Absence of mouse REC8 cohesin promotes synapsis of sister chromatids in meiosis

Cell-autonomous requirement for mammalian target of rapamycin (Mtor) in spermatogonial proliferation and differentiation in the mouse†

Spermatogonial stem cells must balance self-renewal with production of transit-amplifying progenitors that differentiate in response to retinoic acid (RA) before entering meiosis. This self-renewal vs. differentiation fate decision is critical for maintaining tissue homeostasis, as imbalances cause defects that can lead to human testicular cancer or infertility. Little is currently known about the program of differentiation initiated by RA, and the pathways and proteins involved are poorly defined. We recently found that RA stimulation of the Phosphatidylinositol 3-kinase (PI3K)/AKT/Mammalian target of rapamycin (mTOR) kinase signaling pathway is required for differentiation, and that short-term inhibition of mTOR complex 1 (mTORC1) by rapamycin blocked spermatogonial differentiation in vivo and prevented RA-induced translational activation. Since this phenotype resulted from global inhibition of mTORC1, we created conditional germ cell knockout mice to investigate the germ cell-autonomous role of MTOR in spermatogonial differentiation. MTOR germ cell KO mice were viable and healthy, but testes from neonatal (postnatal day (P)8), juvenile (P18), and adult (P > 60) KO mice were smaller than littermate controls, and no sperm were produced in adult testes. Histological and immunostaining analyses revealed that spermatogonial differentiation was blocked, and no spermatocytes were formed at any of the ages examined. Although spermatogonial proliferation was reduced in the neonatal testis, it was blocked altogether in the juvenile and adult testis. Importantly, a small population of self-renewing undifferentiated spermatogonia remained in adult testes. Taken together, these results reveal that MTOR is dispensable for the maintenance of undifferentiated spermatogonia, but is cell autonomously required for their proliferation and differentiation.

Impact of Maternal Age on Oocyte and Embryo Competence

The overall success of human reproduction, either spontaneously or after IVF, is highly dependent upon maternal age. The main reasons for age-related infertility include reduced ovarian reserve and decreased oocyte/embryo competence due to aging insults, especially concerning an increased incidence of aneuploidies and possibly decreased mitochondrial activity. Age-related chromosomal abnormalities mainly arise because of meiotic impairments during oogenesis, following flawed chromosome segregation patterns such as non-disjunction, premature separation of sister chromatids, or the recent reverse segregation. In this review, we briefly discuss the main mechanisms putatively impaired by aging in the oocytes and the deriving embryos. We also report the main strategies proposed to improve the management of advanced maternal age women in IVF: fertility preservation through oocyte cryopreservation to prevent aging; optimization of the ovarian stimulation and enhancement of embryo selection to limit its effects; and oocyte donation to circumvent its consequences.

The impact of recombination on human mutation load and disease

Recombination promotes genomic integrity among cells and tissues through double-strand break repair, and is critical for gamete formation and fertility through a strict regulation of the molecular mechanisms associated with proper chromosomal disjunction. In humans, congenital defects and recurrent structural abnormalities can be attributed to aberrant meiotic recombination. Moreover, mutations affecting genes involved in recombination pathways are directly linked to pathologies including infertility and cancer. Recombination is among the most prominent mechanism shaping genome variation, and is associated with not only the structuring of genomic variability, but is also tightly linked with the purging of deleterious mutations from populations. Together, these observations highlight the multiple roles of recombination in human genetics: its ability to act as a major force of evolution, its molecular potential to maintain genome repair and integrity in cell division and its mutagenic cost impacting disease evolution.This article is part of the themed issue 'Evolutionary causes and consequences of recombination rate variation in sexual organisms'.

Regulation of chromosome segregation in oocytes and the cellular basis for female meiotic errors

BACKGROUND

Meiotic chromosome segregation in human oocytes is notoriously error-prone, especially with ageing. Such errors markedly reduce the reproductive chances of increasing numbers of women embarking on pregnancy later in life. However, understanding the basis for these errors is hampered by limited access to human oocytes.

OBJECTIVE AND RATIONALE

Important new discoveries have arisen from molecular analyses of human female recombination and aneuploidy along with high-resolution analyses of human oocyte maturation and mouse models. Here, we review these findings to provide a contemporary picture of the key players choreographing chromosome segregation in mammalian oocytes and the cellular basis for errors.

SEARCH METHODS

A search of PubMed was conducted using keywords including meiosis, oocytes, recombination, cohesion, cohesin complex, chromosome segregation, kinetochores, spindle, aneuploidy, meiotic cell cycle, spindle assembly checkpoint, anaphase-promoting complex, DNA damage, telomeres, mitochondria, female ageing and female fertility. We extracted papers focusing on mouse and human oocytes that best aligned with the themes of this review and that reported transformative and novel discoveries.

OUTCOMES

Meiosis incorporates two sequential rounds of chromosome segregation executed by a spindle whose component microtubules bind chromosomes via kinetochores. Cohesion mediated by the cohesin complex holds chromosomes together and should be resolved at the appropriate time, in a specific step-wise manner and in conjunction with meiotically programmed kinetochore behaviour. In women, the stage is set for meiotic error even before birth when female-specific crossover maturation inefficiency leads to the formation of at-risk recombination patterns. In adult life, multiple co-conspiring factors interact with at-risk crossovers to increase the likelihood of mis-segregation. Available evidence support that these factors include, but are not limited to, cohesion deterioration, uncoordinated sister kinetochore behaviour, erroneous microtubule attachments, spindle instability and structural chromosomal defects that impact centromeres and telomeres. Data from mice indicate that cohesin and centromere-specific histones are long-lived proteins in oocytes. Since these proteins are pivotal for chromosome segregation, but lack any obvious renewal pathway, their deterioration with age provides an appealing explanation for at least some of the problems in older oocytes.

WIDER IMPLICATIONS

Research in the mouse model has identified a number of candidate genes and pathways that are important for chromosome segregation in this species. However, many of these have not yet been investigated in human oocytes so it is uncertain at this stage to what extent they apply to women. The challenge for the future involves applying emerging knowledge of female meiotic molecular regulation towards improving clinical fertility management.

Centromere pairing precedes meiotic chromosome pairing in plants

Meiosis is a specialized eukaryotic cell division, in which diploid cells undergo a single round of DNA replication and two rounds of nuclear division to produce haploid gametes. In most eukaryotes, the core events of meiotic prophase I are chromosomal pairing, synapsis and recombination. To ensure accurate chromosomal segregation, homologs have to identify and align along each other at the onset of meiosis. Although much progress has been made in elucidating meiotic processes, information on the mechanisms underlying chromosome pairing is limited in contrast to the meiotic recombination and synapsis events. Recent research in many organisms indicated that centromere interactions during early meiotic prophase facilitate homologous chromosome pairing, and functional centromere is a prerequisite for centromere pairing such as in maize. Here, we summarize the recent achievements of chromosome pairing research on plants and other organisms, and outline centromere interactions, nuclear chromosome orientation, and meiotic cohesin, as main determinants of chromosome pairing in early meiotic prophase.

Age-Related Loss of Cohesion: Causes and Effects

Aneuploidy is a leading genetic cause of birth defects and lower implantation rates in humans. Most errors in chromosome number originate from oocytes. Aneuploidy in oocytes increases with advanced maternal age. Recent studies support the hypothesis that cohesion deterioration with advanced maternal age represents a leading cause of age-related aneuploidy. Cohesin generates cohesion, and is established only during the premeiotic S phase of fetal development without any replenishment throughout a female's period of fertility. Cohesion holds sister chromatids together until meiosis resumes at puberty, and then chromosome segregation requires the release of sister chromatid cohesion from chromosome arms and centromeres at anaphase I and anaphase II, respectively. The time of cohesion cleavage plays an important role in correct chromosome segregation. This review focuses specifically on the causes and effects of age-related cohesion deterioration in female meiosis.

