Aurora B/C-dependent phosphorylation promotes Rec8 cleavage in mammalian oocytes

The functional organisation of the centromere and kinetochore during meiosis

Meiosis generates gametes through a specialised cell cycle that reduces the genome by half. Homologous chromosomes are segregated in meiosis I and sister chromatids are segregated in meiosis II. Centromeres and kinetochores play central roles in instructing this specialised chromosome segregation pattern. Accordingly, kinetochores acquire meiosis-specific modifications. Here we contextualise recent highlights in our understanding of how centromeres and kinetochores direct the sorting of chromosomes into gametes via meiosis.

Hybrid female sterility due to cohesin protection errors in oocytes

Hybrid incompatibility can lead to lethality and sterility of F1 hybrids, contributing to speciation. Here we found that female hybrids between Mus musculus domesticus and Mus spicilegus mice are sterile due to the failure of homologous chromosome separation in oocyte meiosis I, producing aneuploid eggs. This non-separation phenotype was driven by the mis- localization of the cohesin protector, SGO2, along the chromosome arms instead of its typical centromeric enrichment, resulting in cohesin over-protection. The upstream kinase, BUB1, showed a significantly higher activity in hybrid oocytes, explaining SGO2 mis-targeting along the chromosome arm. Higher BUB1 activity was not observed in mitosis, consistent with viable hybrid mice. Cohesion defects were also evident in hybrid mice from another genus, Peromyscus , wherein cohesin protection is weakened. Defective cohesion in oocytes is a leading cause of reduced fertility especially with advanced maternal age. Our work provides evidence that a major cause of human infertility may play a positive role in promoting mammalian speciation.

Mutation of the SUMOylation site of Aurora-B disrupts spindle formation and chromosome alignment in oocytes

Aurora-B is a kinase that regulates spindle assembly and kinetochore-microtubule (KT-MT) attachment during mitosis and meiosis. SUMOylation is involved in the oocyte meiosis regulation through promoting spindle assembly and chromosome segregation, but its substrates to support this function is still unknown. It is reported that Aurora-B is SUMOylated in somatic cells, and SUMOylated Aurora-B contributes the process of mitosis. However, whether Aurora-B is SUMOylated in oocytes and how SUMOylation of Aurora-B impacts its function in oocyte meiosis remain poorly understood. In this study, we report that Aurora-B is modified by SUMOylation in mouse oocytes. The results show that Aurora-B colocalized and interacted with SUMO-2/3 in mouse oocytes, confirming that Aurora-B is modified by SUMO-2/3 in this system. Compared with that in young mice, the protein expression of SUMO-2/3 decreased in the oocytes of aged mice, indicating that SUMOylation might be related to mouse aging. Overexpression of Aurora-B SUMOylation site mutants, Aurora-BK207R and Aurora-BK292R, inhibited Aurora-B recruitment and first polar body extrusion, disrupting localization of gamma tubulin, spindle formation and chromosome alignment in oocytes. The results show that it was related to decreased recruitment of p-HDAC6 which induces the high stability of whole spindle microtubules including the microtubules of both correct and wrong KT-MT attachments though increased acetylation of microtubules. Therefore, our results corroborate the notion that Aurora-B activity is regulated by SUMO-2/3 in oocytes, and that SUMOylated Aurora B plays an important role in spindle formation and chromosome alignment.

The Molecular Mechanism of Aurora-B Regulating Kinetochore-Microtubule Attachment in Mitosis and Oocyte Meiosis

BACKGROUND

Aurora kinase B (Aurora-B), a member of the chromosomal passenger complex, is involved in correcting kinetochore-microtubule (KT-MT) attachment errors and regulating sister chromatid condensation and cytoplasmic division during mitosis.

