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

STAG3 is a strong candidate gene for male infertility

Oligo- and azoospermia are severe forms of male infertility. However, known genetic factors account only for a small fraction of the cases. Recently, whole-exome sequencing in a large consanguineous family with inherited premature ovarian failure (POF) identified a homozygous frameshift mutation in the STAG3 gene leading to a premature stop codon. STAG3 encodes a meiosis-specific subunit of the cohesin complex, a large proteinaceous ring with DNA-entrapping ability that ensures sister chromatid cohesion and enables correct synapsis and segregation of homologous chromosomes during meiosis. The pathogenicity of the STAG3 mutations was functionally validated with a loss-of-function mouse model for STAG3 in oogenesis. However, and since none of the male members of this family was homozygous for the mutant allele, we only could hypothesized its putative involvement in male infertility. In this report, we show that male mice devoid of Stag3 display a severe meiotic phenotype that includes a meiotic arrest at zygonema-like shortening of their chromosome axial elements/lateral elements, partial loss of centromeric cohesion at early prophase and maintenance of the ability to initiate but not complete RAD51- and DMC1-mediated double-strand break repair, demonstrating that STAG3 is a crucial cohesin subunit in mammalian gametogenesis and supporting our proposal that STAG3 is a strong candidate gene for human male infertility.

Meiotic cohesin STAG3 is required for chromosome axis formation and sister chromatid cohesion

The cohesin complex is essential for mitosis and meiosis. The specific meiotic roles of individual cohesin proteins are incompletely understood. We report in vivo functions of the only meiosis-specific STAG component of cohesin, STAG3. Newly generated STAG3-deficient mice of both sexes are sterile with meiotic arrest. In these mice, meiotic chromosome architecture is severely disrupted as no bona fide axial elements (AE) form and homologous chromosomes do not synapse. Axial element protein SYCP3 forms dot-like structures, many partially overlapping with centromeres. Asynapsis marker HORMAD1 is diffusely distributed throughout the chromatin, and SYCP1, which normally marks synapsed axes, is largely absent. Centromeric and telomeric sister chromatid cohesion are impaired. Centromere and telomere clustering occurs in the absence of STAG3, and telomere structure is not severely affected. Other cohesin proteins are present, localize throughout the STAG3-devoid chromatin, and form complexes with cohesin SMC1β. No other deficiency in a single meiosis-specific cohesin causes a phenotype as drastic as STAG3 deficiency. STAG3 emerges as the key STAG cohesin involved in major functions of meiotic cohesin.

STAG3-mediated stabilization of REC8 cohesin complexes promotes chromosome synapsis during meiosis

Cohesion between sister chromatids in mitotic and meiotic cells is promoted by a ring-shaped protein structure, the cohesin complex. The cohesin core complex is composed of four subunits, including two structural maintenance of chromosome (SMC) proteins, one α-kleisin protein, and one SA protein. Meiotic cells express both mitotic and meiosis-specific cohesin core subunits, generating cohesin complexes with different subunit composition and possibly separate meiotic functions. Here, we have analyzed the in vivo function of STAG3, a vertebrate meiosis-specific SA protein. Mice with a hypomorphic allele of Stag3, which display a severely reduced level of STAG3, are viable but infertile. We show that meiocytes in homozygous mutant Stag3 mice display chromosome axis compaction, aberrant synapsis, impaired recombination and developmental arrest. We find that the three different α-kleisins present in meiotic cells show different dosage-dependent requirements for STAG3 and that STAG3-REC8 cohesin complexes have a critical role in supporting meiotic chromosome structure and functions.

Meiosis-specific cohesin mediates homolog recognition in mouse spermatocytes

During meiosis, homologous chromosome (homolog) pairing is promoted by several layers of regulation that include dynamic chromosome movement and meiotic recombination. However, the way in which homologs recognize each other remains a fundamental issue in chromosome biology. Here, we show that homolog recognition or association initiates upon entry into meiotic prophase before axis assembly and double-strand break (DSB) formation. This homolog association develops into tight pairing only during or after axis formation. Intriguingly, the ability to recognize homologs is retained in Sun1 knockout spermatocytes, in which telomere-directed chromosome movement is abolished, and this is the case even in Spo11 knockout spermatocytes, in which DSB-dependent DNA homology search is absent. Disruption of meiosis-specific cohesin RAD21L precludes the initial association of homologs as well as the subsequent pairing in spermatocytes. These findings suggest the intriguing possibility that homolog recognition is achieved primarily by searching for homology in the chromosome architecture as defined by meiosis-specific cohesin rather than in the DNA sequence itself.