Mechanisms controlling germline cyst breakdown and primordial follicle formation

In fetal females, oogonia proliferate immediately after sex determination. The progress of mitosis in oogonia proceeds so rapidly that the incompletely divided cytoplasm of the sister cells forms cysts. The oogonia will then initiate meiosis and arrest at the diplotene stage of meiosis I, becoming oocytes. Within each germline cyst, oocytes with Balbiani bodies will survive after cyst breakdown (CBD). After CBD, each oocyte is enclosed by pre-granulosa cells to form a primordial follicle (PF). Notably, the PF pool formed perinatally will be the sole lifelong oocyte source of a female. Thus, elucidating the mechanisms of CBD and PF formation is not only meaningful for solving mysteries related to ovarian development but also contributes to the preservation of reproduction. However, the mechanisms that regulate these phenomena are largely unknown. This review summarizes the progress of cellular and molecular research on these processes in mice and humans.

Environmental influences on ovarian dysgenesis - developmental windows sensitive to chemical exposures

A woman's reproductive health and ability to have children directly affect numerous aspects of her life, from personal well-being and socioeconomic standing, to morbidity and lifespan. In turn, reproductive health depends on the development of correctly functioning ovaries, a process that starts early during fetal life. Early disruption to ovarian programming can have long-lasting consequences, potentially manifesting as disease much later in adulthood. A growing body of evidence suggests that exposure to chemicals early in life, including endocrine-disrupting chemicals, can cause a range of disorders later in life, such as those described in the ovarian dysgenesis syndrome hypothesis. In this Review, we discuss four specific time windows during which the ovary is particularly sensitive to disruption by exogenous insults: gonadal sex determination, meiotic division, follicle assembly and the first wave of follicle recruitment. To date, most evidence points towards the germ cell lineage being the most vulnerable to chemical exposure, particularly meiotic division and follicle assembly. Environmental chemicals and pharmaceuticals, such as bisphenols or mild analgesics (including paracetamol), can also affect the somatic cell lineages. This Review summarizes our current knowledge pertaining to environmental chemicals and pharmaceuticals, and their potential contributions to the development of ovarian dysgenesis syndrome. We also highlight knowledge gaps that need addressing to safeguard female reproductive health.

Differential cohesin loading marks paired and unpaired regions of platypus sex chromosomes at prophase I

Cohesins are vital for chromosome organisation during meiosis and mitosis. In addition to the important function in sister chromatid cohesion, these complexes play key roles in meiotic recombination, DSB repair, homologous chromosome pairing and segregation. Egg-laying mammals (monotremes) feature an unusually complex sex chromosome system, which raises fundamental questions about organisation and segregation during meiosis. We discovered a dynamic and differential accumulation of cohesins on sex chromosomes during platypus prophase I and specific reorganisation of the sex chromosome complex around a large nucleolar body. Detailed analysis revealed a differential loading of SMC3 on the chromatin and chromosomal axis of XY shared regions compared with the chromatin and chromosomal axes of asynapsed X and Y regions during prophase I. At late prophase I, SMC3 accumulation is lost from both the chromatin and chromosome axes of the asynaptic regions of the chain and resolves into subnuclear compartments. This is the first report detailing unpaired DNA specific SMC3 accumulation during meiosis in any species and allows speculation on roles for cohesin in monotreme sex chromosome organisation and segregation.

Reanalysis of BRCA1/2 negative high risk ovarian cancer patients reveals novel germline risk loci and insights into missing heritability

While up to 25% of ovarian cancer (OVCA) cases are thought to be due to inherited factors, the majority of genetic risk remains unexplained. To address this gap, we sought to identify previously undescribed OVCA risk variants through the whole exome sequencing (WES) and candidate gene analysis of 48 women with ovarian cancer and selected for high risk of genetic inheritance, yet negative for any known pathogenic variants in either BRCA1 or BRCA2. In silico SNP analysis was employed to identify suspect variants followed by validation using Sanger DNA sequencing. We identified five pathogenic variants in our sample, four of which are in two genes featured on current multi-gene panels; (RAD51D, ATM). In addition, we found a pathogenic FANCM variant (R1931*) which has been recently implicated in familial breast cancer risk. Numerous rare and predicted to be damaging variants of unknown significance were detected in genes on current commercial testing panels, most prominently in ATM (n = 6) and PALB2 (n = 5). The BRCA2 variant p.K3326*, resulting in a 93 amino acid truncation, was overrepresented in our sample (odds ratio = 4.95, p = 0.01) and coexisted in the germline of these women with other deleterious variants, suggesting a possible role as a modifier of genetic penetrance. Furthermore, we detected loss of function variants in non-panel genes involved in OVCA relevant pathways; DNA repair and cell cycle control, including CHEK1, TP53I3, REC8, HMMR, RAD52, RAD1, POLK, POLQ, and MCM4. In summary, our study implicates novel risk loci as well as highlights the clinical utility for retesting BRCA1/2 negative OVCA patients by genomic sequencing and analysis of genes in relevant pathways.

Sororin is enriched at the central region of synapsed meiotic chromosomes

During meiotic prophase, cohesin complexes mediate cohesion between sister chromatids and promote pairing and synapsis of homologous chromosomes. Precisely how the activity of cohesin is controlled to promote these events is not fully understood. In metazoans, cohesion establishment between sister chromatids during mitotic divisions is accompanied by recruitment of the cohesion-stabilizing protein Sororin. During somatic cell division cycles, Sororin is recruited in response to DNA replication-dependent modification of the cohesin complex by ESCO acetyltransferases. How Sororin is recruited and acts in meiosis is less clear. Here, we have surveyed the chromosomal localization of Sororin and its relationship to the meiotic cohesins and other chromatin modifiers with the objective of determining how Sororin contributes to meiotic chromosome dynamics. We show that Sororin localizes to the cores of meiotic chromosomes in a manner that is dependent on synapsis and the synaptonemal complex protein SYCP1. In contrast, cohesin, with which Sororin interacts in mitotic cells, shows axial enrichment on meiotic chromosomes even in the absence of synapsis between homologs. Using high-resolution microscopy, we show that Sororin is localized to the central region of the synaptonemal complex. These results indicate that Sororin regulation during meiosis is distinct from its regulation in mitotic cells and may suggest that it interacts with a distinctly different partner to ensure proper chromosome dynamics in meiosis.

SMC5/6 is required for the formation of segregation-competent bivalent chromosomes during meiosis I in mouse oocytes

SMC complexes include three major classes: cohesin, condensin and SMC5/6. However, the localization pattern and genetic requirements for the SMC5/6 complex during mammalian oogenesis have not previously been examined. In mouse oocytes, the SMC5/6 complex is enriched at the pericentromeric heterochromatin, and also localizes along chromosome arms during meiosis. The infertility phenotypes of females with a Zp3-Cre-driven conditional knockout (cKO) of Smc5 demonstrated that maternally expressed SMC5 protein is essential for early embryogenesis. Interestingly, protein levels of SMC5/6 complex components in oocytes decline as wild-type females age. When SMC5/6 complexes were completely absent in oocytes during meiotic resumption, homologous chromosomes failed to segregate accurately during meiosis I. Despite what appears to be an inability to resolve concatenation between chromosomes during meiosis, localization of topoisomerase IIα to bivalents was not affected; however, localization of condensin along the chromosome axes was perturbed. Taken together, these data demonstrate that the SMC5/6 complex is essential for the formation of segregation-competent bivalents during meiosis I, and findings suggest that age-dependent depletion of the SMC5/6 complex in oocytes could contribute to increased incidence of oocyte aneuploidy and spontaneous abortion in aging females.

