Studying meiotic cohesin in somatic cells reveals that Rec8-containing cohesin requires Stag3 to function and is regulated by Wapl and sororin

ATM signaling modulates cohesin behavior in meiotic prophase and proliferating cells

Cohesins are ancient and ubiquitous regulators of chromosome architecture and function, but their diverse roles and regulation remain poorly understood. During meiosis, chromosomes are reorganized as linear arrays of chromatin loops around a cohesin axis. This unique organization underlies homolog pairing, synapsis, double-stranded break induction, and recombination. We report that axis assembly in Caenorhabditis elegans is promoted by DNA-damage response (DDR) kinases that are activated at meiotic entry, even in the absence of DNA breaks. Downregulation of the cohesin-destabilizing factor WAPL-1 by ATM-1 promotes axis association of cohesins containing the meiotic kleisins COH-3 and COH-4. ECO-1 and PDS-5 also contribute to stabilizing axis-associated meiotic cohesins. Further, our data suggest that cohesin-enriched domains that promote DNA repair in mammalian cells also depend on WAPL inhibition by ATM. Thus, DDR and Wapl seem to play conserved roles in cohesin regulation in meiotic prophase and proliferating cells.

Meiosis-specific cohesin complexes display essential and distinct roles in mitotic embryonic stem cell chromosomes

BACKGROUND

Cohesin is a chromosome-associated SMC-kleisin complex that mediates sister chromatid cohesion, recombination, and most chromosomal processes during mitosis and meiosis. However, it remains unclear whether meiosis-specific cohesin complexes are functionally active in mitotic chromosomes.

RESULTS

Through high-resolution 3D-structured illumination microscopy (3D-SIM) and functional analyses, we report multiple biological processes associated with the meiosis-specific cohesin components, α-kleisin REC8 and STAG3, and the distinct loss of function of meiotic cohesin during the cell cycle of embryonic stem cells (ESCs). First, we show that STAG3 is required for the efficient localization of REC8 to the nucleus by interacting with REC8. REC8-STAG3-containing cohesin regulates topological properties of chromosomes and maintains sister chromatid cohesion. Second, REC8-cohesin has additional sister chromatid cohesion roles in concert with mitotic RAD21-cohesin on ESC chromosomes. SIM imaging of REC8 and RAD21 co-staining revealed that the two types of α-kleisin subunits exhibited distinct loading patterns along ESC chromosomes. Third, knockdown of REC8 or RAD21-cohesin not only leads to higher rates of premature sister chromatid separation and delayed replication fork progression, which can cause proliferation and developmental defects, but also enhances chromosome compaction by hyperloading of retinoblastoma protein-condensin complexes from the prophase onward.

CONCLUSIONS

Our findings indicate that the delicate balance between mitotic and meiotic cohesins may regulate ESC-specific chromosomal organization and the mitotic program.

Meiotic Cohesin and Variants Associated With Human Reproductive Aging and Disease

Successful human reproduction relies on the well-orchestrated development of competent gametes through the process of meiosis. The loading of cohesin, a multi-protein complex, is a key event in the initiation of mammalian meiosis. Establishment of sister chromatid cohesion via cohesin rings is essential for ensuring homologous recombination-mediated DNA repair and future proper chromosome segregation. Cohesin proteins loaded during female fetal life are not replenished over time, and therefore are a potential etiology of age-related aneuploidy in oocytes resulting in decreased fecundity and increased infertility and miscarriage rates with advancing maternal age. Herein, we provide a brief overview of meiotic cohesin and summarize the human genetic studies which have identified genetic variants of cohesin proteins and the associated reproductive phenotypes including primary ovarian insufficiency, trisomy in offspring, and non-obstructive azoospermia. The association of cohesion defects with cancer predisposition and potential impact on aging are also described. Expansion of genetic testing within clinical medicine, with a focus on cohesin protein-related genes, may provide additional insight to previously unknown etiologies of disorders contributing to gamete exhaustion in females, and infertility and reproductive aging in both men and women.