Mutant cohesin in premature ovarian failure

Premature ovarian failure is a major cause of female infertility. The genetic causes of this disorder remain unknown in most patients. Using whole-exome sequence analysis of a large consanguineous family with inherited premature ovarian failure, we identified a homozygous 1-bp deletion inducing a frameshift mutation in STAG3 on chromosome 7. STAG3 encodes a meiosis-specific subunit of the cohesin ring, which ensures correct sister chromatid cohesion. Female mice devoid of Stag3 are sterile, and their fetal oocytes are arrested at early prophase I, leading to oocyte depletion at 1 week of age.

Chromosome segregation in plant meiosis

Faithful chromosome segregation in meiosis is essential for ploidy stability over sexual life cycles. In plants, defective chromosome segregation caused by gene mutations or other factors leads to the formation of unbalanced or unreduced gametes creating aneuploid or polyploid progeny, respectively. Accurate segregation requires the coordinated execution of conserved processes occurring throughout the two meiotic cell divisions. Synapsis and recombination ensure the establishment of chiasmata that hold homologous chromosomes together allowing their correct segregation in the first meiotic division, which is also tightly regulated by cell-cycle dependent release of cohesin and monopolar attachment of sister kinetochores to microtubules. In meiosis II, bi-orientation of sister kinetochores and proper spindle orientation correctly segregate chromosomes in four haploid cells. Checkpoint mechanisms acting at kinetochores control the accuracy of kinetochore-microtubule attachment, thus ensuring the completion of segregation. Here we review the current knowledge on the processes taking place during chromosome segregation in plant meiosis, focusing on the characterization of the molecular factors involved.

OsRAD51C is essential for double-strand break repair in rice meiosis

RAD51C is one of the RAD51 paralogs that plays an important role in DNA double-strand break repair by homologous recombination. Here, we identified and characterized OsRAD51C, the rice homolog of human RAD51C. The Osrad51c mutant plant is normal in vegetative growth but exhibits complete male and female sterility. Cytological investigation revealed that homologous pairing and synapsis were severely disrupted. Massive chromosome fragmentation occurred during metaphase I in Osrad51c meiocytes, and was fully suppressed by the CRC1 mutation. Immunofluorescence analysis showed that OsRAD51C localized onto the chromosomes from leptotene to early pachytene during prophase I, and that normal loading of OsRAD51C was dependent on OsREC8, PAIR2, and PAIR3. Additionally, ZEP1 did not localize properly in Osrad51c, indicating that OsRAD51C is required for synaptonemal complex assembly. Our study also provided evidence in support of a functional divergence in RAD51C among organisms.

The roles of cohesins in mitosis, meiosis, and human health and disease

Mitosis and meiosis are essential processes that occur during development. Throughout these processes, cohesion is required to keep the sister chromatids together until their separation at anaphase. Cohesion is created by multiprotein subunit complexes called cohesins. Although the subunits differ slightly in mitosis and meiosis, the canonical cohesin complex is composed of four subunits that are quite diverse. The cohesin complexes are also important for DNA repair, gene expression, development, and genome integrity. Here we provide an overview of the roles of cohesins during these different events as well as their roles in human health and disease, including the cohesinopathies. Although the exact roles and mechanisms of these proteins are still being elucidated, this review serves as a guide for the current knowledge of cohesins.

Roles of cohesin and condensin in chromosome dynamics during mammalian meiosis

Meiosis is a key step for sexual reproduction in which chromosome number is halved by two successive meiotic divisions after a single round of DNA replication. In the first meiotic division (meiosis I), homologous chromosomes pair, synapse, and recombine with their partners in prophase I. As a result, homologous chromosomes are physically connected until metaphase I and then segregated from each other at the onset of anaphase I. In the subsequent second meiotic division (meiosis II), sister chromatids are segregated. Chromosomal abnormality arising during meiosis is one of the major causes of birth defects and congenital disorders in mammals including human and domestic animals. Hence understanding of the mechanism underlying these unique chromosome behavior in meiosis is of great importance. This review focuses on the roles of cohesin and condensin, and their regulation in chromosome dynamics during mammalian meiosis.

Cohesin: functions beyond sister chromatid cohesion

Faithful segregation of chromosomes during mitosis and meiosis is the cornerstone process of life. Cohesin, a multi-protein complex conserved from yeast to human, plays a crucial role in this process by keeping the sister chromatids together from S-phase to anaphase onset during mitosis and meiosis. Technological advancements have discovered myriad functions of cohesin beyond its role in sister chromatid cohesion (SCC), such as transcription regulation, DNA repair, chromosome condensation, homolog pairing, monoorientation of sister kinetochore, etc. Here, we have focused on such functions of cohesin that are either independent of or dependent on its canonical role of sister chromatid cohesion. At the end, human diseases associated with malfunctioning of cohesin, albeit with mostly unperturbed sister chromatid cohesion, have been discussed.

