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

AURKA controls oocyte spindle assembly checkpoint and chromosome alignment by HEC1 phosphorylation

In human oocytes, meiosis I is error-prone, causing early miscarriages and developmental disorders. The Aurora protein kinases are key regulators of chromosome segregation in mitosis and meiosis, and their dysfunction is associated with aneuploidy. Oocytes express three Aurora kinase (AURK) proteins, but only AURKA is necessary and sufficient to support oocyte meiosis in mice. However, the unique molecular contributions to ensuring high egg quality of AURKA remain unclear. Here, using a combination of genetic and pharmacological approaches, we evaluated how AURKA phosphorylation regulates outer kinetochore function during oocyte meiosis. We found that the outer kinetochore protein Ndc80/HEC1 is constitutively phosphorylated at multiple residues by Aurora kinases during meiosis I, but that serine 69 is specifically phosphorylated by AURKA in mouse and human oocytes. We further show that serine 69 phosphorylation contributes to spindle assembly checkpoint activation and chromosome alignment during meiosis I. These results provide a fundamental mechanistic understanding of how AURKA regulates meiosis and kinetochore function to ensure meiosis I fidelity.

Genetic interaction mapping of Aurora protein kinases in mouse oocytes

The Aurora Kinases (AURKs) are a family of serine-threonine protein kinases critical for cell division. Somatic cells express only AURKA and AURKB. However, mammalian germ cells and some cancer cells express all three isoforms. A major question in the field has been determining the molecular and cellular changes when cells express three instead of two aurora kinases. Using a systematic genetic approach involving different Aurora kinase oocyte-specific knockout combinations, we completed an oocyte-AURK genetic interaction map and show that one genomic copy of Aurka is necessary and sufficient to support female fertility and oocyte meiosis. We further confirm that AURKB and AURKC alone cannot compensate for AURKA. These results highlight the importance of AURKA in mouse oocytes, demonstrating that it is required for spindle formation and proper chromosome segregation. Surprisingly, a percentage of oocytes that lack AURKB can complete meiosis I, but the quality of those eggs is compromised, suggesting a role in AURKB to regulate spindle assembly checkpoint or control the cell cycle. Together with our previous studies, we wholly define the genetic interplay among the Aurora kinases and reinforce the importance of AURKA expression in oocyte meiosis.

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.

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.

Functional specialization of Aurora kinase homologs during oogenic meiosis in the tunicate Oikopleura dioica

A single Aurora kinase found in non-vertebrate deuterostomes is assumed to represent the ancestor of vertebrate Auroras A/B/C. However, the tunicate Oikopleura dioica, a member of the sister group to vertebrates, possesses two Aurora kinases (Aurora1 and Aurora2) that are expressed in proliferative cells and reproductive organs. Previously, we have shown that Aurora kinases relocate from organizing centers to meiotic nuclei and were enriched on centromeric regions as meiosis proceeds to metaphase I. Here, we assessed their respective functions in oogenic meiosis using dsRNA interferences. We found that Aurora1 (Aur1) was involved in meiotic spindle organization and chromosome congression, probably through the regulation of microtubule dynamics, whereas Aurora2 (Aur2) was crucial for chromosome condensation and meiotic spindle assembly. In vitro kinase assays showed that Aur1 and Aur2 had comparable levels of kinase activities. Using yeast two-hybrid library screening, we identified a few novel interaction proteins for Aur1, including c-Jun-amino-terminal kinase-interacting protein 4, cohesin loader Scc2, and mitochondrial carrier homolog 2, suggesting that Aur1 may have an altered interaction network and participate in the regulation of microtubule motors and cohesin complexes in O. dioica.

Initial spindle positioning at the oocyte center protects against incorrect kinetochore-microtubule attachment and aneuploidy in mice

Spindle positioning within the oocyte must be tightly regulated. In mice, the spindle is predominantly assembled at the oocyte center before its migration toward the cortex to achieve the highly asymmetric division, a characteristic of female meiosis. The significance of the initial central positioning of the spindle is largely unknown. We show that initial spindle positioning at the oocyte center is an insurance mechanism to avoid the premature exposure of the spindle to cortical CDC42 signaling, which perturbs proper kinetochore-microtubule attachments, leading to the formation of aneuploid gametes. These findings contribute to understanding why female gametes are notoriously associated with high rates of aneuploidy, the leading genetic cause of miscarriage and congenital abnormalities.

