTSE1 expression contributes to an increased level of immune cell infiltration and is associated with a worse prognosis, providing a potential target for tumor therapy in ccRCC.

Biomolecular condensation involving the cytoskeleton

Biomolecular condensation of proteins contributes to the organization of the cytoplasm and nucleoplasm. A number of condensation processes appear to be directly involved in regulating the structure, function and dynamics of the cytoskeleton. Liquid-liquid phase separation of cytoskeleton proteins, together with polymerization modulators, promotes cytoskeletal fiber nucleation and branching. Furthermore, the attachment of protein condensates to the cytoskeleton can contribute to cytoskeleton stability and organization, regulate transport, create patterns of functional reaction containers, and connect the cytoskeleton with membranes. Surface-bound condensates can exert and buffer mechanical forces that give stability and flexibility to the cytoskeleton, thus, may play a large role in cell biology. In this review, we introduce the concept and role of cellular biomolecular condensation, explain its special function on cytoskeletal fiber surfaces, and point out potential definition and experimental caveats. We review the current literature on protein condensation processes related to the actin, tubulin, and intermediate filament cytoskeleton, and discuss some of them in the context of neurobiology. In summary, we provide an overview about biomolecular condensation in relation to cytoskeleton structure and function, which offers a base for the exploration and interpretation of cytoskeletal condensates in neurobiology.

Polo-like kinase 1 promotes Cdc42-induced actin polymerization for asymmetric division in oocytes

Polo-like kinase I (Plk1) is a highly conserved seronine/threonine kinase essential in meiosis and mitosis for spindle formation and cytokinesis. Here, through temporal application of Plk1 inhibitors, we identify a new role for Plk1 in the establishment of cortical polarity essential for highly asymmetric cell divisions of oocyte meiosis. Application of Plk1 inhibitors in late metaphase I abolishes pPlk1 from spindle poles and prevents the induction of actin polymerization at the cortex through inhibition of local recruitment of Cdc42 and Neuronal Wiskott-Aldrich Syndrome protein (N-WASP). By contrast, an already established polar actin cortex is insensitive to Plk1 inhibitors, but if the polar cortex is first depolymerized, Plk1 inhibitors completely prevent its restoration. Thus, Plk1 is essential for establishment but not maintenance of cortical actin polarity. These findings indicate that Plk1 regulates recruitment of Cdc42 and N-Wasp to coordinate cortical polarity and asymmetric cell division.

Actin-driven chromosome clustering facilitates fast and complete chromosome capture in mammalian oocytes

Accurate chromosome segregation during meiosis is crucial for reproduction. Human and porcine oocytes transiently cluster their chromosomes before the onset of spindle assembly and subsequent chromosome segregation. The mechanism and function of chromosome clustering are unknown. Here we show that chromosome clustering is required to prevent chromosome losses in the long gap phase between nuclear envelope breakdown and the onset of spindle assembly, and to promote the rapid capture of all chromosomes by the acentrosomal spindle. The initial phase of chromosome clustering is driven by a dynamic network of Formin-2- and Spire-nucleated actin cables. The actin cables form in the disassembling nucleus and migrate towards the nuclear centre, moving the chromosomes centripetally by interacting with their arms and kinetochores as they migrate. A cage of stable microtubule loops drives the late stages of chromosome clustering. Together, our data establish a crucial role for chromosome clustering in accurate progression through meiosis.

Regulation of organelle size and organization during development

During early embryogenesis, as cells divide in the developing embryo, the size of intracellular organelles generally decreases to scale with the decrease in overall cell size. Organelle size scaling is thought to be important to establish and maintain proper cellular function, and defective scaling may lead to impaired development and disease. However, how the cell regulates organelle size and organization are largely unanswered questions. In this review, we summarize the process of size scaling at both the cell and organelle levels and discuss recently discovered mechanisms that regulate this process during early embryogenesis. In addition, we describe how some recently developed techniques and Xenopus as an animal model can be used to investigate the underlying mechanisms of size regulation and to uncover the significance of proper organelle size scaling and organization.