Nociceptin induces Rec8 phosphorylation and meiosis in postnatal murine 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. Yet, the extrinsic factors triggering meiotic chromosome dynamics remain elusive. We have recently found that nociceptin, known as a neuropeptide, is up-regulated by follicle-stimulating hormone in Sertoli cells in postnatal murine testes; however, very little is known about the functional role of nociceptin in spermatogenesis. Here, we show that nociceptin induces Rec8 phosphorylation, triggering chromosome dynamics, in spermatocytes during meiosis in 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 of nociceptin, abolished them. These findings suggest that nociceptin is a novel extrinsic factor that plays a crucial role in the progress of meiosis.

Dynamic localization of SMC5/6 complex proteins during mammalian meiosis and mitosis suggests functions in distinct chromosome processes

Four members of the structural maintenance of chromosome (SMC) protein family have essential functions in chromosome condensation (SMC2/4) and sister-chromatid cohesion (SMC1/3). The SMC5/6 complex has been implicated in chromosome replication, DNA repair and chromosome segregation in somatic cells, but its possible functions during mammalian meiosis are unknown. Here, we show in mouse spermatocytes that SMC5 and SMC6 are located at the central region of the synaptonemal complex from zygotene until diplotene. During late diplotene both proteins load to the chromocenters, where they colocalize with DNA Topoisomerase IIα, and then accumulate at the inner domain of the centromeres during the first and second meiotic divisions. Interestingly, SMC6 and DNA Topoisomerase IIα colocalize at stretched strands that join kinetochores during the metaphase II to anaphase II transition, and both are observed on stretched lagging chromosomes at anaphase II following treatment with Etoposide. During mitosis, SMC6 and DNA Topoisomerase IIα colocalize at the centromeres and chromatid axes. Our results are consistent with the participation of SMC5 and SMC6 in homologous chromosome synapsis during prophase I, chromosome and centromere structure during meiosis I and mitosis and, with DNA Topoisomerase IIα, in regulating centromere cohesion during meiosis II.

Cohesin complexes with a potential to link mammalian meiosis to cancer

Among multiple genes aberrantly activated in cancers, invariably, there is a group related to the capacity of cell to self-renewal. Some of these genes are related to the normal process of development, including the establishment of a germline. This group, a part of growing family of Cancer/Testis (CT) genes, now includes the meiosis specific subunits of cohesin complex. The first reports characterizing the SMC1 and RAD21 genes, encoding subunits of cohesin, were published 20 years ago; however the exact molecular mechanics of cohesin molecular machine in vivo remains rather obscure notwithstanding ample elegant experiments. The matters are complicated by the fact that the evolution of cohesin function, which is served by just two basic types of protein complexes in budding yeast, took an explosive turn in Metazoa. The recent characterization of a new set of genes encoding cohesin subunits specific for meiosis in vertebrates adds several levels of complexity to the task of structure-function analysis of specific cohesin pathways, even more so in relation to their aberrant functionality in cancers. These three proteins, SMC1β, RAD21L and STAG3 are likely involved in a specific function in the first meiotic prophase, genetic recombination, and segregation of homologues. However, at present, it is rather challenging to pinpoint the molecular role of these proteins, particularly in synaptonemal complex or centromere function, due to the multiplicity of different cohesins in meiosis. The roles of these proteins in cancer cell physiology, upon their aberrant activation in tumors, also remain to be elucidated. Nevertheless, as the existence of Cancer/Testis cohesin complexes in tumor cells appears to be all but certain, this brings a promise of a new target for cancer therapy and/or diagnostics.

Oocyte differentiation is genetically dissociable from meiosis in mice

Oogenesis is the process by which ovarian germ cells undertake meiosis and differentiate to become eggs. In mice, Stra8 is required for the chromosomal events of meiosis to occur, but its role in differentiation remains unknown. Here we report Stra8-deficient ovarian germ cells that grow and differentiate into oocyte-like cells that synthesize zonae pellucidae, organize surrounding somatic cells into follicles, are ovulated in response to hormonal stimulation, undergo asymmetric cell division to produce a polar body and cleave to form two-cell embryos upon fertilization. These events occur without premeiotic chromosomal replication, sister chromatid cohesion, synapsis or recombination. Thus, oocyte growth and differentiation are genetically dissociable from the chromosomal events of meiosis. These findings open to study the independent contributions of meiosis and oocyte differentiation to the making of a functional egg.

Ikbkap/Elp1 deficiency causes male infertility by disrupting meiotic progression

Mouse Ikbkap gene encodes IKAP—one of the core subunits of Elongator—and is thought to be involved in transcription. However, the biological function of IKAP, particularly within the context of an animal model, remains poorly characterized. We used a loss-of-function approach in mice to demonstrate that Ikbkap is essential for meiosis during spermatogenesis. Absence of Ikbkap results in defects in synapsis and meiotic recombination, both of which result in increased apoptosis and complete arrest of gametogenesis. In Ikbkap-mutant testes, a few meiotic genes are down-regulated, suggesting IKAP's role in transcriptional regulation. In addition, Ikbkap-mutant testes exhibit defects in wobble uridine tRNA modification, supporting a conserved tRNA modification function from yeast to mammals. Thus, our study not only reveals a novel function of IKAP in meiosis, but also suggests that IKAP contributes to this process partly by exerting its effect on transcription and tRNA modification.