Distinct Aurora B pools at the inner centromere and kinetochore have different contributions to meiotic and mitotic chromosome segregation

Proper chromosome segregation depends on establishment of bioriented kinetochore-microtubule attachments, which often requires multiple rounds of release and reattachment. Aurora B and C kinases phosphorylate kinetochore proteins to release tensionless attachments. Multiple pathways recruit Aurora B/C to the centromere and kinetochore. We studied how these pathways contribute to anaphase onset timing and correction of kinetochore-microtubule attachments in budding yeast meiosis and mitosis. We find that the pool localized by the Bub1/Bub3 pathway sets the normal duration of meiosis and mitosis, in differing ways. Our meiosis data suggests that disruption of this pathway leads to PP1 kinetochore localization, which dephosphorylates Cdc20 for premature anaphase onset. For error correction, the Bub1/Bub3 and COMA pathways are individually important in meiosis but compensatory in mitosis. Finally, we find that the haspin and Bub1/3 pathways function together to ensure error correction in mouse oogenesis. Our results suggest that each recruitment pathway localizes spatially distinct kinetochore-localized Aurora B/C pools that function differently between meiosis and mitosis.

Multi-Photon Laser Ablation of Cytoplasmic Microtubule Organizing Centers in Mouse Oocytes

The fidelity of oocyte meiosis is critical for generating developmentally competent euploid eggs. In mammals, the oocyte undergoes a lengthy arrest at prophase I of the first meiotic division. After puberty and upon meiotic resumption, the nuclear membrane disassembles (nuclear envelope breakdown), and the spindle is assembled mainly at the oocyte center. Initial central spindle positioning is essential to protect against abnormal kinetochore-microtubule (MT) attachments and aneuploidy. The centrally positioned spindle migrates in a time-sensitive manner toward the cortex, and this is a necessary process to extrude a tiny polar body. In mitotic cells, spindle positioning relies on the interaction between centrosome-mediated astral MTs and the cell cortex. On the contrary, mouse oocytes lack classic centrosomes and, instead, contain numerous acentriolar MT organizing centers (MTOCs). At the metaphase I stage, mouse oocytes have two different sets of MTOCs: (1) MTOCs that are clustered and sorted to assemble spindle poles (polar MTOCs), and (2) metaphase cytoplasmic MTOCs (mcMTOCs) that remain in the cytoplasm and do not contribute directly to spindle formation but play a crucial role in regulating spindle positioning and timely spindle migration. Here, a multi-photon laser ablation method is described to selectively deplete endogenously labeled mcMTOCs in oocytes collected from Cep192-eGfp reporter mice. This method contributes to the understanding of the molecular mechanisms underlying spindle positioning and migration in mammalian oocytes.

Cohesin is required for meiotic spindle assembly independent of its role in cohesion in C. elegans

Accurate chromosome segregation requires a cohesin-mediated physical attachment between chromosomes that are to be segregated apart, and a bipolar spindle with microtubule plus ends emanating from exactly two poles toward the paired chromosomes. We asked whether the striking bipolar structure of C. elegans meiotic chromosomes is required for bipolarity of acentriolar female meiotic spindles by time-lapse imaging of mutants that lack cohesion between chromosomes. Both a spo-11 rec-8 coh-4 coh-3 quadruple mutant and a spo-11 rec-8 double mutant entered M phase with separated sister chromatids lacking any cohesion. However, the quadruple mutant formed an apolar spindle whereas the double mutant formed a bipolar spindle that segregated chromatids into two roughly equal masses. Residual non-cohesive COH-3/4-dependent cohesin on separated sister chromatids of the double mutant was sufficient to recruit haspin-dependent Aurora B kinase, which mediated bipolar spindle assembly in the apparent absence of chromosomal bipolarity. We hypothesized that cohesin-dependent Aurora B might activate or inhibit spindle assembly factors in a manner that would affect their localization on chromosomes and found that the chromosomal localization patterns of KLP-7 and CLS-2 correlated with Aurora B loading on chromosomes. These results demonstrate that cohesin is essential for spindle assembly and chromosome segregation independent of its role in sister chromatid cohesion.

