CEP215 and AURKA regulate spindle pole focusing and aMTOC organization in mouse 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.

Single-cell multi-omics profiling of human preimplantation embryos identifies cytoskeletal defects during embryonic arrest

Human in vitro fertilized embryos exhibit low developmental capabilities, and the mechanisms that underlie embryonic arrest remain unclear. Here using a single-cell multi-omics sequencing approach, we simultaneously analysed alterations in the transcriptome, chromatin accessibility and the DNA methylome in human embryonic arrest due to unexplained reasons. Arrested embryos displayed transcriptome disorders, including a distorted microtubule cytoskeleton, increased genomic instability and impaired glycolysis, which were coordinated with multiple epigenetic reprogramming defects. We identified Aurora A kinase (AURKA) repression as a cause of embryonic arrest. Mechanistically, arrested embryos induced through AURKA inhibition resembled the reprogramming abnormalities of natural embryonic arrest in terms of the transcriptome, the DNA methylome, chromatin accessibility and H3K4me3 modifications. Mitosis-independent sequential activation of the zygotic genome in arrested embryos showed that YY1 contributed to human major zygotic genome activation. Collectively, our study decodes the reprogramming abnormalities and mechanisms of human embryonic arrest and the key regulators of zygotic genome activation.

Pericentric major satellite transcription is essential for meiotic chromosome stability and spindle pole organization

In somatic cells, mitotic transcription of major satellite non-coding RNAs is tightly regulated and essential for heterochromatin formation and the maintenance of genome integrity. We recently demonstrated that major satellite transcripts are expressed, and chromatin-bound during mouse oocyte meiosis. Pericentric satellite RNAs are also expressed in human oocytes. However, the specific biological function(s) during oocyte meiosis remain to be established. Here, we use validated locked nucleic acid gapmers for major satellite RNA depletion followed by live cell imaging, and superresolution analysis to determine the role of pericentric non-coding RNAs during female meiosis. Depletion of satellite RNA induces mesoscale changes in pericentric heterochromatin structure leading to chromosome instability, kinetochore attachment errors and abnormal chromosome alignment. Chromosome misalignment is associated with spindle defects, microtubule instability and, unexpectedly, loss of acentriolar microtubule organizing centre (aMTOC) tethering to spindle poles. Pericentrin fragmentation and failure to assemble ring-like aMTOCs with loss of associated polo-like kinase 1 provide critical insight into the mechanisms leading to impaired spindle pole integrity. Inhibition of transcription or RNA splicing phenocopies the chromosome alignment errors and spindle defects, suggesting that pericentric transcription during oocyte meiosis is required to regulate heterochromatin structure, chromosome segregation and maintenance of spindle organization.

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.

Effects of Ran-GTP/importin β inhibition on the meiotic division of porcine oocytes

The Ran-GTP/importin β pathway has been implicated in a diverse array of mitotic functions in somatic mitosis; however, the possible meiotic roles of Ran-GTP/importin β in mammalian oocyte meiosis are still not fully understood. In the present study, importazole (IPZ), a small molecule inhibitor of the interaction between Ran and importin β was used to explore the potential meiotic roles of Ran-GTP/importin β in porcine oocytes undergoing meiosis. After IPZ treatment, the extrusion rate of the first polar body (PB1) was significantly decreased, and a higher proportion of the oocytes were arrested at the germinal vesicle breakdown (GVBD) stage. Moreover, IPZ treatment led to severe defects in metaphase I (MI) spindle assembly and chromosome alignment during the germinal vesicle (GV)-to-MI stage, as well as failure of metaphase II (MII) spindle reassembly and homologous chromosome segregation during the MI-to-MII stage. Notably, IPZ treatment decreased TPX2 expression and abnormal subcellular localization. Furthermore, the expression levels of aurora kinase A (AURKA) and transforming acidic coiled-coil 3 (TACC3) were significantly reduced after IPZ treatment. Collectively, these data indicate that the interaction of Ran-GTP and importin β is essential for proper spindle assembly and successful chromosome segregation during two consecutive meiotic divisions in porcine oocytes, and regulation of this complex might be related to its effect on the TPX2 signaling cascades.