Ectopic expression of meiotic cohesin RAD21L promotes adjacency of homologous chromosomes in somatic cells

Pairing, synapsis, and crossover recombination of homologous chromosomes (homologs) are prerequisite for the proper segregation of homologs during meiosis I. The meiosis-specific cohesin subunit, RAD21L, is essential for such meiotic chromosomal events, but it is unknown to what extent RAD21L by itself contributes to the process since various meiotic genes are also involved. To reveal the exclusive contribution of RAD21L to the specific regulation of homologs, we examined the effects of ectopic RAD21L expression on chromosome dynamics in somatic cells. We found that expression of GFP-fused RAD21L by plasmid transfection significantly shortened the distance between two FISH signals representing a pair of homologs for chromosome X or chromosome 11 in the nuclei compared to that in control (non-transfected) cells whereas expression of GFP-fused RAD21, a mitotic counterpart of RAD21L, showed no detectable effects. This indicates that RAD21L, when ectopically expressed in somatic cells, can promote homolog adjacency, which resembles the homolog pairing normally seen during meiosis. Furthermore, deletion of the N-terminal winged helix domain from RAD21L, prevented its association with another cohesin subunit, SMC3, and abolished the phenomenon of homolog adjacency upon ectopic expression. Our findings suggest that RAD21L-containing cohesin can promote homolog adjacency independently of other meiotic gene products, which might be central to the process of meiotic homolog paring.

Prophase I: Preparing Chromosomes for Segregation in the Developing Oocyte

Formation of an oocyte involves a specialized cell division termed meiosis. In meiotic prophase I (the initial stage of meiosis), chromosomes undergo elaborate events to ensure the proper segregation of their chromosomes into gametes. These events include processes leading to the formation of a crossover that, along with sister chromatid cohesion, forms the physical link between homologous chromosomes. Crossovers are formed as an outcome of recombination. This process initiates with programmed double-strand breaks that are repaired through the use of homologous chromosomes as a repair template. The accurate repair to form crossovers takes place in the context of the synaptonemal complex, a protein complex that links homologous chromosomes in meiotic prophase I. To allow proper execution of meiotic prophase I events, signaling processes connect different steps in recombination and synapsis. The events occurring in meiotic prophase I are a prerequisite for proper chromosome segregation in the meiotic divisions. When these processes go awry, chromosomes missegregate. These meiotic errors are thought to increase with aging and may contribute to the increase in aneuploidy observed in advanced maternal age female oocytes.

Meiotic cohesin subunits RAD21L and REC8 are positioned at distinct regions between lateral elements and transverse filaments in the synaptonemal complex of mouse spermatocytes

Cohesins containing a meiosis-specific α-kleisin subunit, RAD21L or REC8, play roles in diverse aspects of meiotic chromosome dynamics including formation of axial elements (AEs), assembly of the synaptonemal complex (SC), recombination of homologous chromosomes (homologs), and cohesion of sister chromatids. However, the exact functions of individual α-kleisins remain to be elucidated. Here, we examined the localization of RAD21L and REC8 within the SC by super-resolution microscopy, 3D-SIM. We found that both RAD21L and REC8 were localized at the connection sites between lateral elements (LEs) and transverse filaments (TFs) of pachynema with RAD21L locating interior to REC8 sites. RAD21L and REC8 were not symmetrical in terms of synaptic homologs, suggesting that the arrangement of different cohesins is not strictly fixed along all chromosome axes. Intriguingly, some RAD21L signals, but not REC8 signals, were observed between unsynapsed regions of AEs of zygonema as if they formed a bridge between homologs. Furthermore, the signals of recombination intermediates overlapped with those of RAD21L to a greater degree than with those of REC8. These results highlight the different properties of two meiotic α-kleisins, and strongly support the previous proposition that RAD21L is an atypical cohesin that establishes the association between homologs rather than sister chromatids.

Genetics of primary ovarian insufficiency

Primary ovarian insufficiency (POI) is characterized by a loss of ovarian function before the age of 40 and account for one major cause of female infertility. POI relevance is continuously growing because of the increasing number of women desiring conception beyond 30 years of age, when POI prevalence is >1%. POI is highly heterogeneous and can present with ovarian dysgenesis and primary amenorrhea, or with secondary amenorrhea, and it can be associated with other congenital or acquired abnormalities. In most cases POI remains classified as idiopathic. However, the age of menopause is an inheritable trait and POI has a strong genetic component. This is confirmed by the existence of several candidate genes, experimental and natural models. The variable expressivity of POI defect may indicate that, this disease may frequently be considered as a multifactorial or oligogenic defect. The most common genetic contributors to POI are the X chromosome-linked defects. Here, we review the principal X-linked and autosomal genes involved in syndromic and non-syndromic forms of POI with the expectation that this list will soon be upgraded, thus allowing the possibility to predict the risk of an early age at menopause in families with POI.

Identification of Multiple Gene Mutations Accounts for a new Genetic Architecture of Primary Ovarian Insufficiency

CONTEXT

Idiopathic primary ovarian insufficiency (POI) is a major cause of amenorrhea and infertility. POI affects 1% of women before age 40 years, and several genetic causes have been reported. To date, POI has been considered a monogenic disorder.

OBJECTIVE

The aim of this study was to identify novel gene variations and to investigate if individuals with POI harbor mutation in multiple loci.

PATIENTS AND METHODS

One hundred well-phenotyped POI patients were systematically screened for variants in 19 known POI loci (and potential candidate genes) using next-generation sequencing.

RESULTS

At least one rare protein-altering gene variant was identified in 19 patients, including missense mutations in new candidate genes, namely SMC1β and REC8 (involved in the cohesin complex) and LHX8, a gene encoding a transcription factor. Novel or recurrent deleterious mutations were also detected in the known POI candidate genes NOBOX, FOXL2, SOHLH1, FIGLA, GDF9, BMP15, and GALT. Seven patients harbor mutations in two loci, and this digenicity seems to influence the age of symptom onset.

CONCLUSIONS

Genetic anomalies in women with POI are more frequent than previously believed. Digenic findings in several cases suggest that POI is not a purely monogenic disorder and points to a role of digenicity. The genotype-phenotype correlations in some kindreds suggest that a synergistic effect of several mutations may underlie the POI phenotype.

C14ORF39/SIX6OS1 is a constituent of the synaptonemal complex and is essential for mouse fertility

Meiotic recombination generates crossovers between homologous chromosomes that are essential for genome haploidization. The synaptonemal complex is a ‘zipper'-like protein assembly that synapses homologue pairs together and provides the structural framework for processing recombination sites into crossovers. Humans show individual differences in the number of crossovers generated across the genome. Recently, an anonymous gene variant in C14ORF39/SIX6OS1 was identified that influences the recombination rate in humans. Here we show that C14ORF39/SIX6OS1 encodes a component of the central element of the synaptonemal complex. Yeast two-hybrid analysis reveals that SIX6OS1 interacts with the well-established protein synaptonemal complex central element 1 (SYCE1). Mice lacking SIX6OS1 are defective in chromosome synapsis at meiotic prophase I, which provokes an arrest at the pachytene-like stage and results in infertility. In accordance with its role as a modifier of the human recombination rate, SIX6OS1 is essential for the appropriate processing of intermediate recombination nodules before crossover formation.

The synaptonemal complex is a meiosis-specific proteinaceous structure that supports homologous chromosome pairs during meiosis. Here, the authors show that SIX6OS1 (of previously unknown function) is part of the synaptonemal complex central element and upon deletion in mice, causes defective chromosome synapsis and infertility.