Haspin participates in AURKB recruitment to centromeres and contributes to chromosome congression in male mouse meiosis

Chromosome segregation requires that centromeres properly attach to spindle microtubules. This essential step regulates the accuracy of cell division and therefore must be precisely regulated. One of the main centromeric regulatory signaling pathways is the Haspin-H3T3ph-chromosomal passenger complex (CPC) cascade, which is responsible for the recruitment of the CPC to the centromeres. In mitosis, Haspin kinase phosphorylates histone H3 at threonine 3 (H3T3ph), an essential epigenetic mark that recruits the CPC, whose catalytic component is Aurora B kinase. However, the centromeric Haspin-H3T3ph-CPC pathway remains largely uncharacterized in mammalian male meiosis. We have analyzed Haspin functions by either its chemical inhibition in cultured spermatocytes using LDN-192960, or the ablation of Haspin gene in Haspin-/-. Our studies suggest that Haspin kinase activity is required for proper chromosome congression during both meiotic divisions and for the recruitment of Aurora B and kinesin MCAK to meiotic centromeres. However, the absence of H3T3ph histone mark does not alter Borealin and SGO2 centromeric localization. These results add new and relevant information regarding the regulation of the Haspin-H3T3ph-CPC pathway and centromere function during meiosis.

Genetic analysis of C. elegans Haspin-like genes shows that hasp-1 plays multiple roles in the germline

Haspin is a histone kinase that promotes error-free chromosome segregation by recruiting the Chromosomal Passenger Complex (CPC) to mitotic and meiotic chromosomes. Haspin remains less well studied than other M-phase kinases and the models explaining Haspin function have been developed primarily in mitotic cells. Here, we generate strains containing new conditional or nonsense mutations in the C. elegans Haspin homologs hasp-1 and hasp-2 and characterize their phenotypes. We show that hasp-1 is responsible for all predicted functions of Haspin and that loss of function of hasp-1 using classical and conditional alleles produces defects in germline stem cell proliferation, spermatogenesis, and confirms its role in oocyte meiosis. Genetic analysis suggests hasp-1 acts downstream of the Polo-like kinase plk-2 and shows synthetic interactions between hasp-1 and two genes expected to promote recruitment of the CPC by a parallel pathway that depends on the kinase Bub1. This work adds to the growing understanding of Haspin function by characterizing a variety of roles in an intact animal.

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.

Aurora kinase B inhibits aurora kinase A to control maternal mRNA translation in mouse oocytes

Mammalian oocytes are transcriptionally quiescent, and meiosis and early embryonic divisions rely on translation of stored maternal mRNAs. Activation of these mRNAs is mediated by polyadenylation. Cytoplasmic polyadenylation binding element 1 (CPEB1) regulates mRNA polyadenylation. One message is aurora kinase C (Aurkc), encoding a protein that regulates chromosome segregation. We previously demonstrated that AURKC levels are upregulated in oocytes lacking aurora kinase B (AURKB), and this upregulation caused increased aneuploidy rates, a role we investigate here. Using genetic and pharmacologic approaches, we found that AURKB negatively regulates CPEB1-dependent translation of many messages. To determine why translation is increased, we evaluated aurora kinase A (AURKA), a kinase that activates CPEB1 in other organisms. We find that AURKA activity is increased in Aurkb knockout mouse oocytes and demonstrate that this increase drives the excess translation. Importantly, removal of one copy of Aurka from the Aurkb knockout strain background reduces aneuploidy rates. This study demonstrates that AURKA is required for CPEB1-dependent translation, and it describes a new AURKB requirement to maintain translation levels through AURKA, a function crucial to generating euploid eggs.