Predicting embryonic aneuploidy rate in IVF patients using whole-exome sequencing

Infertility is a major reproductive health issue that affects about 12% of women of reproductive age in the United States. Aneuploidy in eggs accounts for a significant proportion of early miscarriage and in vitro fertilization failure. Recent studies have shown that genetic variants in several genes affect chromosome segregation fidelity and predispose women to a higher incidence of egg aneuploidy. However, the exact genetic causes of aneuploid egg production remain unclear, making it difficult to diagnose infertility based on individual genetic variants in mother's genome. In this study, we evaluated machine learning-based classifiers for predicting the embryonic aneuploidy risk in female IVF patients using whole-exome sequencing data. Using two exome datasets, we obtained an area under the receiver operating curve of 0.77 and 0.68, respectively. High precision could be traded off for high specificity in classifying patients by selecting different prediction score cutoffs. For example, a strict prediction score cutoff of 0.7 identified 29% of patients as high-risk with 94% precision. In addition, we identified MCM5, FGGY, and DDX60L as potential aneuploidy risk genes that contribute the most to the predictive power of the model. These candidate genes and their molecular interaction partners are enriched for meiotic-related gene ontology categories and pathways, such as microtubule organizing center and DNA recombination. In summary, we demonstrate that sequencing data can be mined to predict patients' aneuploidy risk thus improving clinical diagnosis. The candidate genes and pathways we identified are promising targets for future aneuploidy studies.

Frequent Spindle Assembly Errors Require Structural Rearrangement to Complete Meiosis in Zea mays

The success of an organism is contingent upon its ability to faithfully pass on its genetic material. In the meiosis of many species, the process of chromosome segregation requires that bipolar spindles be formed without the aid of dedicated microtubule organizing centers, such as centrosomes. Here, we describe detailed analyses of acentrosomal spindle assembly and disassembly in time-lapse images, from live meiotic cells of Zea mays. Microtubules organized on the nuclear envelope with a perinuclear ring structure until nuclear envelope breakdown, at which point microtubules began bundling into a bipolar form. However, the process and timing of spindle assembly was highly variable, with frequent assembly errors in both meiosis I and II. Approximately 61% of cells formed incorrect spindle morphologies, with the most prevalent being tripolar spindles. The erroneous spindles were actively rearranged to bipolar through a coalescence of poles before proceeding to anaphase. Spindle disassembly occurred as a two-state process with a slow depolymerization, followed by a quick collapse. The results demonstrate that maize meiosis I and II spindle assembly is remarkably fluid in the early assembly stages, but otherwise proceeds through a predictable series of events.

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.

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.

Human centrosome organization and function in interphase and mitosis

Centrosomes were first described by Edouard Van Beneden and named and linked to chromosome segregation by Theodor Boveri around 1870. In the 1960-1980s, electron microscopy studies have revealed the remarkable ultrastructure of a centriole -- a nine-fold symmetrical microtubular assembly that resides within a centrosome and organizes it. Less than two decades ago, proteomics and genomic screens conducted in multiple species identified hundreds of centriole and centrosome core proteins and revealed the evolutionarily conserved nature of the centriole assembly pathway. And now, super resolution microscopy approaches and improvements in cryo-tomography are bringing an unparalleled nanoscale-detailed picture of the centriole and centrosome architecture. In this chapter, we summarize the current knowledge about the architecture of human centrioles. We discuss the structured organization of centrosome components in interphase, focusing on localization/function relationship. We discuss the process of centrosome maturation and mitotic spindle pole assembly in centriolar and acentriolar cells, emphasizing recent literature.

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.

The impact of transcription inhibition during in vitro maturation on the proteome of bovine oocytes†

Proper oocyte maturation is a prerequisite for successful reproduction and requires the resumption of meiosis to the metaphase II stage (MII). In bovine oocytes, nuclear maturation has been shown to occur in in vitro maturing cumulus-enclosed oocytes (COCs) in the absence of transcription, but their developmental capacity is reduced compared to transcriptionally competent COCs. To assess the impact of transcription during in vitro maturation of bovine COCs on the quantitative oocyte proteome, a holistic nano-LC-MS/MS analysis of germinal vesicle oocytes and MII oocytes matured with or without addition of the transcription inhibitor actinomycin D (ActD) was carried out. Analyzing eight biological replicates for each of the three groups, a total of 2018 proteins was identified. These could be clearly classified into proteins depending or not depending on transcription during oocyte maturation. Proteins whose abundance increased after maturation irrespective of transcription inhibition - and hence independent of transcription - were related to the cell cycle, reflecting the progression of meiosis, and to cellular component organization, which is crucial for cytoplasmic maturation. In contrast, transcription-dependent proteins were associated with cell-cell adhesion and translation. Since a high rate of protein synthesis in oocytes has been shown to correlate with their developmental competence, oocyte maturation in transcriptionally impaired COCs is apparently disturbed. Our experiments reveal that impaired transcription during in vitro maturation of COCs has a substantial effect on specific components of the oocyte proteome, and that transcription is required for specific classes of oocyte proteins predominantly involved in translation.