Distinct Roles of Meiosis-Specific Cohesin Complexes in Mammalian Spermatogenesis

Mammalian meiocytes feature four meiosis-specific cohesin proteins in addition to ubiquitous ones, but the roles of the individual cohesin complexes are incompletely understood. To decipher the functions of the two meiosis-specific kleisins, REC8 or RAD21L, together with the only meiosis-specific SMC protein SMC1β, we generated Smc1β-/-Rec8-/- and Smc1β-/-Rad21L-/- mouse mutants. Analysis of spermatocyte chromosomes revealed that besides SMC1β complexes, SMC1α/RAD21 and to a small extent SMC1α/REC8 contribute to chromosome axis length. Removal of SMC1β and RAD21L almost completely abolishes all chromosome axes. The sex chromosomes do not pair in single or double mutants, and autosomal synapsis is impaired in all mutants. Super resolution microscopy revealed synapsis-associated SYCP1 aberrantly deposited between sister chromatids and on single chromatids in Smc1β-/-Rad21L-/- cells. All mutants show telomere length reduction and structural disruptions, while wild-type telomeres feature a circular TRF2 structure reminiscent of t-loops. There is no loss of centromeric cohesion in both double mutants at leptonema/early zygonema, indicating that, at least in the mutant backgrounds, an SMC1α/RAD21 complex provides centromeric cohesion at this early stage. Thus, in early prophase I the most prominent roles of the meiosis-specific cohesins are in axis-related features such as axis length, synapsis and telomere integrity rather than centromeric cohesion.

Ectopic Expression of Testis Germ Cell Proteins in Cancer and Its Potential Role in Genomic Instability

Genomic instability is a hallmark of human cancer and an enabling factor for the genetic alterations that drive cancer development. The processes involved in genomic instability resemble those of meiosis, where genetic material is interchanged between homologous chromosomes. In most types of human cancer, epigenetic changes, including hypomethylation of gene promoters, lead to the ectopic expression of a large number of proteins normally restricted to the germ cells of the testis. Due to the similarities between meiosis and genomic instability, it has been proposed that activation of meiotic programs may drive genomic instability in cancer cells. Some germ cell proteins with ectopic expression in cancer cells indeed seem to promote genomic instability, while others reduce polyploidy and maintain mitotic fidelity. Furthermore, oncogenic germ cell proteins may indirectly contribute to genomic instability through induction of replication stress, similar to classic oncogenes. Thus, current evidence suggests that testis germ cell proteins are implicated in cancer development by regulating genomic instability during tumorigenesis, and these proteins therefore represent promising targets for novel therapeutic strategies.

Genetic Interactions Between the Meiosis-Specific Cohesin Components, STAG3, REC8, and RAD21L

Cohesin is an essential structural component of chromosomes that ensures accurate chromosome segregation during mitosis and meiosis. Previous studies have shown that there are cohesin complexes specific to meiosis, required to mediate homologous chromosome pairing, synapsis, recombination, and segregation. Meiosis-specific cohesin complexes consist of two structural maintenance of chromosomes proteins (SMC1α/SMC1β and SMC3), an α-kleisin protein (RAD21, RAD21L, or REC8), and a stromal antigen protein (STAG1, 2, or 3). STAG3 is exclusively expressed during meiosis, and is the predominant STAG protein component of cohesin complexes in primary spermatocytes from mouse, interacting directly with each α-kleisin subunit. REC8 and RAD21L are also meiosis-specific cohesin components. Stag3 mutant spermatocytes arrest in early prophase ("zygotene-like" stage), displaying failed homolog synapsis and persistent DNA damage, as a result of unstable loading of cohesin onto the chromosome axes. Interestingly, Rec8, Rad21L double mutants resulted in an earlier "leptotene-like" arrest, accompanied by complete absence of STAG3 loading. To assess genetic interactions between STAG3 and α-kleisin subunits RAD21L and REC8, our lab generated Stag3, Rad21L, and Stag3, Rec8 double knockout mice, and compared them to the Rec8, Rad21L double mutant. These double mutants are phenotypically distinct from one another, and more severe than each single knockout mutant with regards to chromosome axis formation, cohesin loading, and sister chromatid cohesion. The Stag3, Rad21L, and Stag3, Rec8 double mutants both progress further into prophase I than the Rec8, Rad21L double mutant. Our genetic analysis demonstrates that cohesins containing STAG3 and REC8 are the main complex required for centromeric cohesion, and RAD21L cohesins are required for normal clustering of pericentromeric heterochromatin. Furthermore, the STAG3/REC8 and STAG3/RAD21L cohesins are the primary cohesins required for axis formation.

The cohesin REC8 prevents illegitimate inter-sister synaptonemal complex assembly

During meiosis, a specialized chromosome structure is assembled to promote pairing/synapsis of homologous chromosomes and meiotic recombination, a process yielding chiasmata between homologs to ensure accurate segregation. Meiosis‐specific cohesin complexes mediating sister chromatid cohesion play pivotal roles in almost all these events, including synaptonemal complex (SC ) formation. In this issue of EMBO Reports , Agostinho and colleagues have examined chromosome axes and SC structures by taking advantage of a hypomorphic Stag3 mutant in which the levels of the cohesin subunit REC 8 are partly reduced 6 . Using super‐resolution microscopy, the authors illuminate previously unforeseen chromosome axis structures, showing locally separated axes in regions where REC 8 is absent, regardless of RAD 21L or RAD 21 cohesin localization. Furthermore, they assessed the relationship between sister chromatid cohesion and inter‐sister SC formation, demonstrating that “axial opening” in the REC 8‐free region is accompanied by illegitimate SC formation between sister chromatids. This study highlights the physiological importance of REC 8 in sister chromatid cohesion and proper SC formation during meiosis, suggesting a new model in which a high density of REC 8 deposition along the chromosome prevents illegitimate inter‐sister SC formation.

High density of REC8 constrains sister chromatid axes and prevents illegitimate synaptonemal complex formation

During meiosis, cohesin complexes mediate sister chromatid cohesion (SCC), synaptonemal complex (SC) assembly and synapsis. Here, using super-resolution microscopy, we imaged sister chromatid axes in mouse meiocytes that have normal or reduced levels of cohesin complexes, assessing the relationship between localization of cohesin complexes, SCC and SC formation. We show that REC8 foci are separated from each other by a distance smaller than 15% of the total chromosome axis length in wild-type meiocytes. Reduced levels of cohesin complexes result in a local separation of sister chromatid axial elements (LSAEs), as well as illegitimate SC formation at these sites. REC8 but not RAD21 or RAD21L cohesin complexes flank sites of LSAEs, whereas RAD21 and RAD21L appear predominantly along the separated sister-chromatid axes. Based on these observations and a quantitative distribution analysis of REC8 along sister chromatid axes, we propose that the high density of randomly distributed REC8 cohesin complexes promotes SCC and prevents illegitimate SC formation.

The width of the lateral element of the synaptonemal complex is determined by a multilayered organization of its components

The synaptonemal complex (SC) is a proteinaceous structure that holds the homologous chromosomes in close proximity while they exchange genetic material in a process known as meiotic recombination. This meiotic recombination leads to genetic variability in sexually reproducing organisms. The ultrastructure of the SC is studied by electron microscopy and it is observed as a tripartite structure. Two lateral elements (LE) separated by a central region (CR) confer its classical tripartite organization. The LEs are the anchoring platform for the replicated homologous chromosomes to properly exchange genetic material with one another. An accurate assembly of the LE is indispensable for the proper completion of meiosis. Ultrastructural studies suggested that the LE is organized as a multilayered unit. However, no validation of this model has been previously provided. In this ultrastructural study, by using mice with different genetic backgrounds that affect the LE width, we provide further evidence that support a multilayered organization of the LE. Additionally, we provide data suggesting additional roles of the different cohesin complex components in the structure of the LEs of the SC.