SUMMARY

However, few reviews have discussed its mechanism in oocyte meiosis and the differences between its role in mitosis and meiosis. Therefore, in this review, we summarize the localization, recruitment, activation, and functions of Aurora-B in mitosis and oocyte meiosis. The accurate regulation of Aurora-B is essential for ensuring accurate chromosomal segregation and correct KT-MT attachments. Aurora-B regulates the stability of KT-MT attachments by competing with cyclin-dependent kinase 1 to control the phosphorylation of the SILK and RVSF motifs on kinetochore scaffold 1 and by competing with protein phosphatase 1 to influence the phosphorylation of NDC80 which is the substrate of Aurora-B. In addition, Aurora-B regulates the spindle assembly checkpoint by promoting the recruitment and activation of mitotic arrest deficient 2.

KEY MESSAGES

This review provides a theoretical foundation for elucidating the mechanism of cell division and understanding oocyte chromosomal aneuploidy.

Age-dependent loss of cohesion protection in human oocytes

Aneuploid human eggs (oocytes) are a major cause of infertility, miscarriage, and chromosomal disorders. Such aneuploidies increase greatly as women age, with defective linkages between sister chromatids (cohesion) in meiosis as a common cause. We found that loss of a specific pool of the cohesin protector protein, shugoshin 2 (SGO2), may contribute to this phenomenon. Our data indicate that SGO2 preserves sister chromatid cohesion in meiosis by protecting a "cohesin bridge" between sister chromatids. In human oocytes, SGO2 localizes to both sub-centromere cups and the pericentromeric bridge, which spans the sister chromatid junction. SGO2 normally colocalizes with cohesin; however, in meiosis II oocytes from older women, SGO2 is frequently lost from the pericentromeric bridge and sister chromatid cohesion is weakened. MPS1 and BUB1 kinase activities maintain SGO2 at sub-centromeres and the pericentromeric bridge. Removal of SGO2 throughout meiosis I by MPS1 inhibition reduces cohesion protection, increasing the incidence of single chromatids at meiosis II. Therefore, SGO2 deficiency in human oocytes can exacerbate the effects of maternal age by rendering residual cohesin at pericentromeres vulnerable to loss in anaphase I. Our data show that impaired SGO2 localization weakens cohesion integrity and may contribute to the increased incidence of aneuploidy observed in human oocytes with advanced maternal age.

The molecular mechanisms of human separase regulation

Sister chromatid segregation is the final irreversible step of mitosis. It is initiated by a complex regulatory system that ultimately triggers the timely activation of a conserved cysteine protease named separase. Separase cleaves the cohesin protein ring that links the sister chromatids and thus facilitates their separation and segregation to the opposite poles of the dividing cell. Due to the irreversible nature of this process, separase activity is tightly controlled in all eukaryotic cells. In this mini-review, we summarize the latest structural and functional findings on the regulation of separase, with an emphasis on the regulation of the human enzyme by two inhibitors, the universal inhibitor securin and the vertebrate-specific inhibitor CDK1-cyclin B. We discuss the two fundamentally different inhibitory mechanisms by which these inhibitors block separase activity by occluding substrate binding. We also describe conserved mechanisms that facilitate substrate recognition and point out open research questions that will guide studies of this fascinating enzyme for years to come.

A receptor-like kinase controls the amplitude of secondary cell wall synthesis in rice

Cell wall expansion is a key element in determining plant morphology and growth, and cell wall integrity changes are relayed to the cell to fine-tune growth responses. Here, we show that variations in the ectodomain of a cell wall-associated receptor-like kinase, WAK10, in temperate Oryza japonica accessions differentially amplify fluctuations in cell wall integrity to control rice stem height. Mutation in the WAK10 gene exhibited increased cell wall thickening in stem sclerenchyma and reduced cell expansion in the stem. Two WAK10 ectodomain variants bound pectic oligosaccharides with different affinities. The pectic oligosaccharide binding regulated WAK10 phosphorylation activity, the amplitude of secondary wall deposition, and ultimately, stem height. Rice population analyses revealed active enrichment of the short-stem WAK10 ectodomain alleles in japonica subspecies during domestication. Our study outlines not only a mechanism for how variations in ligand affinities of a receptor kinase control cell wall biosynthesis and plant growth, but it also provides breeding targets for new semi-dwarf rice cultivars.