Multiple Roles of PLK1 in Mitosis and Meiosis

Cells are equipped with a diverse network of signaling and regulatory proteins that function as cell cycle regulators and checkpoint proteins to ensure the proper progression of cell division. A key regulator of cell division is polo-like kinase 1 (PLK1), a member of the serine/threonine kinase family that plays an important role in regulating the mitotic and meiotic cell cycle. The phosphorylation of specific substrates mediated by PLK1 controls nuclear envelope breakdown (NEBD), centrosome maturation, proper spindle assembly, chromosome segregation, and cytokinesis. In mammalian oogenesis, PLK1 is essential for resuming meiosis before ovulation and for establishing the meiotic spindle. Among other potential roles, PLK1 regulates the localized translation of spindle-enriched mRNAs by phosphorylating and thereby inhibiting the translational repressor 4E-BP1, a downstream target of the mTOR (mammalian target of rapamycin) pathway. In this review, we summarize the functions of PLK1 in mitosis, meiosis, and cytokinesis and focus on the role of PLK1 in regulating mRNA translation. However, knowledge of the role of PLK1 in the regulation of meiosis remains limited.

C. elegans XMAP215/ZYG-9 and TACC/TAC-1 act at multiple times during oocyte meiotic spindle assembly and promote both spindle pole coalescence and stability

The conserved two-component XMAP215/TACC modulator of microtubule stability is required in multiple animal phyla for acentrosomal spindle assembly during oocyte meiotic cell division. In C. elegans, XMAP215/zyg-9 and TACC/tac-1 mutant oocytes exhibit multiple and indistinguishable oocyte spindle assembly defects beginning early in meiosis I. To determine if these defects represent one or more early requirements with additional later and indirect consequences, or multiple temporally distinct and more direct requirements, we have used live cell imaging and fast-acting temperature-sensitive zyg-9 and tac-1 alleles to dissect their requirements at high temporal resolution. Temperature upshift and downshift experiments indicate that the ZYG-9/TAC-1 complex has multiple temporally distinct and separable requirements throughout oocyte meiotic cell division. First, we show that during prometaphase ZYG-9 and TAC-1 promote the coalescence of early pole foci into a bipolar structure, stabilizing pole foci as they grow and limiting their growth rate, with these requirements being independent of an earlier defect in microtubule organization that occurs upon nuclear envelope breakdown. Second, during metaphase, ZYG-9 and TAC-1 maintain spindle bipolarity by suppressing ectopic pole formation. Third, we show that ZYG-9 and TAC-1 also are required for spindle assembly during meiosis II, independently of their meiosis I requirements. The metaphase pole stability requirement appears to be important for maintaining chromosome congression, and we discuss how negative regulation of microtubule stability by ZYG-9/TAC-1 during oocyte meiotic cell division might account for the observed defects in spindle pole coalescence and stability.

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.

Proteomic Profiling Reveals the Molecular Control of Oocyte Maturation

Meiotic maturation is an intricate and precisely regulated process orchestrated by various pathways and numerous proteins. However, little is known about the proteome landscape during oocytes maturation. Here, we obtained the temporal proteomic profiles of mouse oocytes during in vivo maturation. We successfully quantified 4694 proteins from 4500 oocytes in three key stages (germinal vesicle, germinal vesicle breakdown, and metaphase II). In particular, we discovered the novel proteomic features during oocyte maturation, such as the active Skp1-Cullin-Fbox pathway and an increase in mRNA decay-related proteins. Using functional approaches, we further identified the key factors controlling the histone acetylation state in oocytes and the vital proteins modulating meiotic cell cycle. Taken together, our data serve as a broad resource on the dynamics occurring in oocyte proteome and provide important knowledge to better understand the molecular mechanisms during germ cell development.