Sororin loads to the synaptonemal complex central region independently of meiotic cohesin complexes

The distribution and regulation of the cohesin complexes have been extensively studied during mitosis. However, the dynamics of their different regulators in vertebrate meiosis is largely unknown. In this work, we have analyzed the distribution of the regulatory factor Sororin during male mouse meiosis. Sororin is detected at the central region of the synaptonemal complex during prophase I, in contrast with the previously reported localization of other cohesin components in the lateral elements. This localization of Sororin depends on the transverse filaments protein SYCP1, but not on meiosis-specific cohesin subunits REC8 and SMC1β. By late prophase I, Sororin accumulates at centromeres and remains there up to anaphase II The phosphatase activity of PP2A seems to be required for this accumulation. We hypothesize that Sororin function at the central region of the synaptonemal complex could be independent on meiotic cohesin complexes. In addition, we suggest that Sororin participates in the regulation of centromeric cohesion during meiosis in collaboration with SGO2-PP2A.

Sordaria, a model system to uncover links between meiotic pairing and recombination

The mycelial fungus Sordaria macrospora was first used as experimental system for meiotic recombination. This review shows that it provides also a powerful cytological system for dissecting chromosome dynamics in wild-type and mutant meioses. Fundamental cytogenetic findings include: (1) the identification of presynaptic alignment as a key step in pairing of homologous chromosomes. (2) The discovery that biochemical complexes that mediate recombination at the DNA level concomitantly mediate pairing of homologs. (3) This pairing process involves not only resolution but also avoidance of chromosomal entanglements and the resolution system includes dissolution of constraining DNA recombination interactions, achieved by a unique role of Mlh1. (4) Discovery that the central components of the synaptonemal complex directly mediate the re-localization of the recombination proteins from on-axis to in-between homologue axis positions. (5) Identification of putative STUbL protein Hei10 as a structure-based signal transduction molecule that coordinates progression and differentiation of recombinational interactions at multiple stages. (6) Discovery that a single interference process mediates both nucleation of the SC and designation of crossover sites, thereby ensuring even spacing of both features. (7) Discovery of local modulation of sister-chromatid cohesion at sites of crossover recombination.

Cohesin-interacting protein WAPL-1 regulates meiotic chromosome structure and cohesion by antagonizing specific cohesin complexes

Wapl induces cohesin dissociation from DNA throughout the mitotic cell cycle, modulating sister chromatid cohesion and higher-order chromatin structure. Cohesin complexes containing meiosis-specific kleisin subunits govern most aspects of meiotic chromosome function, but whether Wapl regulates these complexes remains unknown. We show that during C. elegans oogenesis WAPL-1 antagonizes binding of cohesin containing COH-3/4 kleisins, but not REC-8, demonstrating that sensitivity to WAPL-1 is dictated by kleisin identity. By restricting the amount of chromosome-associated COH-3/4 cohesin, WAPL-1 controls chromosome structure throughout meiotic prophase. In the absence of REC-8, WAPL-1 inhibits COH-3/4-mediated cohesion, which requires crossover-fated events formed during meiotic recombination. Thus, WAPL-1 promotes functional specialization of meiotic cohesin: WAPL-1-sensitive COH-3/4 complexes modulate higher-order chromosome structure, while WAPL-1-refractory REC-8 complexes provide stable cohesion. Surprisingly, a WAPL-1-independent mechanism removes cohesin before metaphase I. Our studies provide insight into how meiosis-specific cohesin complexes are regulated to ensure formation of euploid gametes.

Involvement of the Cohesin Cofactor PDS5 (SPO76) During Meiosis and DNA Repair in Arabidopsis thaliana

Maintenance and precise regulation of sister chromatid cohesion is essential for faithful chromosome segregation during mitosis and meiosis. Cohesin cofactors contribute to cohesin dynamics and interact with cohesin complexes during cell cycle. One of these, PDS5, also known as SPO76, is essential during mitosis and meiosis in several organisms and also plays a role in DNA repair. In yeast, the complex Wapl-Pds5 controls cohesion maintenance and colocalizes with cohesin complexes into chromosomes. In Arabidopsis, AtWAPL proteins are essential during meiosis, however, the role of AtPDS5 remains to be ascertained. Here we have isolated mutants for each of the five AtPDS5 genes (A-E) and obtained, after different crosses between them, double, triple, and even quadruple mutants (Atpds5a Atpds5b Atpds5c Atpds5e). Depletion of AtPDS5 proteins has a weak impact on meiosis, but leads to severe effects on development, fertility, somatic homologous recombination (HR) and DNA repair. Furthermore, this cohesin cofactor could be important for the function of the AtSMC5/AtSMC6 complex. Contrarily to its function in other species, our results suggest that AtPDS5 is dispensable during the meiotic division of Arabidopsis, although it plays an important role in DNA repair by HR.

Cohesin and human disease: lessons from mouse models

Cohesin is an evolutionarily conserved large ring-like multi-subunit protein structure that can encircle DNA. Cohesin affects many processes that occur on chromosomes such as segregation, DNA replication, double-strand break repair, condensation, chromosome organization, and gene expression. Mutations in the genes that encode cohesin and its regulators cause human developmental disorders and cancer. Several mouse models have been established with the aim of understanding the cohesin mediated processes that are disrupted in these diseases. Mouse models support the idea that cohesin is essential for cell division, but partial loss of function can alter gene expression, DNA replication and repair, gametogenesis, and nuclear organization.

Genetics of the ovarian reserve

Primordial follicles or non-growing follicles (NGFs) are the functional unit of reproduction, each comprising a single germ cell surrounded by supporting somatic cells. NGFs constitute the ovarian reserve (OR), prerequisite for germ cell ovulation and the continuation of the species. The dynamics of the reserve is determined by the number of NGFs formed and their complex subsequent fates. During the reproductive lifespan, the OR progressively diminishes due to follicle atresia as well as recruitment, maturation, and ovulation. The depletion of the OR is the major determining driver of menopause, which ensues when the number of primordial follicles falls below a threshold of ∼1,000. Therefore, genes and processes involved in follicle dynamics are particularly important to understand the process of menopause, both in the typical reproductive lifespan and in conditions like primary ovarian insufficiency, defined as menopause before age 40. Genes and their variants that affect the timing of menopause thereby provide candidates for diagnosis of and intervention in problems of reproductive lifespan. We review the current knowledge of processes and genes involved in the development of the OR and in the dynamics of ovarian follicles.

Mammalian target of rapamycin complex 1 (mTORC1) Is required for mouse spermatogonial differentiation in vivo

Spermatogonial stem cells (SSCs) must balance self-renewal with production of transit-amplifying progenitors that differentiate in response to retinoic acid (RA) before entering meiosis. This self-renewal vs. differentiation spermatogonial fate decision is critical for maintaining tissue homeostasis, as imbalances cause spermatogenesis defects that can lead to human testicular cancer or infertility. A great deal of effort has been exerted to understand how the SSC population is maintained. In contrast, little is known about the essential program of differentiation initiated by retinoic acid (RA) that precedes meiosis, and the pathways and proteins involved are poorly defined. We recently reported a novel role for RA in stimulating the PI3/AKT/mTOR kinase signaling pathway to activate translation of repressed mRNAs such as Kit. Here, we examined the requirement for mTOR complex 1 (mTORC1) in mediating the RA signal to direct spermatogonial differentiation in the neonatal testis. We found that in vivo inhibition of mTORC1 by rapamycin blocked spermatogonial differentiation, which led to an accumulation of undifferentiated spermatogonia. In addition, rapamycin also blocked the RA-induced translational activation of mRNAs encoding KIT, SOHLH1, and SOHLH2 without affecting expression of STRA8. These findings highlight dual roles for RA in germ cell development - transcriptional activation of genes, and kinase signaling to stimulate translation of repressed messages required for spermatogonial differentiation.