Newfound features of meiotic chromosome organization that promote efficient congression and segregation in Caenorhabditis elegans oocytes

Although end-on microtubule-kinetochore attachments typically drive chromosome alignment, Caenorhabditis elegans oocytes do not form these connections. Instead, microtubule bundles run laterally alongside chromosomes and a ring-shaped protein complex facilitates congression (the "ring complex", RC). Here, we report new aspects of RC and chromosome structure that are required for congression and segregation. First, we found that in addition to encircling the outside of each homologous chromosome pair (bivalent), the RC also forms internal subloops that wrap around the domains where cohesion is lost during the first meiotic division; cohesin removal could therefore disengage these subloops in anaphase, enabling RC removal from chromosomes. Additionally, we discovered new features of chromosome organization that facilitate congression. Analysis of a mutant that forms bivalents with a fragile, unresolved homolog interface revealed that these bivalents are usually able to biorient on the spindle, with lateral microtubule bundles running alongside them and constraining the chromosome arms so that the two homologs are pointed to opposite spindle poles. This biorientation facilitates congression, as monooriented bivalents exhibited reduced polar ejection forces that resulted in congression defects. Thus, despite not forming end-on attachments, chromosome biorientation promotes congression in C. elegans oocytes. Our work therefore reveals novel features of chromosome organization in oocytes and highlights the importance of proper chromosome structure for faithful segregation during meiotic divisions.

Separase Control and Cohesin Cleavage in Oocytes: Should I Stay or Should I Go?

The key to gametogenesis is the proper execution of a specialized form of cell division named meiosis. Prior to the meiotic divisions, the recombination of maternal and paternal chromosomes creates new genetic combinations necessary for fitness and adaptation to an ever-changing environment. Two rounds of chromosome segregation -meiosis I and II- have to take place without intermediate S-phase and lead to the creation of haploid gametes harboring only half of the genetic material. Importantly, the segregation patterns of the two divisions are fundamentally different and require adaptation of the mitotic cell cycle machinery to the specificities of meiosis. Separase, the enzyme that cleaves Rec8, a subunit of the cohesin complex constituting the physical connection between sister chromatids, has to be activated twice: once in meiosis I and immediately afterwards, in meiosis II. Rec8 is cleaved on chromosome arms in meiosis I and in the centromere region in meiosis II. This step-wise cohesin removal is essential to generate gametes of the correct ploidy and thus, embryo viability. Hence, separase control and Rec8 cleavage must be perfectly controlled in time and space. Focusing on mammalian oocytes, this review lays out what we know and what we still ignore about this fascinating mechanism.

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.

A field guide to Aurora kinase inhibitors: an oocyte perspective

In brief

The Aurora protein kinases have critical functions in controlling oocyte meiotic maturation. In this study, we describe an assay for examining their activation state in oocytes and establish the best working doses of three commonly used inhibitors.

Abstract

Several small molecule inhibitors exist for targeting Aurora kinase proteins in somatic cells. From this point of view, we evaluate the specificity of these inhibitors in mouse oocytes, and we demonstrate that MLN 8237 and AZD 1152 are specific for Aurora kinase A and Aurora kinase C, respectively, only when used at low concentrations.

Protocol to measure cleavage efficiency of the meiotic cohesin subunit Rec8 by separase in mouse oocytes using a biosensor

Here, we describe a biosensor to assess meiotic cohesin subunit Rec8 cleavage in mouse oocytes. We detail oocyte collection and microinjection of the mRNA expressing the biosensor. The biosensor is targeted to chromosomes and consists of two fluorophores flanking a Rec8 fragment containing separase cleavage sites. Cleavage leads to dissociation of one fluorophore from chromosomes, and the efficiency can be estimated by live imaging. We detail the use of this biosensor in mouse oocytes with or without Aurora B/C inhibitor. For complete details on the use and execution of this protocol, please refer to Nikalayevich et al. (2022).