Mechanistic insights into HuR inhibitor MS-444 arresting embryonic development revealed by low-input RNA-seq and STORM

With improvements in the survival rate of patients with cancer, fertility maintenance has become a major concern in terms of cancer treatment for women of reproductive age. Thus, it is important to examine the impact on fertility of anticancer drugs that are used clinically or are undergoing trials. The HuR small-molecule inhibitor MS-444 has been used in many cancer treatment studies, but its reproductive toxicity in females is unknown. Here, we reported that MS-444 blocked the nucleocytoplasmic transport of Agbl2 mRNA by inhibiting HuR dimerization, resulting in the developmental arrest of 2-cell stage embryos in mouse. Combining analysis of low-input RNA-seq for MS-444-treated 2-cell embryos and mapping binding sites of RNA-binding protein, Agbl2 was predicted to be the target gene of MS-444. For further confirmation, RNAi experiment in wild-type zygotes showed that Agbl2 knockdown reduced the proportion of embryos successfully developed to the blastocyst stage: from 71% in controls to 23%. Furthermore, RNA-FISH and luciferase reporter analyses showed that MS-444 blocked the nucleocytoplasmic transport of Agbl2 mRNA and reduced its stability by inhibiting HuR dimerization. In addition, optimized stochastic optical reconstruction microscopy (STORM) imaging showed that MS-444 significantly reduced the HuR dimerization, and HuR mainly existed in cluster form in 2-cell stage embryos. In conclusion, this study provides clinical guidance for maintaining fertility during the treatment of cancer with MS-444 in women of reproductive age. And also, our research provides guidance for the application of STORM in nanometer scale studies of embryonic cells. HuR inhibitor MS-444 arrested embryonic development at 2-cell stage. Low-input RNA-seq revealed that Agbl2 was the target gene of MS-444. MS-444 blocked the nucleocytoplasmic transport of Agbl2 mRNA by inhibiting HuR dimerization and reduced the stability of Agbl2 mRNA. STORM with our optimized protocol showed that HuR tended to form elliptical and dense clusters in 2-cell stage embryos.

The mechanism of acentrosomal spindle assembly in human oocytes

Meiotic spindle assembly ensures proper chromosome segregation in oocytes. However, the mechanisms behind spindle assembly in human oocytes remain largely unknown. We used three-dimensional high-resolution imaging of more than 2000 human oocytes to identify a structure that we named the human oocyte microtubule organizing center (huoMTOC). The proteins TACC3, CCP110, CKAP5, and DISC1 were found to be essential components of the huoMTOC. The huoMTOC arises beneath the oocyte cortex and migrates adjacent to the nuclear envelope before nuclear envelope breakdown (NEBD). After NEBD, the huoMTOC fragments and relocates on the kinetochores to initiate microtubule nucleation and spindle assembly. Disrupting the huoMTOC led to spindle assembly defects and oocyte maturation arrest. These results reveal a physiological mechanism of huoMTOC-regulated spindle assembly in human oocytes.

Mammalian oocytes store mRNAs in a mitochondria-associated membraneless compartment

Full-grown oocytes are transcriptionally silent and must stably maintain the messenger RNAs (mRNAs) needed for oocyte meiotic maturation and early embryonic development. However, where and how mammalian oocytes store maternal mRNAs is unclear. Here, we report that mammalian oocytes accumulate mRNAs in a mitochondria-associated ribonucleoprotein domain (MARDO). MARDO assembly around mitochondria was promoted by the RNA-binding protein ZAR1 and directed by an increase in mitochondrial membrane potential during oocyte growth. MARDO foci coalesced into hydrogel-like matrices that clustered mitochondria. Maternal mRNAs stored in the MARDO were translationally repressed. Loss of ZAR1 disrupted the MARDO, dispersed mitochondria, and caused a premature loss of MARDO-localized mRNAs. Thus, a mitochondria-associated membraneless compartment controls mitochondrial distribution and regulates maternal mRNA storage, translation, and decay to ensure fertility in mammals.

Meiotic defects in human oocytes: Potential causes and clinical implications

Meiotic defects cause abnormal chromosome segregation leading to aneuploidy in mammalian oocytes. Chromosome segregation is particularly error-prone in human oocytes, but the mechanisms behind such errors remain unclear. To explain the frequent chromosome segregation errors, recent investigations have identified multiple meiotic defects and explained how these defects occur in female meiosis. In particular, we review the causes of cohesin exhaustion, leaky spindle assembly checkpoint (SAC), inherently unstable meiotic spindle, fragmented kinetochores or centromeres, abnormal aurora kinases (AURK), and clinical genetic variants in human oocytes. We mainly focus on meiotic defects in human oocytes, but also refer to the potential defects of female meiosis in mouse models.