Functions of the MRE11 complex in the development and maintenance of oocytes

The MRE11 complex (MRE11, RAD50, and NBS1) is a central component of the DNA damage response, governing both double-strand break repair and DNA damage response signaling. To determine the functions of the MRE11 complex in the development and maintenance of oocytes, we analyzed ovarian phenotypes of mice harboring the hypomorphic Mre11^ATLD1 allele. Mre11^ATLD1/ATLD1 females exhibited premature oocyte elimination attributable to defects in homologous chromosome pairing and double-strand break repair during meiotic prophase. Other aspects of meiotic progression, including attachment of telomeres to the nuclear envelope and recruitment of RAD21L, a component of the meiotic cohesin complex to the synaptonemal complex, were normal. Unlike Dmc1^−/− and Trp13^Gt/Gt mice which exhibit comparable defects in double-strand break repair and oocyte depletion by 5 days post-partum, we found that oocyte attrition occurred by 12 weeks in Mre11^ATLD1/ATLD1. Disruption of the oocyte checkpoint pathway governed by Chk2 gene further enhanced the survival of Mre11^ATLD1/ATLD1 follicles. Together our data suggest that the MRE11 complex influences the elimination of oocytes with unrepaired meiotic double-strand breaks post-natally, in addition to its previously described role in double-strand break repair and homologous synapsis during female meiosis.

Bone morphogenetic protein 2 promotes primordial follicle formation in the ovary

Primordial follicles (PF) are formed when somatic cells differentiate into flattened pregranulosa cells, invaginate into the oocyte nests and encircle individual oocytes. We hypothesize that BMP2 regulates PF formation by promoting the transition of germ cells into oocytes and somatic cells into pregranulosa cells. E15 hamster ovaries were cultured for 8 days corresponding to postnatal day 8 (P8) in vivo, with or without BMP2, and the formation of PF was examined. BMP2 was expressed in the oocytes as well as ovarian somatic cells during development. BMP2 exposure for the first two days or the last two days or the entire 8 days of culture led to increase in PF formation suggesting that BMP2 affected both germ cell transition and somatic cell differentiation. Whereas an ALK2/3 inhibitor completely blocked BMP2-induced PF formation, an ALK2-specific inhibitor was partially effective, suggesting that BMP2 affected PF formation via both ALK2 and ALK3. BMP2 also reduced apoptosis in vitro. Further, more meiotic oocytes were present in BMP2 exposed ovaries. In summary, the results provide the first evidence that BMP2 regulates primordial follicle formation by promoting germ cell to oocyte transition and somatic cell to pre-granulosa cells formation and it acts via both ALK2 and ALK3.

Expression of the CTCFL Gene during Mouse Embryogenesis Causes Growth Retardation, Postnatal Lethality, and Dysregulation of the Transforming Growth Factor β Pathway

CTCFL, a paralog of CTCF, also known as BORIS (brother of regulator of imprinted sites), is a testis-expressed gene whose function is largely unknown. Its product is a cancer testis antigen (CTA), and it is often expressed in tumor cells and also seen in two benign human vascular malformations, juvenile angiofibromas and infantile hemangiomas. To understand the function of Ctcfl, we created tetracycline-inducible Ctcfl transgenic mice. We show that Ctcfl expression during embryogenesis results in growth retardation, eye malformations, multiorgan pathologies, vascular defects, and neonatal death. This phenotype resembles prior mouse models that perturb the transforming growth factor β (TGFB) pathway. Embryonic stem (ES) cells with the Ctcfl transgene reproduce the phenotype in ES cell-tetraploid chimeras. Transcriptome sequencing of the Ctcfl ES cells revealed 14 genes deregulated by Ctcfl expression. Bioinformatic analysis revealed the TGFB pathway as most affected by embryonic Ctcfl expression. Understanding the consequence of Ctcfl expression in nontesticular cells and elucidating downstream targets of Ctcfl could explain the role of its product as a CTA and its involvement in two, if not more, human vascular malformations.

Complex elaboration: making sense of meiotic cohesin dynamics

In mitotically dividing cells, the cohesin complex tethers sister chromatids, the products of DNA replication, together from the time they are generated during S phase until anaphase. Cohesion between sister chromatids ensures accurate chromosome segregation, and promotes normal gene regulation and certain kinds of DNA repair. In somatic cells, the core cohesin complex is composed of four subunits: Smc1, Smc3, Rad21 and an SA subunit. During meiotic cell divisions meiosis-specific isoforms of several of the cohesin subunits are also expressed and incorporated into distinct meiotic cohesin complexes. The relative contributions of these meiosis-specific forms of cohesin to chromosome dynamics during meiotic progression have not been fully worked out. However, the localization of these proteins during chromosome pairing and synapsis, and their unique loss-of-function phenotypes, suggest non-overlapping roles in controlling meiotic chromosome behavior. Many of the proteins that regulate cohesin function during mitosis also appear to regulate cohesin during meiosis. Here we review how cohesin contributes to meiotic chromosome dynamics, and explore similarities and differences between cohesin regulation during the mitotic cell cycle and meiotic progression. A deeper understanding of the regulation and function of cohesin in meiosis will provide important new insights into how the cohesin complex is able to promote distinct kinds of chromosome interactions under diverse conditions.

STAG3 truncating variant as the cause of primary ovarian insufficiency

Primary ovarian insufficiency (POI) is a distressing cause of infertility in young women. POI is heterogeneous with only a few causative genes having been discovered so far. Our objective was to determine the genetic cause of POI in a consanguineous Lebanese family with two affected sisters presenting with primary amenorrhoea and an absence of any pubertal development. Multipoint parametric linkage analysis was performed. Whole-exome sequencing was done on the proband. Linkage analysis identified a locus on chromosome 7 where exome sequencing successfully identified a homozygous two base pair duplication (c.1947_48dupCT), leading to a truncated protein p.(Y650Sfs*22) in the STAG3 gene, confirming it as the cause of POI in this family. Exome sequencing combined with linkage analyses offers a powerful tool to efficiently find novel genetic causes of rare, heterogeneous disorders, even in small single families. This is only the second report of a STAG3 variant; the first STAG3 variant was recently described in a phenotypically similar family with extreme POI. Identification of an additional family highlights the importance of STAG3 in POI pathogenesis and suggests it should be evaluated in families affected with POI.

Recombination, Pairing, and Synapsis of Homologs during Meiosis

Recombination is a prominent feature of meiosis in which it plays an important role in increasing genetic diversity during inheritance. Additionally, in most organisms, recombination also plays mechanical roles in chromosomal processes, most notably to mediate pairing of homologous chromosomes during prophase and, ultimately, to ensure regular segregation of homologous chromosomes when they separate at the first meiotic division. Recombinational interactions are also subject to important spatial patterning at both early and late stages. Recombination-mediated processes occur in physical and functional linkage with meiotic axial chromosome structure, with interplay in both directions, before, during, and after formation and dissolution of the synaptonemal complex (SC), a highly conserved meiosis-specific structure that links homolog axes along their lengths. These diverse processes also are integrated with recombination-independent interactions between homologous chromosomes, nonhomology-based chromosome couplings/clusterings, and diverse types of chromosome movement. This review provides an overview of these diverse processes and their interrelationships.

MEI4 – a central player in the regulation of meiotic DNA double-strand break formation in the mouse

The formation of programmed DNA double-strand breaks (DSBs) at the beginning of meiotic prophase marks the initiation of meiotic recombination. Meiotic DSB formation is catalyzed by SPO11 and their repair takes place on meiotic chromosome axes. The evolutionarily conserved MEI4 protein is required for meiotic DSB formation and is localized on chromosome axes. Here, we show that HORMAD1, one of the meiotic chromosome axis components, is required for MEI4 localization. Importantly, the quantitative correlation between the level of axis-associated MEI4 and DSB formation suggests that axis-associated MEI4 could be a limiting factor for DSB formation. We also show that MEI1, REC8 and RAD21L are important for proper MEI4 localization. These findings on MEI4 dynamics during meiotic prophase suggest that the association of MEI4 to chromosome axes is required for DSB formation, and that the loss of this association upon DSB repair could contribute to turning off meiotic DSB formation.