The process of meiosis is responsible for gamete formation and ensures that offspring will inherit a complete set of chromosomes from each parent. Errors arising during this process generally result in spontaneous abortions, birth defects, or infertility. Many genes that are essential in regulating meiosis have also been implicated in DNA repair. Importantly, defects in DNA repair are common causes of cancers. Therefore, identification of genes important for normal meiosis contributes not only to the field of reproduction but also to the field of cancer biology. We studied the effects of deleting mouse Ikbkap, a gene that encodes one of the subunit of the Elongator complex initially described as an RNA polymerase II–associated transcription elongation factor. We demonstrate that Ikbkap mutant mice exhibit infertility and defects in meiotic progression. Specifically, homologous and sex chromosomes fail to synapse (become associated), DNA double-strand breaks are inefficiently repaired, and DNA crossovers are significantly decreased in Ikbkap males. We also demonstrate that the requirement for Elongator in tRNA modification, which has been shown in lower eukaryotes, is conserved in mammals. Our findings suggest novel roles for Ikbkap in meiosis progression and tRNA modification, which have not been reported previously.

PRC1 coordinates timing of sexual differentiation of female primordial germ cells

In mammals, sex differentiation of primordial germ cells (PGCs) is determined by extrinsic cues from the environment. In mouse female PGCs, expression of stimulated by retinoic acid gene 8 (Stra8) and meiosis are induced in response to retinoic acid provided from the mesonephroi. Given the widespread role of retinoic acid signalling during development, the molecular mechanisms that enable PGCs to express Stra8 and enter meiosis in a timely manner are unknown. Here we identify gene-dosage-dependent roles in PGC development for Ring1 and Rnf2, two central components of the Polycomb repressive complex 1 (PRC1). Both paralogues are essential for PGC development between days 10.5 and 11.5 of gestation. Rnf2 is subsequently required in female PGCs to maintain high levels of Oct4 (also known as Pou5f1) and Nanog expression, and to prevent premature induction of meiotic gene expression and entry into meiotic prophase. Chemical inhibition of retinoic acid signalling partially suppresses precocious Oct4 downregulation and Stra8 activation in Rnf2-deficient female PGCs. Chromatin immunoprecipitation analyses show that Stra8 is a direct target of PRC1 and PRC2 in PGCs. These data demonstrate the importance of PRC1 gene dosage in PGC development and in coordinating the timing of sex differentiation of female PGCs by antagonizing extrinsic retinoic acid signalling.

HECT-E3 ligase ETC-1 regulates securin and cyclin B1 cytoplasmic abundance to promote timely anaphase during meiosis in C. elegans

The anaphase inhibitor securin plays a crucial role in regulating the timing of sister chromatid separation during mitosis. When sister chromatid pairs become bioriented, the E3 ligase anaphase promoting complex/cyclosome (APC/C) ubiquitylates securin for proteolysis, triggering sister chromatid separation. Securin is also implicated in regulating meiotic progression. Securin protein levels change sharply during cell cycle progression, enabling its timely action. To understand the mechanism underlying the tightly regulated dynamics of securin, we analyzed the subcellular localization of the securin IFY-1 during C. elegans development. IFY-1 was highly expressed in the cytoplasm of germ cells. The cytoplasmic level of IFY-1 declined immediately following meiosis I division and remained low during meiosis II and following mitoses. We identified a C. elegans homolog of another type of E3 ligase, UBE3C, designated ETC-1, as a regulator of the cytoplasmic IFY-1 level. RNAi-mediated depletion of ETC-1 stabilized IFY-1 and CYB-1 (cyclin B1) in post-meiosis I embryos. ETC-1 knockdown in a reduced APC function background caused an embryonic lethal phenotype. In vitro, ETC-1 ubiquitylates IFY-1 and CYB-1 in the presence of the E2 enzyme UBC-18, which functions in pharyngeal development. Genetic analysis revealed that UBC-18 plays a distinct role together with ETC-1 in regulating the cytoplasmic level of IFY-1 during meiosis. Our study reports a novel mechanism, mediated by ETC-1, that co-operates with APC/C to maintain the meiotic arrest required for proper cell cycle timing during reproduction.