The oncogenic role of meiosis-specific Aurora kinase C in mitotic cells

Aberrant expression of meiosis-specific genes in cancer has recently emerged as a driver of some cancer formation. Aurora kinase C (AURKC) is a member of the Aurora kinase family of proteins known to regulate chromosome segregation during cell divisions. AURKC is normally expressed in meiotic cells; however, elevated levels of AURKC mRNA and protein are frequently measured in cancer cells. To understand the function of AURKC in cancer cells, expression was induced in noncancerous, human retina pigmented epithelial cells. While AURKC expression did not alter cell proliferation over 72 h, it did increase cell migration and anchorage independent growth in soft agar suggesting an oncogenic role in mitotically dividing cells. To evaluate AURKC as a potential therapeutic target, a frameshift mutation in the gene was introduced in U2OS osteosarcoma cells using CRISPR-Cas9 technology resulting in a premature stop codon. Cancer cells lacking AURKC displayed no change in cell proliferation over 72 h but did migrate less and formed fewer colonies in soft agar. Whole transcriptome sequencing analysis uncovered over 400 differentially expressed genes in U2OS cells with and without AURKC. GO analysis revealed alterations in proteinaceous extracellular matrix genes including COL1A1. These data indicate that therapeutics targeting AURKC could decrease cancer cell metastasis and disease progression. Because AURKC is transcriptionally silenced in normal mitotic cells, its disruption could specifically target cancer cells limiting the toxic side effects associated with current therapeutics.

Distinct roles of haspin in stem cell division and male gametogenesis

The kinase haspin phosphorylates histone H3 at threonine-3 (H3T3ph) during mitosis. H3T3ph provides a docking site for the Chromosomal Passenger Complex at the centromere, enabling correction of erratic microtubule-chromosome contacts. Although this mechanism is operational in all dividing cells, haspin-null mice do not exhibit developmental anomalies, apart from aberrant testis architecture. Investigating this problem, we show here that mouse embryonic stem cells that lack or overexpress haspin, albeit prone to chromosome misalignment during metaphase, can still divide, expand and differentiate. RNA sequencing reveals that haspin dosage affects severely the expression levels of several genes that are involved in male gametogenesis. Consistent with a role in testis-specific expression, H3T3ph is detected not only in mitotic spermatogonia and meiotic spermatocytes, but also in non-dividing cells, such as haploid spermatids. Similarly to somatic cells, the mark is erased in the end of meiotic divisions, but re-installed during spermatid maturation, subsequent to methylation of histone H3 at lysine-4 (H3K4me₃) and arginine-8 (H3R8me₂). These serial modifications are particularly enriched in chromatin domains containing histone H3 trimethylated at lysine-27 (H3K27me₃), but devoid of histone H3 trimethylated at lysine-9 (H3K9me₃). The unique spatio-temporal pattern of histone H3 modifications implicates haspin in the epigenetic control of spermiogenesis.

Phosphorylation of H3-Thr3 by Haspin Is Required for Primary Cilia Regulation

Primary cilia are commonly found on most quiescent, terminally differentiated cells and play a major role in the regulation of the cell cycle, cell motility, sensing, and cell–cell communication. Alterations in ciliogenesis and cilia maintenance are causative of several human diseases, collectively known as ciliopathies. A key determinant of primary cilia is the histone deacetylase HDAC6, which regulates their length and resorption and whose distribution is regulated by the death inducer-obliterator 3 (Dido3). Here, we report that the atypical protein kinase Haspin is a key regulator of cilia dynamics. Cells defective in Haspin activity exhibit longer primary cilia and a strong delay in cilia resorption upon cell cycle reentry. We show that Haspin is active in quiescent cells, where it phosphorylates threonine 3 of histone H3, a known mitotic Haspin substrate. Forcing Dido3 detachment from the chromatin prevents Haspin inhibition from impacting cilia dynamics, suggesting that Haspin activity is required for the relocalization of Dido3–HDAC6 to the basal body. Exploiting the zebrafish model, we confirmed the physiological relevance of this mechanism. Our observations shed light on a novel player, Haspin, in the mechanisms that govern the determination of cilia length and the homeostasis of mature cilia.

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.

Hexestrol Deteriorates Oocyte Quality via Perturbation of Mitochondrial Dynamics and Function

Hexestrol (HES) is a synthetic non-steroidal estrogen that was widely used illegally to boost the growth rate in livestock production and aquaculture. HES can also be transferred to humans from treated animals and the environment. HES has been shown to have an adverse effect on ovarian function and oogenesis, but the potential mechanism has not been clearly defined. To understand the potential mechanisms regarding how HES affect female ovarian function, we assessed oocyte quality by examining the critical events during oocyte maturation. We found that HES has an adverse effect on oocyte quality, indicated by the decreased capacity of oocyte maturation and early embryo development competency. Specifically, HES-exposed oocytes exhibited aberrant microtubule nucleation and spindle assembly, resulting in meiotic arrest. In addition, HES exposure disrupted mitochondrial distribution and the balance of mitochondrial fission and fusion, leading to aberrant mitochondrial membrane potential and accumulation of reactive oxygen species. Lastly, we found that HES exposure can increase cytosolic Ca²⁺ levels and induce DNA damage and early apoptosis. In summary, these results demonstrate that mitochondrial dysfunction and perturbation of normal mitochondrial fission and fusion dynamics could be major causes of reduced oocyte quality after HES exposure.