Loss of Cep72 affects the morphology of spermatozoa in mice

The centrosome regulates mammalian meiosis by affecting recombination, synapsis, chromosome segregation, and spermiogenesis. Cep72 is one of the critical components of the centrosome. However, the physiological role of Cep72 in spermatogenesis and fertility remains unclear. In this study, we identify Cep72 as a testis-specific expression protein. Although Cep72 knockout mice were viable and fertile, their sperms were morphologically abnormal with incomplete flagellum structures. Transcriptome analysis reveals significant differences in six genes (Gm49527, Hbb-bt, Hba-a2, Rps27a-ps2, Gm29647, and Gm8430), which were not previously associated with spermatogenesis. Overall, these results indicate that Cep72 participates in regulating sperm morphology and yet is dispensable for fertility in mice.

Transient inhibition of CDK2 activity prevents oocyte meiosis I completion and egg activation in mouse

Mammalian oocytes are arrested at the diplotene stage of prophase I during fetal or postnatal development. It was reported that cyclin-dependent kinases (CDK1) was the sole CDK to drive the resumption of meiosis and CDK2 was dispensable for meiosis progression in mouse oocytes according to the conditional knockout studies. However, a recent study showed that CDK2 activity is essential for meiotic division and gametogenesis by means of gene-directed mutagenesis, which avoids the compensatory activation of other CDKs. Taken the compensatory effect between CDKs after gene knockout, the physiological function of CDK2 activity in oocyte maturation remains unclear. To address this issue, we applied a specific small-molecule inhibitor to restrain CDK2 activity transiently during oocyte meiotic maturation. Surprisingly, transient inhibition of CDK2 activity severely prevented the meiosis I completion although the meiotic resumption was not affected. Then we found that CDK2 activity was required for establishment of normal spindle and chromosome dynamics. Notably, CDK2 inhibition interrupted the anaphase-promoting complex/cyclosome (APC/C)-dependent degradation pathway by maintaining the activation of spindle assembly checkpoint (SAC). Interestingly, CDK2 inhibition prevented the egg activation as well. Overall, our data demonstrate that CDK2 kinase activity is required for proper dynamics of spindle and chromosomes, whose disturbance induces the continuous SAC activation and subsequent inactivation of APC/C activity in oocyte meiosis.

Female meiosis II and pronuclear fusion require the microtubule transport factor Bicaudal D

Bicaudal D (BicD) is a dynein adaptor that transports different cargoes along microtubules. Reducing the activity of BicD specifically in freshly laid Drosophila eggs by acute protein degradation revealed that BicD is needed to produce normal female meiosis II products, to prevent female meiotic products from re-entering the cell cycle, and for pronuclear fusion. Given that BicD is required to localize the spindle assembly checkpoint (SAC) components Mad2 and BubR1 to the female meiotic products, it appears that BicD functions to localize these components to control metaphase arrest of polar bodies. BicD interacts with Clathrin heavy chain (Chc), and both proteins localize to centrosomes, mitotic spindles and the tandem spindles during female meiosis II. Furthermore, BicD is required to localize clathrin and the microtubule-stabilizing factors transforming acidic coiled-coil protein (D-TACC/Tacc) and Mini spindles (Msps) correctly to the meiosis II spindles, suggesting that failure to localize these proteins may perturb SAC function. Furthermore, immediately after the establishment of the female pronucleus, D-TACC and Caenorhabditis elegans BicD, tacc and Chc are also needed for pronuclear fusion, suggesting that the underlying mechanism might be more widely used across species.