Sister chromatid segregation in meiosis II: deprotection through phosphorylation

Meiotic divisions (meiosis I and II) are specialized cell divisions to generate haploid gametes. The first meiotic division with the separation of chromosomes is named reductional division. The second division, which takes place immediately after meiosis I without intervening S-phase, is equational, with the separation of sister chromatids, similar to mitosis. This meiotic segregation pattern requires the two-step removal of the cohesin complex holding sister chromatids together: cohesin is removed from chromosome arms that have been subjected to homologous recombination in meiosis I and from the centromere region in meiosis II. Cohesin in the centromere region is protected from removal in meiosis I, but this protection has to be removed--deprotected--for sister chromatid segregation in meiosis II. Whereas the mechanisms of cohesin protection are quite well understood, the mechanisms of deprotection have been largely unknown until recently. In this review I summarize our current knowledge on cohesin deprotection.

Chromosome pairing and synapsis during Caenorhabditis elegans meiosis

Meiosis is the specialized cell division cycle that produces haploid gametes to enable sexual reproduction. Reduction of chromosome number by half requires elaborate chromosome dynamics that occur in meiotic prophase to establish physical linkages between each pair of homologous chromosomes. Caenorhabditis elegans has emerged as an excellent model organism for molecular studies of meiosis, enabling investigators to combine the power of molecular genetics, cytology, and live analysis. Here we focus on recent studies that have shed light on how chromosomes find and identify their homologous partners, and the structural changes that accompany and mediate these interactions.

Altered cohesin gene dosage affects Mammalian meiotic chromosome structure and behavior

Based on studies in mice and humans, cohesin loss from chromosomes during the period of protracted meiotic arrest appears to play a major role in chromosome segregation errors during female meiosis. In mice, mutations in meiosis-specific cohesin genes cause meiotic disturbances and infertility. However, the more clinically relevant situation, heterozygosity for mutations in these genes, has not been evaluated. We report here evidence from the mouse that partial loss of gene function for either Smc1b or Rec8 causes perturbations in the formation of the synaptonemal complex (SC) and affects both synapsis and recombination between homologs during meiotic prophase. Importantly, these defects increase the frequency of chromosomally abnormal eggs in the adult female. These findings have important implications for humans: they suggest that women who carry mutations or variants that affect cohesin function have an elevated risk of aneuploid pregnancies and may even be at increased risk of transmitting structural chromosome abnormalities.

RanBPM, a scaffolding protein for gametogenesis

RanBPM is a multimodular scaffold protein that interacts with a great variety of molecules including nuclear, cytoplasmic, and membrane proteins. By building multiprotein complexes, RanBPM is thought to regulate various signaling pathways, especially in the immune and nervous system. However, the diversity of these interactions does not facilitate the identification of its precise mechanism of action, and therefore the physiological role of RanBPM still remains unclear. Recently, RanBPM has been shown to be critical for the fertility of both genders in mouse. Although mechanistically it is still unclear how RanBPM affects gametogenesis, the data collected so far suggest that it is a key player in this process. Here, we examine the RanBPM sterility phenotype in the context of other genetic mutations affecting mouse gametogenesis to investigate whether this scaffold protein affects the function of other known proteins whose deficiency results in similar sterility phenotypes.

A specific family of interspersed repeats (SINEs) facilitates meiotic synapsis in mammals

Background

Errors during meiosis that affect synapsis and recombination between homologous chromosomes contribute to aneuploidy and infertility in humans. Despite the clinical relevance of these defects, we know very little about the mechanisms by which homologous chromosomes interact with one another during mammalian meiotic prophase. Further, we remain ignorant of the way in which chromosomal DNA complexes with the meiosis-specific structure that tethers homologs, the synaptonemal complex (SC), and whether specific DNA elements are necessary for this interaction.

Results

In the present study we utilized chromatin immunoprecipitation (ChIP) and DNA sequencing to demonstrate that the axial elements of the mammalian SC are markedly enriched for a specific family of interspersed repeats, short interspersed elements (SINEs). Further, we refine the role of the repeats to specific sub-families of SINEs, B1 in mouse and AluY in old world monkey (Macaca mulatta).

Conclusions

Because B1 and AluY elements are the most actively retrotransposing SINEs in mice and rhesus monkeys, respectively, our observations imply that they may serve a dual function in axial element binding; i.e., as the anchoring point for the SC but possibly also as a suppressor/regulator of retrotransposition.

Cohesin in gametogenesis

Sister chromatid cohesion depends on cohesin, a tripartite complex that forms ring structures to hold sister chromatids together in mitosis and meiosis. Meiocytes feature a multiplicity of distinct cohesin proteins and complexes, some meiosis specific, which serve additional functions such as supporting synapsis of two pairs of sister chromatids and determining the loop-axis architecture of prophase I chromosomes. Despite considerable new insights gained in the past few years into the localization and function of some cohesin proteins, and the recent identification of yet another meiosis-specific cohesin subunit, a plethora of open questions remains, which concern not only fundamental germ cell biology but also the consequences of cohesin impairment for human reproductive health.