Compromised MPS1 Activity Induces Multipolar Spindle Formation in Oocytes From Aged Mares: Establishing the Horse as a Natural Animal Model to Study Age-Induced Oocyte Meiotic Spindle Instability

Aneuploidy originating during meiosis in oocytes is the major cause of reduced fertility, implantation failure and miscarriage in women beyond their mid-thirties. Loss of chromosome cohesion, and defective microtubule dynamics and spindle assembly are, in turn, the major contributors to the error-prone nature of chromosome segregation in the oocytes of older women. However, the underlying molecular defects are not well understood. Altered function of MPS1 and AURKC have been shown to induce multipolar spindle phenotypes in murine oocytes and cancer cells, however, their role in reproductive aging associated oocyte aneuploidy is not known. Although age-related gamete and embryonic aneuploidy has been studied in female rodents, the horse may be a more appropriate animal model. Similar to women, aged mares suffer from reduced fertility and an increased incidence of oocyte aneuploidy. Moreover, mares show a long interval (decades) to reproductive senescence and, unlike rodents but similar to women, horse oocytes assemble the meiotic spindle in a slow and unstable manner, independent of microtubule organizing centers. In this study we found that oocytes from aged mares have lower expression of mRNA for Mps1, Spc25 and AurkC than oocytes from young mares while gene expression for other meiosis regulators did not differ. To assess the ability of horse oocytes to correctly form a bipolar spindle, in vitro matured MII oocytes were allowed to re-form their spindle after nocodazole-induced microtubule depolymerization. To investigate the importance of MPS1 and AURKC function in spindle (re)assembly, various concentrations of a MPS1 inhibitor (MPS1i, Compound 5) or an AURK inhibitor (AURKi, ZM447439) were included after nocodazole washout. MII oocytes from aged mares showed a higher incidence of spindle abnormalities after exposure to MPS1i. In contrast, Aurora kinase inhibition severely impaired microtubule organization and spindle formation in all oocytes, irrespective of mare age. In conclusion, gene expression for the kinases Mps1, Spc25, and AurkC is reduced in oocytes from aged mares. Moreover, spindle (re)assembly in aged mares’ oocytes is more unstable when Mps1 is inhibited. Overall, this suggests that compromised Mps1 activity predisposes to meiotic spindle instability in aged mare oocytes. This spindle instability could predispose to chromosome segregation errors.

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.

Establishing correct kinetochore-microtubule attachments in mitosis and meiosis

Faithful chromosome segregation in mitosis and meiosis requires that chromosomes properly attach to spindle microtubules. Initial kinetochore-microtubule attachments are often incorrect and rely on error correction mechanisms to release improper attachments, allowing the formation of new attachments. Aurora B kinase and, in mammalian germ cells, Aurora C kinase function as the enzymatic component of the Chromosomal Passenger Complex (CPC), which localizes to the inner centromere/kinetochore and phosphorylates kinetochore proteins for microtubule release during error correction. In this review, we discuss recent findings of the molecular pathways that regulate the chromosomal localization of Aurora B and C kinases in human cell lines, mice, fission yeast, and budding yeast. We also discuss differences in the importance of localization pathways between mitosis and meiosis.