Nuclear localization of PRDM9 and its role in meiotic chromatin modifications and homologous synapsis

Developmental progress of germ cells through meiotic phases is closely tied to ongoing meiotic recombination. In mammals, recombination preferentially occurs in genomic regions known as hotspots; the protein that activates these hotspots is PRDM9, containing a genetically variable zinc finger (ZNF) domain and a PR-SET domain with histone H3K4 trimethyltransferase activity. PRDM9 is required for fertility in mice, but little is known about its localization and developmental dynamics. Application of spermatogenic stage-specific markers demonstrates that PRDM9 accumulates in male germ cell nuclei at pre-leptonema to early leptonema but is no longer detectable in nuclei by late zygonema. By the pachytene stage, PRDM9-dependent histone H3K4 trimethyl marks on hotspots also disappear. PRDM9 localizes to nuclei concurrently with the deposition of meiotic cohesin complexes, but is not required for incorporation of cohesin complex proteins into chromosomal axial elements, or accumulation of normal numbers of RAD51 foci on meiotic chromatin by late zygonema. Germ cells lacking PRDM9 exhibit inefficient homology recognition and synapsis, with aberrant repair of meiotic DNA double-strand breaks and transcriptional abnormalities characteristic of meiotic silencing of unsynapsed chromatin. Together, these results on the developmental time course for nuclear localization of PRDM9 establish its direct window of function and demonstrate the independence of chromosome axial element formation from the concurrent PRDM9-mediated activation of recombination hotspots.

A novel transcriptional factor Nkapl is a germ cell-specific suppressor of Notch signaling and is indispensable for spermatogenesis

Spermatogenesis is an elaborately regulated system dedicated to the continuous production of spermatozoa via the genesis of spermatogonia. In this process, a variety of genes are expressed that are relevant to the differentiation of germ cells at each stage. Although Notch signaling plays a critical role in germ cell development in Drosophila and Caenorhabditis elegans, its function and importance for spermatogenesis in mammals is controversial. We report that Nkapl is a novel germ cell-specific transcriptional suppressor in Notch signaling. It is also associated with several molecules of the Notch corepressor complex such as CIR, HDAC3, and CSL. It was expressed robustly in spermatogonia and early spermatocytes after the age of 3 weeks. Nkapl-deleted mice showed complete arrest at the level of pachytene spermatocytes. In addition, apoptosis was observed in this cell type. Overexpression of NKAPL in germline stem cells demonstrated that Nkapl induced changes in spermatogonial stem cell (SSC) markers and the reduction of differentiation factors through the Notch signaling pathway, whereas testes with Nkapl deleted showed inverse changes in those markers and factors. Therefore, Nkapl is indispensable because aberrantly elevated Notch signaling has negative effects on spermatogenesis, affecting SSC maintenance and differentiation factors. Notch signaling should be properly regulated through the transcriptional factor Nkapl.

Structural and functional adaptations of the mammalian nuclear envelope to meet the meiotic requirements

Numerous studies in the past years provided definite evidence that the nuclear envelope is much more than just a simple barrier. It rather constitutes a multifunctional platform combining structural and dynamic features to fulfill many fundamental functions such as chromatin organization, regulation of transcription, signaling, but also structural duties like maintaining general nuclear architecture and shape. One additional and, without doubt, highly impressive aspect is the recently identified key function of selected nuclear envelope components in driving meiotic chromosome dynamics, which in turn is essential for accurate recombination and segregation of the homologous chromosomes. Here, we summarize the recent work identifying new key players in meiotic telomere attachment and movement and discuss the latest advances in our understanding of the actual function of the meiotic nuclear envelope.

Phosphorylation of cohesin Rec11/SA3 by casein kinase 1 promotes homologous recombination by assembling the meiotic chromosome axis

In meiosis, cohesin is required for sister chromatid cohesion, as well as meiotic chromosome axis assembly and recombination. However, mechanisms underlying the multifunctional nature of cohesin remain elusive. Here, we show that fission yeast casein kinase 1 (CK1) plays a crucial role in assembling the meiotic chromosome axis (so-called linear element: LinE) and promoting recombination. An in vitro phosphorylation screening assay identified meiotic cohesin subunit Rec11/SA3 as an excellent substrate of CK1. The phosphorylation of Rec11 by CK1 mediates the interaction with the Rec10/Red1/SCP2 axis component, a key step in meiotic chromosome axis assembly, and is dispensable for sister chromatid cohesion. Crucially, the expression of Rec11-Rec10 fusion protein nearly completely bypasses the requirement for CK1 or cohesin phosphorylation for LinE assembly and recombination. This study uncovers a central mechanism of the cohesin-dependent assembly of the meiotic chromosome axis and recombination apparatus that acts independently of sister chromatid cohesion.

Nociceptin and meiosis during spermatogenesis in postnatal testes

Phosphorylated Rec8, a key component of cohesin, mediates the association and disassociation, "dynamics," of chromosomes occurring in synaptonemal complex formation, crossover recombination, and sister chromatid cohesion during meiosis in germ cells. Yet, the extrinsic factors triggering meiotic chromosome dynamics remained unclear. In postnatal testes, follicle-stimulating hormone (FSH) acts directly on somatic Sertoli cells to activate gene expression via an intracellular signaling pathway composed of cAMP, cAMP-dependent protein kinase (PKA), and cAMP-response element-binding protein (CREB), and promotes germ cell development and spermatogenesis indirectly. Yet, the paracrine factors mediating the FSH effects to germ cells remained elusive. We have shown that nociceptin, known as a neuropeptide, is upregulated by FSH signaling through cAMP/PKA/CREB pathway in Sertoli cells of postnatal murine testes. Chromatin immunoprecipitation from Sertoli cells demonstrated that CREB phosphorylated at Ser133 associates with prepronociceptin gene encoding nociceptin. Analyses with Sertoli cells and testes revealed that both prepronociceptin mRNA and the nociceptin peptide are induced after FSH signaling is activated. In addition, the nociceptin peptide is induced in testes after 9 days post partum following FSH surge. Thus, our findings may identify nociceptin as a novel paracrine mediator of the FSH effects in the regulation of spermatogenesis; however, very little has known about the functional role of nociceptin in spermatogenesis. We have shown that nociceptin induces Rec8 phosphorylation, triggering chromosome dynamics, during meiosis in spermatocytes of postnatal murine testes. The nociceptin receptor Oprl-1 is exclusively expressed in the plasma membrane of testicular germ cells, mostly spermatocytes. Treatment of testes with nociceptin resulted in a rapid phosphorylation of Rec8. Injection of nociceptin into mice stimulated Rec8 phosphorylation and meiotic chromosome dynamics in testes, whereas injection of nocistatin, a specific inhibitor for nociceptin, abolished them. Therefore, our findings suggest that nociceptin is a novel extrinsic factor that plays a crucial role in the progress of meiosis during spermatogenesis.