Analysis of STAG3 variants in Chinese non-obstructive azoospermia patients with germ cell maturation arrest

STAG3 is essential for male meiosis and testis of male Stag3^-/- mice shows the histopathological type of germ cell maturation arrest (MA). Whether variants of the STAG3 gene exist in Chinese idiopathic non-obstructive azoospermia (NOA) patients needs to be determined. We recruited 58 Chinese NOA men with MA who underwent testis biopsy and 192 fertile men as the control group. The 34 exons of the STAG3 gene were amplified using polymerase chain reaction (PCR) and sequenced. We identified eight novel single nucleotide polymorphisms (SNPs), including two missense SNPs (c.433T > C in exon2 and c.553A > G in exon3), three synonymous SNPs (c.539G > A, c.569C > T in exon3, and c.1176C > G in exon8), and three SNPs in introns. The allele and genotype frequencies of the novel and other SNPs have no significant differences between two groups. Our results indicated that variants in the coding sequence of the STAG3 gene were uncommon in NOA patients with MA in Chinese population. Future studies in large cohorts of different ethnic populations will be needed to determine the association between the STAG3 gene and NOA.

Kinetochore individualization in meiosis I is required for centromeric cohesin removal in meiosis II

Partitioning of the genome in meiosis occurs through two highly specialized cell divisions, named meiosis I and meiosis II. Step-wise cohesin removal is required for chromosome segregation in meiosis I, and sister chromatid segregation in meiosis II. In meiosis I, mono-oriented sister kinetochores appear as fused together when examined by high-resolution confocal microscopy, whereas they are clearly separated in meiosis II, when attachments are bipolar. It has been proposed that bipolar tension applied by the spindle is responsible for the physical separation of sister kinetochores, removal of cohesin protection, and chromatid separation in meiosis II. We show here that this is not the case, and initial separation of sister kinetochores occurs already in anaphase I independently of bipolar spindle forces applied on sister kinetochores, in mouse oocytes. This kinetochore individualization depends on separase cleavage activity. Crucially, without kinetochore individualization in meiosis I, bivalents when present in meiosis II oocytes separate into chromosomes and not sister chromatids. This shows that whether centromeric cohesin is removed or not is determined by the kinetochore structure prior to meiosis II.

Separase-triggered apoptosis enforces minimal length of mitosis

Prolonged mitosis often results in apoptosis¹. Shortened mitosis causes tumorigenic aneuploidy, but it is unclear whether it also activates the apoptotic machinery². Separase, a cysteine protease and trigger of all eukaryotic anaphases, has a caspase-like catalytic domain but has not previously been associated with cell death^3,4. Here we show that human cells that enter mitosis with already active separase rapidly undergo death in mitosis owing to direct cleavage of anti-apoptotic MCL1 and BCL-XL by separase. Cleavage not only prevents MCL1 and BCL-XL from sequestering pro-apoptotic BAK, but also converts them into active promoters of death in mitosis. Our data strongly suggest that the deadliest cleavage fragment, the C-terminal half of MCL1, forms BAK/BAX-like pores in the mitochondrial outer membrane. MCL1 and BCL-XL are turned into separase substrates only upon phosphorylation by NEK2A. Early mitotic degradation of this kinase is therefore crucial for preventing apoptosis upon scheduled activation of separase in metaphase. Speeding up mitosis by abrogation of the spindle assembly checkpoint results in a temporal overlap of the enzymatic activities of NEK2A and separase and consequently in cell death. We propose that NEK2A and separase jointly check on spindle assembly checkpoint integrity and eliminate cells that are prone to chromosome missegregation owing to accelerated progression through early mitosis.

Wapl releases Scc1-cohesin and regulates chromosome structure and segregation in mouse oocytes

Cohesin is essential for genome folding and inheritance. In somatic cells, these functions are both mediated by Scc1-cohesin, which in mitosis is released from chromosomes by Wapl and separase. In mammalian oocytes, cohesion is mediated by Rec8-cohesin. Scc1 is expressed but neither required nor sufficient for cohesion, and its function remains unknown. Likewise, it is unknown whether Wapl regulates one or both cohesin complexes and chromosome segregation in mature oocytes. Here, we show that Wapl is required for accurate meiosis I chromosome segregation, predominantly releases Scc1-cohesin from chromosomes, and promotes production of euploid eggs. Using single-nucleus Hi-C, we found that Scc1 is essential for chromosome organization in oocytes. Increasing Scc1 residence time on chromosomes by Wapl depletion leads to vermicelli formation and intra-loop structures but, unlike in somatic cells, does not increase loop size. We conclude that distinct cohesin complexes generate loops and cohesion in oocytes and propose that the same principle applies to all cell types and species.