HASPIN kinase mediates histone deacetylation to regulate oocyte meiotic maturation in pigs

HASPIN kinase-catalyzed phosphorylation of histone H3 on threonine 3 (H3T3p) directs the activity and localization of chromosomal passenger complex (CPC) and spindle assembly checkpoint (SAC) to regulate chromosome condensation and segregation in both mitosis and meiosis. However, the function of HASPIN kinase in the meiotic maturation of porcine oocytes is not yet known. Here, we found that HASPIN mRNA is constantly expressed in porcine oocyte maturation and subsequent early embryo development. H3T3p is highly enriched on chromosomes at germinal vesicle breakdown (GVBD) stage and thereafter maintains a low level in progression through metaphase I (MI) to metaphase II (MII). Correspondingly, H3T3p was completely abolished in oocytes treated with an inhibitor of HASPIN kinase. Functionally, inhibition of HASPIN activity led to a significant reduction in the rate of oocyte meiotic maturation and the limited cumulus expansion. Additionally, HASPIN inhibition caused both spindle disorganization and chromosome misalignment in oocytes at MI and MII stage. Importantly, HASPIN inhibition severely prevented deacetylation of several highly conserved lysine (K) residues of histone H3 and H4 including H3K9, H3K14, H4K5, H4K8, H4K12 and H4K16 on the metaphase chromosomes during oocyte meiotic maturation. Taken together, these results demonstrate that HASPIN kinase regulates porcine oocyte meiotic maturation via modulating histone deacetylation.

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.

Tetraploidy causes chromosomal instability in acentriolar mouse embryos

Tetraploidisation is considered a common event in the evolution of chromosomal instability (CIN) in cancer cells. The current model for how tetraploidy drives CIN in mammalian cells is that a doubling of the number of centrioles that accompany the genome doubling event leads to multipolar spindle formation and chromosome segregation errors. By exploiting the unusual scenario of mouse blastomeres, which lack centrioles until the ~64-cell stage, we show that tetraploidy can drive CIN by an entirely distinct mechanism. Tetraploid blastomeres assemble bipolar spindles dictated by microtubule organising centres, and multipolar spindles are rare. Rather, kinetochore-microtubule turnover is altered, leading to microtubule attachment defects and anaphase chromosome segregation errors. The resulting blastomeres become chromosomally unstable and exhibit a dramatic increase in whole chromosome aneuploidies. Our results thus reveal an unexpected mechanism by which tetraploidy drives CIN, in which the acquisition of chromosomally-unstable microtubule dynamics contributes to chromosome segregation errors following tetraploidisation.

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.

Aurora A Protein Kinase: To the Centrosome and Beyond

Accurate chromosome segregation requires the perfect spatiotemporal rearrangement of the cellular cytoskeleton. Isolated more than two decades ago from Drosophila, Aurora A is a widespread protein kinase that plays key roles during cell division. Numerous studies have described the localisation of Aurora A at centrosomes, the mitotic spindle, and, more recently, at mitotic centromeres. In this review, we will summarise the cytoskeletal rearrangements regulated by Aurora A during cell division. We will also discuss the recent discoveries showing that Aurora A also controls not only the dynamics of the cortical proteins but also regulates the centromeric proteins, revealing new roles for this kinase during cell division.

Genetic Interactions between the Aurora Kinases Reveal New Requirements for AURKB and AURKC during Oocyte Meiosis

Errors in chromosome segregation during female meiosis I occur frequently, and aneuploid embryos account for 1/3 of all miscarriages in humans [1]. Unlike mitotic cells that require two Aurora kinase (AURK) homologs to help prevent aneuploidy (AURKA and AURKB), mammalian germ cells also require a third (AURKC) [2, 3]. AURKA is the spindle-pole-associated homolog, and AURKB/C are the chromosome-localized homologs. In mitosis, AURKB has essential roles as the catalytic subunit of the chromosomal passenger complex (CPC), regulating chromosome alignment, kinetochore-microtubule attachments, cohesion, the spindle assembly checkpoint, and cytokinesis [4, 5]. In mouse oocyte meiosis, AURKC takes over as the predominant CPC kinase [6], although the requirement for AURKB remains elusive [7]. In the absence of AURKC, AURKB compensates, making defining potential non-overlapping functions difficult [6, 8]. To investigate the role(s) of AURKB and AURKC in oocytes, we analyzed oocyte-specific Aurkb and Aurkc single- and double-knockout (KO) mice. Surprisingly, we find that double KO female mice are fertile. We demonstrate that, in the absence of AURKC, AURKA localizes to chromosomes in a CPC-dependent manner. These data suggest that AURKC prevents AURKA from localizing to chromosomes by competing for CPC binding. This competition is important for adequate spindle length to support meiosis I. We also describe a unique requirement for AURKB to negatively regulate AURKC to prevent aneuploidy. Together, our work reveals oocyte-specific roles for the AURKs in regulating each other's localization and activity. This inter-kinase regulation is critical to support wild-type levels of fecundity in female mice.