Absence of SUN-domain protein Slp1 blocks karyogamy and switches meiotic recombination and synapsis from homologs to sister chromatids

Karyogamy, the process of nuclear fusion is required for two haploid gamete nuclei to form a zygote. Also, in haplobiontic organisms, karyogamy is required to produce the diploid nucleus/cell that then enters meiosis. We identify sun like protein 1 (Slp1), member of the mid-Sad1p, UNC-84-domain ubiquitous family, as essential for karyogamy in the filamentous fungus Sordaria macrospora, thus uncovering a new function for this protein family. Slp1 is required at the last step, nuclear fusion, not for earlier events including nuclear movements, recognition, and juxtaposition. Correspondingly, like other family members, Slp1 localizes to the endoplasmic reticulum and also to its extensions comprising the nuclear envelope. Remarkably, despite the absence of nuclear fusion in the slp1 null mutant, meiosis proceeds efficiently in the two haploid "twin" nuclei, by the same program and timing as in diploid nuclei with a single dramatic exception: the normal prophase program of recombination and synapsis between homologous chromosomes, including loading of recombination and synaptonemal complex proteins, occurs instead between sister chromatids. Moreover, the numbers of recombination-initiating double-strand breaks (DSBs) and ensuing recombinational interactions, including foci of the essential crossover factor Homo sapiens enhancer of invasion 10 (Hei10), occur at half the diploid level in each haploid nucleus, implying per-chromosome specification of DSB formation. Further, the distribution of Hei10 foci shows interference like in diploid meiosis. Centromere and spindle dynamics, however, still occur in the diploid mode during the two meiotic divisions. These observations imply that the prophase program senses absence of karyogamy and/or absence of a homolog partner and adjusts the interchromosomal interaction program accordingly.

Divergent kleisin subunits of cohesin specify mechanisms to tether and release meiotic chromosomes

We show that multiple, functionally specialized cohesin complexes mediate the establishment and two-step release of sister chromatid cohesion that underlies the production of haploid gametes. In C. elegans, the kleisin subunits REC-8 and COH-3/4 differ between meiotic cohesins and endow them with distinctive properties that specify how cohesins load onto chromosomes and then trigger and release cohesion. Unlike REC-8 cohesin, COH-3/4 cohesin becomes cohesive through a replication-independent mechanism initiated by the DNA double-stranded breaks that induce crossover recombination. Thus, break-induced cohesion also tethers replicated meiotic chromosomes. Later, recombination stimulates separase-independent removal of REC-8 and COH-3/4 cohesins from reciprocal chromosomal territories flanking the crossover site. This region-specific removal likely underlies the two-step separation of homologs and sisters. Unexpectedly, COH-3/4 performs cohesion-independent functions in synaptonemal complex assembly. This new model for cohesin function diverges from that established in yeast but likely applies directly to plants and mammals, which utilize similar meiotic kleisins.

Retinoic acid activates two pathways required for meiosis in mice

In all sexually reproducing organisms, cells of the germ line must transition from mitosis to meiosis. In mice, retinoic acid (RA), the extrinsic signal for meiotic initiation, activates transcription of Stra8, which is required for meiotic DNA replication and the subsequent processes of meiotic prophase. Here we report that RA also activates transcription of Rec8, which encodes a component of the cohesin complex that accumulates during meiotic S phase, and which is essential for chromosome synapsis and segregation. This RA induction of Rec8 occurs in parallel with the induction of Stra8, and independently of Stra8 function, and it is conserved between the sexes. Further, RA induction of Rec8, like that of Stra8, requires the germ-cell-intrinsic competence factor Dazl. Our findings strengthen the importance of RA and Dazl in the meiotic transition, provide important details about the Stra8 pathway, and open avenues to investigate early meiosis through analysis of Rec8 induction and function.

The transition from mitosis to meiosis is a defining feature of germ cells, the precursors of eggs and sperm. In mice, retinoic acid (RA), a vitamin A derivative, induces expression of the gene Stra8, which in turn is required for the first critical steps of meiosis. The timing of Stra8 expression in mammalian germ cells is influenced by an RA-degrading enzyme, CYP26B1, that is normally expressed in fetal testes to delay meiosis in males. It is unknown if Stra8 is RA's only meiosis-inducing target in germ cells or if other such genes are regulated by RA independently of Stra8. To investigate this question, we generated two lines of mice: Cyp26b1 mutants and Stra8 mutants. Our genetic experiments comparing germ cell development in these two mutants revealed a new RA target, Rec8. We demonstrate that Rec8 upregulation by RA occurs in the same temporal and spatial manner as Stra8, but Rec8 expression is independent of Stra8. Rec8, like Stra8, plays a critical role during early meiotic processes, suggesting that RA induces meiosis in at least two independent pathways. These findings expand our understanding of the gene regulatory network involved in meiotic initiation in mammals.

The cohesin subunit Rad21 is required for synaptonemal complex maintenance, but not sister chromatid cohesion, during Drosophila female meiosis

Replicated sister chromatids are held in close association from the time of their synthesis until their separation during the next mitosis. This association is mediated by the ring-shaped cohesin complex that appears to embrace the sister chromatids. Upon proteolytic cleavage of the α-kleisin cohesin subunit at the metaphase-to-anaphase transition by separase, sister chromatids are separated and segregated onto the daughter nuclei. The more complex segregation of chromosomes during meiosis is thought to depend on the replacement of the mitotic α-kleisin cohesin subunit Rad21/Scc1/Mcd1 by the meiotic paralog Rec8. In Drosophila, however, no clear Rec8 homolog has been identified so far. Therefore, we have analyzed the role of the mitotic Drosophila α-kleisin Rad21 during female meiosis. Inactivation of an engineered Rad21 variant by premature, ectopic cleavage during oogenesis results not only in loss of cohesin from meiotic chromatin, but also in precocious disassembly of the synaptonemal complex (SC). We demonstrate that the lateral SC component C(2)M can interact directly with Rad21, potentially explaining why Rad21 is required for SC maintenance. Intriguingly, the experimentally induced premature Rad21 elimination, as well as the expression of a Rad21 variant with destroyed separase consensus cleavage sites, do not interfere with chromosome segregation during meiosis, while successful mitotic divisions are completely prevented. Thus, chromatid cohesion during female meiosis does not depend on Rad21-containing cohesin.

Meiosis-specific cohesin component, Stag3 is essential for maintaining centromere chromatid cohesion, and required for DNA repair and synapsis between homologous chromosomes

Cohesins are important for chromosome structure and chromosome segregation during mitosis and meiosis. Cohesins are composed of two structural maintenance of chromosomes (SMC1-SMC3) proteins that form a V-shaped heterodimer structure, which is bridged by a α-kleisin protein and a stromal antigen (STAG) protein. Previous studies in mouse have shown that there is one SMC1 protein (SMC1β), two α-kleisins (RAD21L and REC8) and one STAG protein (STAG3) that are meiosis-specific. During meiosis, homologous chromosomes must recombine with one another in the context of a tripartite structure known as the synaptonemal complex (SC). From interaction studies, it has been shown that there are at least four meiosis-specific forms of cohesin, which together with the mitotic cohesin complex, are lateral components of the SC. STAG3 is the only meiosis-specific subunit that is represented within all four meiosis-specific cohesin complexes. In Stag3 mutant germ cells, the protein level of other meiosis-specific cohesin subunits (SMC1β, RAD21L and REC8) is reduced, and their localization to chromosome axes is disrupted. In contrast, the mitotic cohesin complex remains intact and localizes robustly to the meiotic chromosome axes. The instability of meiosis-specific cohesins observed in Stag3 mutants results in aberrant DNA repair processes, and disruption of synapsis between homologous chromosomes. Furthermore, mutation of Stag3 results in perturbation of pericentromeric heterochromatin clustering, and disruption of centromere cohesion between sister chromatids during meiotic prophase. These defects result in early prophase I arrest and apoptosis in both male and female germ cells. The meiotic defects observed in Stag3 mutants are more severe when compared to single mutants for Smc1β, Rec8 and Rad21l, however they are not as severe as the Rec8, Rad21l double mutants. Taken together, our study demonstrates that STAG3 is required for the stability of all meiosis-specific cohesin complexes. Furthermore, our data suggests that STAG3 is required for structural changes of chromosomes that mediate chromosome pairing and synapsis, DNA repair and progression of meiosis.