Meiotic sex chromosome cohesion and autosomal synapsis are supported by Esco2

In mitotic cells, establishment of sister chromatid cohesion requires acetylation of the cohesin subunit SMC3 (acSMC3) by ESCO1 and/or ESCO2. Meiotic cohesin plays additional but poorly understood roles in the formation of chromosome axial elements (AEs) and synaptonemal complexes. Here, we show that levels of ESCO2, acSMC3, and the pro-cohesion factor sororin increase on meiotic chromosomes as homologs synapse. These proteins are less abundant on the largely unsynapsed sex chromosomes, whose sister chromatid cohesion appears weaker throughout the meiotic prophase. Using three distinct conditional Esco2 knockout mouse strains, we demonstrate that ESCO2 is essential for male gametogenesis. Partial depletion of ESCO2 in prophase I spermatocytes delays chromosome synapsis and further weakens cohesion along sex chromosomes, which show extensive separation of AEs into single chromatids. Unsynapsed regions of autosomes are associated with the sex chromatin and also display split AEs. This study provides the first evidence for a specific role of ESCO2 in mammalian meiosis, identifies a particular ESCO2 dependence of sex chromosome cohesion and suggests support of autosomal synapsis by acSMC3-stabilized cohesion.

Is age-related increase of chromosome segregation errors in mammalian oocytes caused by cohesin deterioration?

BACKGROUND

Mammalian oocytes initiate meiosis in fetal ovary and are arrested at dictyate stage in prophase I for a long period. It is known that incidence of chromosome segregation errors in oocytes increases with advancing age, but the molecular mechanism underlying this phenomenon has not been clarified.

METHODS

Cohesin, a multi-subunit protein complex, mediates sister chromatid cohesion in both mitosis and meiosis. In this review, molecular basis of meiotic chromosome cohesion and segregation is summarized. Further, the relationship between chromosome segregation errors and cohesin deterioration in aged oocytes is discussed.

RESULTS

Recent studies show that chromosome-associated cohesin decreases in an age-dependent manner in mouse oocytes. Furthermore, conditional knockout or activation of cohesin in oocytes indicates that only the cohesin expressed before premeiotic S phase can establish and maintain sister chromatic cohesion and that cohesin does not turnover during the dictyate arrest.

CONCLUSION

In mice, the accumulating evidence suggests that deterioration of cohesin due to the lack of turnover during dictyate arrest is one of the major causes of chromosome segregation errors in aged oocytes. However, whether the same is true in human remains elusive since even the deterioration of cohesin during dictyate arrest has not been demonstrated in human oocytes.

In-Frame Variants in STAG3 Gene Cause Premature Ovarian Insufficiency

Premature ovarian insufficiency (POI) is a severe clinical syndrome defined by ovarian dysfunction in women less than 40 years old who generally manifest with infertility, menstrual disturbance, elevated gonadotrophins, and low estradiol levels. STAG3 is considered a genetic aetiology of POI, which facilitates entry of REC8 into the nucleus of a cell and plays an essential role in gametogenesis. At present, only six truncated variants associated with POI have been reported; there have been no reports of an in-frame variant of STAG3 causing POI. In this study, two novel homozygous in-frame variants (c.877_885del, p.293_295del; c.891_893dupTGA, p.297_298insAsp) in STAG3 were identified in two sisters with POI from a five-generation consanguineous Han Chinese family. To evaluate the effects of these two variants, we performed fluorescence localization and co-immunoprecipitation analyses using in vitro cell model. The two variants were shown to be pathogenic, as neither STAG3 nor REC8 entered nuclei, and interactions between mutant STAG3 and REC8 or SMC1A were absent. To the best of our knowledge, this is the first report on in-frame variants of STAG3 that cause POI. This finding extends the spectrum of variants in STAG3 and sheds new light on the genetic origins of POI.