Cdk1 inactivation induces post-anaphase-onset spindle migration and membrane protrusion required for extreme asymmetry in mouse oocytes

Female meiotic divisions are extremely asymmetric, producing large oocytes and small polar bodies (PBs). In mouse oocytes, the spindle relocates to the cortex before anaphase of meiosis I (MI). It is presumed that by displacing the future midzone, pre-anaphase spindle repositioning alone ensures asymmetry. But how subsequent anaphase events might contribute to asymmetric PB extrusion (PBE) is unknown. Here, we find that inactivation of cyclin-dependent kinase 1 (Cdk1) induces anaphase and simultaneously triggers cytoplasmic formin-mediated F-actin polymerisation that propels the spindle into the cortex causing it to protrude while anaphase progresses. Significantly, if post-anaphase-onset spindle migration fails, protrusion and asymmetry are severely threatened even with intact pre-anaphase migration. Conversely, post-anaphase migration can completely compensate for failed pre-anaphase migration. These data identify a cell-cycle-triggered phase of spindle displacement occurring after anaphase-onset, which, by inducing protrusion, is necessary for extreme asymmetry in mouse oocytes and uncover a pathway for maximising unequal division.

CFP1 coordinates histone H3 lysine-4 trimethylation and meiotic cell cycle progression in mouse oocytes

Trimethylation of histone H3 on lysine-4 (H3K4me3) is associated with gene-regulatory elements, but its transcription-independent function in cell division is unclear. CxxC-finger protein-1 (CFP1) is a major mediator of H3K4 trimethylation in mouse oocytes. Here we report that oocyte-specific knockout of Cxxc1, inhibition of CFP1 function, or abrogation of H3K4 methylation in oocytes each causes a delay of meiotic resumption as well as metaphase I arrest owing to defective spindle assembly and chromosome misalignment. These phenomena are partially attributed to insufficient phosphorylation of histone H3 at threonine-3. CDK1 triggers cell division–coupled degradation and inhibitory phosphorylation of CFP1. Preventing CFP1 degradation and phosphorylation causes CFP1 accumulation on chromosomes and impairs meiotic maturation and preimplantation embryo development. Therefore, CFP1-mediated H3K4 trimethylation provides 3a permission signal for the G2–M transition. Dual inhibition of CFP1 removes the SETD1–CFP1 complex from chromatin and ensures appropriate chromosome configuration changes during meiosis and mitosis.

The transcription-independent function of trimethylation of histone H3 (H3K4me) in cell division is unclear. Here, Heng-Yu Fan and colleagues report that CFP1, a subunit of the H3K4 methyltransferase, is required for oocyte meiosis, being phosphorylated and degraded during cell cycle transition.

Aurora A Kinase Function at Kinetochores

One of the most important regulatory aspects of chromosome segregation is the ability of kinetochores to precisely control their attachment strength to spindle microtubules. Central to this regulation is Aurora B, a mitotic kinase that phosphorylates kinetochore substrates to promote microtubule turnover. A critical target of Aurora B is the kinetochore protein Ndc80/Hec1, which is a component of the NDC80 complex, the primary force-transducing link between kinetochores and microtubules. Although Aurora B is regarded as the "master regulator" of kinetochore-microtubule attachment, it is becoming clear that this kinase is not solely responsible for phosphorylating Hec1 and other kinetochore substrates to facilitate microtubule turnover. In particular, there is growing evidence that Aurora A kinase, whose activities at spindle poles have been extensively described, has additional roles at kinetochores in regulating the kinetochore-microtubule interface.

Maternal RNA regulates Aurora C kinase during mouse oocyte maturation in a translation-independent fashion

During oocyte meiotic maturation, Aurora kinase C (AURKC) is required to accomplish many critical functions including destabilizing erroneous kinetochore-microtubule (K-MT)attachments and regulating bipolar spindle assembly. How localized activity of AURKC is regulated in mammalian oocytes, however, is not fully understood. Female gametes from many species, including mouse, contain stores of maternal transcripts that are required for downstream developmental events. We show here that depletion of maternal RNA in mouse oocytes resulted in impaired meiotic progression, increased incidence of chromosome misalignment and abnormal spindle formation at metaphase I (Met I), and cytokinesis defects. Importantly, depletion of maternal RNA perturbed the localization and activity of AURKC within the chromosomal passenger complex (CPC). These perturbations were not observed when translation was inhibited by cycloheximide (CHX) treatment. These results demonstrate a translation-independent function of maternal RNA to regulate AURKC-CPC function in mouse oocytes.