Phase separation in controlling meiotic chromosome dynamics

Sexually reproducing organisms produce haploid gametes through meiotic cell division, during which a single round of DNA replication is followed by two consecutive chromosome segregation. A series of meiosis-specific events take place during the meiotic prophase to ensure successful chromosome segregation. These events include programmed DNA double-strand break formation, chromosome movement driven by cytoplasmic forces, homologous pairing, synaptonemal complex installation, and inter-homolog crossover formation. Phase separation has emerged as a key principle controlling cellular biomolecular material organization and biological processes. Recent studies have revealed the involvements of phase separation in assembling meiotic chromosome-associated structures. Here we review and discuss how phase separation may participate in meiotic chromosome dynamics and propose that it may provide opportunities to understand the mysteries in meiotic regulations.

Spindle Assembly and Mitosis in Plants

In contrast to well-studied fungal and animal cells, plant cells assemble bipolar spindles that exhibit a great deal of plasticity in the absence of structurally defined microtubule-organizing centers like the centrosome. While plants employ some evolutionarily conserved proteins to regulate spindle morphogenesis and remodeling, many essential spindle assembly factors found in vertebrates are either missing or not required for producing the plant bipolar microtubule array. Plants also produce proteins distantly related to their fungal and animal counterparts to regulate critical events such as the spindle assembly checkpoint. Plant spindle assembly initiates with microtubule nucleation on the nuclear envelope followed by bipolarization into the prophase spindle. After nuclear envelope breakdown, kinetochore fibers are assembled and unified into the spindle apparatus with convergent poles. Of note, compared to fungal and animal systems, relatively little is known about how plant cells remodel the spindle microtubule array during anaphase. Uncovering mitotic functions of novel proteins for spindle assembly in plants will illuminate both common and divergent mechanisms employed by different eukaryotic organisms to segregate genetic materials.

Cytoskeletal form and function in mammalian oocytes and zygotes

The actin and microtubule cytoskeletons of mammalian oocytes and zygotes exist in distinct forms at various subcellular locations. This enables each cytoskeletal system to perform vastly different functions in time and space within the same cell. In recent years, key discovery enabling tools including light-sensitive microscopy assays have helped to illuminate cytoskeletal form and function in female reproductive cell biology. New findings include unexpected participation of F-actin in oocyte chromosome segregation, oocyte specific modes of spindle self-organization as well as existence of nuclear actin polymers whose functions are only starting to emerge. Functional actin-microtubule interactions have also been identified as an important feature that supports mammalian embryo development. Other advances have revealed reproductive age-related changes in chromosome structure and dynamics that predispose mammalian eggs to aneuploidy.

Aurora kinase genetic polymorphisms: an association study in Down syndrome and spontaneous abortion

Aneuploidies, such as Down syndrome (DS), are the leading cause of pregnancy loss. Abnormalities in aurora kinase proteins result in genomic instability and aneuploidy, mainly in tumors. Thus, polymorphisms in Aurora kinase genes could influence the occurrence of DS and spontaneous abortion. A case-control study was conducted including 124 mothers of DS children (DSM) and 219 control mothers (CM) to investigate DS risk according to AURKA and AURKC polymorphisms. Genotyping was performed using TaqMan real-time PCR. The minor allele frequency (MAF) observed in AURKA rs2273535 was, respectively, 0.23 in DSM and 0.20 in CM, whereas the frequency of the AURKC rs758099 T allele was 0.32 in case and 0.33 in control mothers. Statistical analysis showed no significant difference in the distribution of genotypes and allele frequencies between DSM and CM. According to previous history of spontaneous abortion, the AURKA rs2273535 genotypes (TT + AT vs. AA: OR 2.54, 95% CI 1.13-5.71, p = 0.02; AT vs. AA: OR 2.39, 95% CI 1.03-5.51, p = 0.04; T vs. A: OR 2.08, 95% CI 1.12-3.90, p = 0.02) and AURKC rs758099 (TT vs. CC: OR 4.34, 95% CI 1.03-18.02, p = 0.04; TT + CT vs. CC: OR 2.52, 95% CI 1.02-6.23, p = 0.04; T vs. C: OR 2.03, 95% CI 1.09-3.80, p = 0.02) were observed as risk factors for spontaneous abortion in case mothers. Our study suggests a possible relationship between AURKA/AURKC variants and increased risk of spontaneous abortion within Down syndrome mothers.