Disruption of CHTF18 causes defective meiotic recombination in male mice

CHTF18 (chromosome transmission fidelity factor 18) is an evolutionarily conserved subunit of the Replication Factor C-like complex, CTF18-RLC. CHTF18 is necessary for the faithful passage of chromosomes from one daughter cell to the next during mitosis in yeast, and it is crucial for germline development in the fruitfly. Previously, we showed that mouse Chtf18 is expressed throughout the germline, suggesting a role for CHTF18 in mammalian gametogenesis. To determine the role of CHTF18 in mammalian germ cell development, we derived mice carrying null and conditional mutations in the Chtf18 gene. Chtf18-null males exhibit 5-fold decreased sperm concentrations compared to wild-type controls, resulting in subfertility. Loss of Chtf18 results in impaired spermatogenesis; spermatogenic cells display abnormal morphology, and the stereotypical arrangement of cells within seminiferous tubules is perturbed. Meiotic recombination is defective and homologous chromosomes separate prematurely during prophase I. Repair of DNA double-strand breaks is delayed and incomplete; both RAD51 and γH2AX persist in prophase I. In addition, MLH1 foci are decreased in pachynema. These findings demonstrate essential roles for CHTF18 in mammalian spermatogenesis and meiosis, and suggest that CHTF18 may function during the double-strand break repair pathway to promote the formation of crossovers.

Meiosis is the specialized process of cell division during germ cell development that results in formation of eggs and sperm. Genetic exchange between maternal and paternal chromosomes occurs during meiosis in a process called homologous recombination, in which DNA double- strand breaks are made and then repaired to allow DNA crossovers to form. These are essential processes that keep homologous chromosomes joined until anaphase I and ensure proper chromosome segregation. Errors in meiotic recombination lead to chromosome mis-segregation and ultimately aneuploidy, an abnormal chromosome number. Although it is well known that defects in these processes contribute greatly to infertility, birth defects, and pregnancy loss in humans, their molecular basis is not well understood. We demonstrate here a Chtf18 mutant mouse that exhibits subfertility and defects in meiotic recombination. Specifically, DNA double-strand breaks are incompletely repaired, DNA crossovers are significantly decreased, and homologous chromosomes separate during prophase I in Chtf18-null males. Our findings suggest roles for CHTF18 in DNA double-strand break repair and crossover formation, functions in mammals not previously known.

HAL-2 promotes homologous pairing during Caenorhabditis elegans meiosis by antagonizing inhibitory effects of synaptonemal complex precursors

During meiosis, chromosomes align with their homologous pairing partners and stabilize this alignment through assembly of the synaptonemal complex (SC). Since the SC assembles cooperatively yet is indifferent to homology, pairing and SC assembly must be tightly coordinated. We identify HAL-2 as a key mediator in this coordination, showing that HAL-2 promotes pairing largely by preventing detrimental effects of SC precursors (SYP proteins). hal-2 mutants fail to establish pairing and lack multiple markers of chromosome movement mediated by pairing centers (PCs), chromosome sites that link chromosomes to cytoplasmic microtubules through nuclear envelope-spanning complexes. Moreover, SYP proteins load inappropriately along individual unpaired chromosomes in hal-2 mutants, and markers of PC-dependent movement and function are restored in hal-2; syp double mutants. These and other data indicate that SYP proteins can impede pairing and that HAL-2 promotes pairing predominantly but not exclusively by counteracting this inhibition, thereby enabling activation and regulation of PC function. HAL-2 concentrates in the germ cell nucleoplasm and colocalizes with SYP proteins in nuclear aggregates when SC assembly is prevented. We propose that HAL-2 functions to shepherd SYP proteins prior to licensing of SC assembly, preventing untimely interactions between SC precursors and chromosomes and allowing sufficient accumulation of precursors for rapid cooperative assembly upon homology verification.

For successful segregation of homologous chromosomes during sexual reproduction, homologs must first identify and pair with their correct partners. Further, many organisms stabilize and maintain alignment between paired homologs through assembly of a highly ordered structure known as the synaptonemal complex (SC). Pairing and synapsis must be tightly coordinated to ensure that SC assembly only occurs in a productive manner, linking the axes of correctly aligned homologous chromosomes. In this work, we identify HAL-2, a protein that concentrates in the nucleoplasm of germ cells, as a key player in mediating this coordination. We find that precursors of the SC have the potential to inhibit homolog pairing, interfering with the very process that the SC normally serves to stabilize. Moreover, we show that HAL-2 promotes homolog pairing and associated chromosome movement primarily by counteracting these detrimental inhibitory effects of SC precursors. Our data suggest that HAL-2 serves to prevent inappropriate association of SC precursors with chromosomes prior to licensing of SC assembly, and we propose that HAL-2 may enable precursors to accumulate in a manner that allows rapid, cooperative SC assembly upon homology verification.