Meiotic prophase-like pathway for cleavage-independent removal of cohesin for chromosome morphogenesis

Sister chromatid cohesion is essential for chromosome segregation both in mitosis and meiosis. Cohesion between two chromatids is mediated by a protein complex called cohesin. The loading and unloading of the cohesin are tightly regulated during the cell cycle. In vertebrate cells, cohesin is released from chromosomes by two distinct pathways. The best characterized pathway occurs at the onset of anaphase, when the kleisin component of the cohesin is destroyed by a protease, separase. The cleavage of the cohesin by separase releases entrapped sister chromatids allowing anaphase to commence. In addition, prior to the metaphase-anaphase transition, most of cohesin is removed from chromosomes in a cleavage-independent manner. This cohesin release is referred to as the prophase pathway. In meiotic cells, sister chromatid cohesion is essential for the segregation of homologous chromosomes during meiosis I. Thus, it was assumed that the prophase pathway for cohesin removal from chromosome arms would be suppressed during meiosis to avoid errors in chromosome segregation. However, recent studies revealed the presence of a meiosis-specific prophase-like pathway for cleavage-independent removal of cohesin during late prophase I in different organisms. In budding yeast, the cleavage-independent removal of cohesin is mediated through meiosis-specific phosphorylation of cohesin subunits, Rec8, the meiosis-specific kleisin, and the yeast Wapl ortholog, Rad61/Wpl1. This pathway plays a role in chromosome morphogenesis during late prophase I, promoting chromosome compaction. In this review, we give an overview of the prophase pathway for cohesin dynamics during meiosis, which has a complex regulation leading to differentially localized populations of cohesin along meiotic chromosomes.

Meiotic gene activation in somatic and germ cell tumours

Background

Germ cell tumours are uniquely associated with the gametogenic tissues of males and females. A feature of these cancers is that they can express genes that are normally tightly restricted to meiotic cells. This aberrant gene expression has been used as an indicator that these cancer cells are attempting a programmed germ line event, meiotic entry. However, work in non‐germ cell cancers has also indicated that meiotic genes can become aberrantly activated in a wide range of cancer types and indeed provide functions that serve as oncogenic drivers. Here, we review the activation of meiotic factors in cancers and explore commonalities between meiotic gene activation in germ cell and non‐germ cell cancers.

Objectives

The objectives of this review are to highlight key questions relating to meiotic gene activation in germ cell tumours and to offer possible interpretations as to the biological relevance in this unique cancer type.

Materials and Methods

PubMed and the GEPIA database were searched for papers in English and for cancer gene expression data, respectively.

Results

We provide a brief overview of meiotic progression, with a focus on the unique mechanisms of reductional chromosome segregation in meiosis I. We then offer detailed insight into the role of meiotic chromosome regulators in non‐germ cell cancers and extend this to provide an overview of how this might relate to germ cell tumours.

Conclusions

We propose that meiotic gene activation in germ cell tumours might not indicate an unscheduled attempt to enter a full meiotic programme. Rather, it might simply reflect either aberrant activation of a subset of meiotic genes, with little or no biological relevance, or aberrant activation of a subset of meiotic genes as positive tumour evolutionary/oncogenic drivers. These postulates provide the provocation for further studies in this emerging field.

The cohesin complex in mammalian meiosis

Cohesin is an evolutionary conserved multi-protein complex that plays a pivotal role in chromosome dynamics. It plays a role both in sister chromatid cohesion and in establishing higher order chromosome architecture, in somatic and germ cells. Notably, the cohesin complex in meiosis differs from that in mitosis. In mammalian meiosis, distinct types of cohesin complexes are produced by altering the combination of meiosis-specific subunits. The meiosis-specific subunits endow the cohesin complex with specific functions for numerous meiosis-associated chromosomal events, such as chromosome axis formation, homologue association, meiotic recombination and centromeric cohesion for sister kinetochore geometry. This review mainly focuses on the cohesin complex in mammalian meiosis, pointing out the differences in its roles from those in mitosis. Further, common and divergent aspects of the meiosis-specific cohesin complex between mammals and other organisms are discussed.