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.

A monoastral mitotic spindle determines lineage fate and position in the mouse embryo

During mammalian development, the first asymmetric cell divisions segregate cells into inner and outer positions of the embryo to establish the pluripotent and trophectoderm lineages. Typically, polarity components differentially regulate the mitotic spindle via astral microtubule arrays to trigger asymmetric division patterns. However, early mouse embryos lack centrosomes, the microtubule-organizing centres (MTOCs) that usually generate microtubule asters. Thus, it remains unknown whether spindle organization regulates lineage segregation. Here we find that heterogeneities in cell polarity in the early 8-cell-stage mouse embryo trigger the assembly of a highly asymmetric spindle organization. This spindle arises in an unusual modular manner, forming a single microtubule aster from an apically localized, non-centrosomal MTOC, before joining it to the rest of the spindle apparatus. When fully assembled, this 'monoastral' spindle triggers spatially asymmetric division patterns to segregate cells into inner and outer positions. Moreover, the asymmetric inheritance of spindle components causes differential cell polarization to determine pluripotent versus trophectoderm lineage fate.

Microtubule organizing centers regulate spindle positioning in mouse oocytes

During female meiosis I (MI), spindle positioning must be tightly regulated to ensure the fidelity of the first asymmetric division and faithful chromosome segregation. Although the role of F-actin in regulating these critical processes has been studied extensively, little is known about whether microtubules (MTs) participate in regulating these processes. Using mouse oocytes as a model system, we characterize a subset of MT organizing centers that do not contribute directly to spindle assembly, termed mcMTOCs. Using laser ablation, STED super-resolution microscopy, and chemical manipulation, we show that mcMTOCs are required to regulate spindle positioning and faithful chromosome segregation during MI. We discuss how forces exerted by F-actin on the spindle are balanced by mcMTOC-nucleated MTs to anchor the spindle centrally and to regulate its timely migration. Our findings provide a model for asymmetric cell division, complementing the current F-actin-based models, and implicate mcMTOCs as a major player in regulating spindle positioning.

Molecular determinants of phase separation for Drosophila DNA replication licensing factors

Liquid-liquid phase separation (LLPS) of intrinsically disordered regions (IDRs) in proteins can drive the formation of membraneless compartments in cells. Phase-separated structures enrich for specific partner proteins and exclude others. Previously, we showed that the IDRs of metazoan DNA replication initiators drive DNA-dependent phase separation in vitro and chromosome binding in vivo, and that initiator condensates selectively recruit replication-specific partner proteins (Parker et al., 2019). How initiator IDRs facilitate LLPS and maintain compositional specificity is unknown. Here, using Drosophila melanogaster (Dm) Cdt1 as a model initiation factor, we show that phase separation results from a synergy between electrostatic DNA-bridging interactions and hydrophobic inter-IDR contacts. Both sets of interactions depend on sequence composition (but not sequence order), are resistant to 1,6-hexanediol, and do not depend on aromaticity. These findings demonstrate that distinct sets of interactions drive condensate formation and specificity across different phase-separating systems and advance efforts to predict IDR LLPS propensity and partner selection a priori.

NuMA regulates mitotic spindle assembly, structural dynamics and function via phase separation

A functional mitotic spindle is essential for accurate chromosome congression and segregation during cell proliferation; however, the underlying mechanisms of its assembly remain unclear. Here we show that NuMA regulates this assembly process via phase separation regulated by Aurora A. NuMA undergoes liquid-liquid phase separation during mitotic entry and KifC1 facilitates NuMA condensates concentrating on spindle poles. Phase separation of NuMA is mediated by its C-terminus, whereas its dynein-dynactin binding motif also facilitates this process. Phase-separated NuMA droplets concentrate tubulins, bind microtubules, and enrich crucial regulators, including Kif2A, at the spindle poles, which then depolymerizes spindle microtubules and promotes poleward spindle microtubule flux for spindle assembly and structural dynamics. In this work, we show that NuMA orchestrates mitotic spindle assembly, structural dynamics and function via liquid-liquid phase separation regulated by Aurora A phosphorylation.

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.