Genetic variants in REC8, RNF212, and PRDM9 influence male recombination in cattle

We use >250,000 cross-over events identified in >10,000 bovine sperm cells to perform an extensive characterization of meiotic recombination in male cattle. We map Quantitative Trait Loci (QTL) influencing genome-wide recombination rate, genome-wide hotspot usage, and locus-specific recombination rate. We fine-map three QTL and present strong evidence that genetic variants in REC8 and RNF212 influence genome-wide recombination rate, while genetic variants in PRDM9 influence genome-wide hotspot usage.

Homologous recombination is an essential cellular process that determines proper chromosome segregation during meiosis, affects fertility, and influences evolvability. Nevertheless, the components of the recombination apparatus remain incompletely characterized in mammals. One approach to identify such components is to identify the genes that underlie inherited variation in recombination phenotypes. In addition to providing mechanistic insights, this would allow the study of the evolutionary forces that shape the recombination process. In this paper, we take advantage of genotypes for 50,000 genome-wide SNP markers to measure four recombination phenotypes (genome-wide recombination rate, genome-wide hotspot usage, locus-specific recombination rate, genome-wide cross-over interference) for >750 bulls on the basis of >250,000 cross-overs detected in sperm cells transmitted to >10,000 sons. We quantify the heritability and scan the genome for Quantitative Trait Loci (QTL) influencing each one of these recombination phenotypes. We perform a detailed genetic analysis of three such QTL, thereby providing evidence that genetic variants in REC8 and RNF212 influence genome-wide recombination rate, while genetic variants in an X-linked PRDM9 paralogue influence genome-wide hotspot usage.

Cohesin in determining chromosome architecture

Cells use ring-like structured protein complexes for various tasks in DNA dynamics. The tripartite cohesin ring is particularly suited to determine chromosome architecture, for it is large and dynamic, may acquire different forms, and is involved in several distinct nuclear processes. This review focuses on cohesin's role in structuring chromosomes during mitotic and meiotic cell divisions and during interphase.

Age-related aneuploidy through cohesion exhaustion

The trend of women to become pregnant when older than in previous generations poses a paramount medical problem, for oocytes are particularly prone to chromosome missegregation, and aneuploidy increases with age. Recent data strongly suggest that as oocyte age increases sister chromatid cohesion is weakened or lost. Cohesin deterioration seems to contribute significantly to age-dependent aneuploidy, as discussed in this review.

Geometry and force behind kinetochore orientation: lessons from meiosis

During mitosis, replicated chromosomes (sister chromatids) become attached at the kinetochore by spindle microtubules emanating from opposite poles and segregate equationally. In the first division of meiosis, however, sister chromatids become attached from the same pole and co-segregate, whereas homologous chromosomes connected by chiasmata segregate to opposite poles. Disorder in this specialized chromosome attachment in meiosis is the leading cause of miscarriage in humans. Recent studies have elucidated the molecular mechanisms determining chromosome orientation, and consequently segregation, in meiosis. Comparative studies of meiosis and mitosis have led to the general principle that kinetochore geometry and tension exerted by microtubules synergistically generate chromosome orientation.

Cohesin-SA1 deficiency drives aneuploidy and tumourigenesis in mice due to impaired replication of telomeres

Cohesin is a protein complex originally identified for its role in sister chromatid cohesion, although increasing evidence portrays it also as a major organizer of interphase chromatin. Vertebrate cohesin consists of Smc1, Smc3, Rad21/Scc1 and either stromal antigen 1 (SA1) or SA2. To explore the functional specificity of these two versions of cohesin and their relevance for embryonic development and cancer, we generated a mouse model deficient for SA1. Complete ablation of SA1 results in embryonic lethality, while heterozygous animals have shorter lifespan and earlier onset of tumourigenesis. SA1-null mouse embryonic fibroblasts show decreased proliferation and increased aneuploidy as a result of chromosome segregation defects. These defects are not caused by impaired centromeric cohesion, which depends on cohesin-SA2. Instead, they arise from defective telomere replication, which requires cohesion mediated specifically by cohesin-SA1. We propose a novel mechanism for aneuploidy generation that involves impaired telomere replication upon loss of cohesin-SA1, with clear implications in tumourigenesis.

Molecular control of rodent spermatogenesis

Spermatogenesis is a complex developmental process that ultimately generates mature spermatozoa. This process involves a phase of proliferative expansion, meiosis, and cytodifferentiation. Mouse models have been widely used to study spermatogenesis and have revealed many genes and molecular mechanisms that are crucial in this process. Although meiosis is generally considered as the most crucial phase of spermatogenesis, mouse models have shown that pre-meiotic and post-meiotic phases are equally important. Using knowledge generated from mouse models and in vitro studies, the current review provides an overview of the molecular control of rodent spermatogenesis. Finally, we briefly relate this knowledge to fertility problems in humans and discuss implications for future research. This article is part of a Special Issue entitled: Molecular Genetics of Human Reproductive Failure.