Triple-Color Live Imaging of Mouse Oocytes

Meiotic division is a dynamic process that exhibits active interactive behaviors amongst different intracellular structures and components for spindle assembly and chromosome segregation. Understanding the mechanisms of meiotic spindle assembly and chromosome segregation therefore requires a quantitative analysis of spatiotemporal relationships among different structures and components. In this chapter, we describe a method for triple-color live imaging of meiotic division in mouse oocytes. This approach combines the microinjection of RNAs encoding proteins tagged with green and red fluorescent proteins and the visualization of microtubules with the fluorogenic far-red probe SiR-Tubulin. This method enables the simultaneous spatiotemporal mapping of three different components of the spindle and chromosomes, which opens the way to quantitative analysis of their interactive behaviors.

Haspin inhibition reveals functional differences of interchromatid axis-localized AURKB and AURKC

Use of mouse oocytes that only express Aurora kinase B as the catalytic subunit of the chromosomal passenger complex (CPC) provides evidence indicating differential capacities of AURKB– and AURKC–CPC complexes at a distinct localization.

Aneuploidy is the leading genetic abnormality contributing to infertility, and chromosome segregation errors are common during female mammalian meiosis I (MI). Previous results indicate that haspin kinase regulates resumption of meiosis from prophase arrest, chromosome condensation, and kinetochore–microtubule attachments during early prometaphase of MI. Here we report that haspin inhibition in late prometaphase I causes acceleration of MI, bypass of the spindle assembly checkpoint (SAC), and loss of interchromatid axis–localized Aurora kinase C. Meiotic cells contain a second chromosomal passenger complex (CPC) population, with Aurora kinase B (AURKB) bound to INCENP. Haspin inhibition in oocytes from Aurkc^−/− mice, where AURKB is the sole CPC kinase, does not alter MI completion timing, and no change in localization of the SAC protein, MAD2, is observed. These data suggest that AURKB on the interchromatid axis is not needed for SAC activation and illustrate a key difference between the functional capacities of the two AURK homologues.

Specialize and Divide (Twice): Functions of Three Aurora Kinase Homologs in Mammalian Oocyte Meiotic Maturation

The aurora kinases (AURKs) comprise an evolutionarily conserved family of serine/threonine kinases involved in mitosis and meiosis. While most mitotic cells express two AURK isoforms (AURKA and AURKB), mammalian germ cells also express a third, AURKC. Although much is known about the functions of the kinases in mitosis, less is known about how the three isoforms function to coordinate meiosis. This review is aimed at describing what is known about the three isoforms in female meiosis, the similarities and differences between kinase functions, and speculates as to why mammalian germ cells require expression of three AURKs instead of two.

The chromosomal basis of meiotic acentrosomal spindle assembly and function in oocytes

Several aspects of meiosis are impacted by the absence of centrosomes in oocytes. Here, we review four aspects of meiosis I that are significantly affected by the absence of centrosomes in oocyte spindles. One, microtubules tend to assemble around the chromosomes. Two, the organization of these microtubules into a bipolar spindle is directed by the chromosomes. Three, chromosome bi-orientation and attachment to microtubules from the correct pole require modification of the mechanisms used in mitotic cells. Four, chromosome movement to the poles at anaphase cannot rely on polar anchoring of spindle microtubules by centrosomes. Overall, the chromosomes are more active participants during acentrosomal spindle assembly in oocytes, compared to mitotic and male meiotic divisions where centrosomes are present. The chromosomes are endowed with information that can direct the meiotic divisions and dictate their own behavior in oocytes. Processes beyond those known from mitosis appear to be required for their bi-orientation at meiosis I. As mitosis occurs without centrosomes in many systems other than oocytes, including all plants, the concepts discussed here may not be limited to oocytes. The study of meiosis in oocytes has revealed mechanisms that are operating in mitosis and will probably continue to do so.