Phosphorylation of chromosome core components may serve as axis marks for the status of chromosomal events during mammalian meiosis

Meiotic recombination and chromosome synapsis between homologous chromosomes are essential for proper chromosome segregation at the first meiotic division. While recombination and synapsis, as well as checkpoints that monitor these two events, take place in the context of a prophase I-specific axial chromosome structure, it remains unclear how chromosome axis components contribute to these processes. We show here that many protein components of the meiotic chromosome axis, including SYCP2, SYCP3, HORMAD1, HORMAD2, SMC3, STAG3, and REC8, become post-translationally modified by phosphorylation during the prophase I stage. We found that HORMAD1 and SMC3 are phosphorylated at a consensus site for the ATM/ATR checkpoint kinase and that the phosphorylated forms of HORMAD1 and SMC3 localize preferentially to unsynapsed chromosomal regions where synapsis has not yet occurred, but not to synapsed or desynapsed regions. We investigated the genetic requirements for the phosphorylation events and revealed that the phosphorylation levels of HORMAD1, HORMAD2, and SMC3 are dramatically reduced in the absence of initiation of meiotic recombination, whereas BRCA1 and SYCP3 are required for normal levels of phosphorylation of HORMAD1 and HORMAD2, but not of SMC3. Interestingly, reduced HORMAD1 and HORMAD2 phosphorylation is associated with impaired targeting of the MSUC (meiotic silencing of unsynapsed chromatin) machinery to unsynapsed chromosomes, suggesting that these post-translational events contribute to the regulation of the synapsis surveillance system. We propose that modifications of chromosome axis components serve as signals that facilitate chromosomal events including recombination, checkpoint control, transcription, and synapsis regulation.

SOHLH1 and SOHLH2 coordinate spermatogonial differentiation

Spermatogonial self-renewal and differentiation are essential for male fertility and reproduction. We discovered that germ cell specific genes Sohlh1 and Sohlh2, encode basic helix-loop-helix (bHLH) transcriptional regulators that are essential in spermatogonial differentiation. Sohlh1 and Sohlh2 individual mouse knockouts show remarkably similar phenotypes. Here we show that SOHLH1 and SOHLH2 proteins are co-expressed in the entire spermatogonial population except in the GFRA1(+) spermatogonia, which includes spermatogonial stem cells (SSCs). SOHLH1 and SOHLH2 are expressed in both KIT negative and KIT positive spermatogonia, and overlap Ngn3/EGFP and SOX3 expression. SOHLH1 and SOHLH2 heterodimerize with each other in vivo, as well as homodimerize. The Sohlh1/Sohlh2 double mutant phenocopies single mutants, i.e., spermatogonia continue to proliferate but do not differentiate properly. Further analysis revealed that GFRA1(+) population was increased, while meiosis commenced prematurely in both single and double knockouts. Sohlh1 and Sohlh2 double deficiency has a synergistic effect on gene expression patterns as compared to the single knockouts. SOHLH proteins affect spermatogonial development by directly regulating Gfra1, Sox3 and Kit gene expression. SOHLH1 and SOHLH2 suppress genes involved in SSC maintenance, and induce genes important for spermatogonial differentiation.

Genetic evidence that synaptonemal complex axial elements govern recombination pathway choice in mice

Chiasmata resulting from interhomolog recombination are critical for proper chromosome segregation at meiotic metaphase I, thus preventing aneuploidy and consequent deleterious effects. Recombination in meiosis is driven by programmed induction of double strand breaks (DSBs), and the repair of these breaks occurs primarily by recombination between homologous chromosomes, not sister chromatids. Almost nothing is known about the basis for recombination partner choice in mammals. We addressed this problem using a genetic approach. Since meiotic recombination is coupled with synaptonemal complex (SC) morphogenesis, we explored the role of axial elements--precursors to the lateral element in the mature SC--in recombination partner choice, DSB repair pathways, and checkpoint control. Female mice lacking the SC axial element protein SYCP3 produce viable, but often aneuploid, oocytes. We describe genetic studies indicating that while DSB-containing Sycp3-/- oocytes can be eliminated efficiently, those that survive have completed repair before the execution of an intact DNA damage checkpoint. We find that the requirement for DMC1 and TRIP13, proteins normally essential for recombination repair of meiotic DSBs, is substantially bypassed in Sycp3 and Sycp2 mutants. This bypass requires RAD54, a functionally conserved protein that promotes intersister recombination in yeast meiosis and mammalian mitotic cells. Immunocytological and genetic studies indicated that the bypass in Sycp3-/- Dmc1-/- oocytes was linked to increased DSB repair. These experiments lead us to hypothesize that axial elements mediate the activities of recombination proteins to favor interhomolog, rather than intersister recombinational repair of genetically programmed DSBs in mice. The elimination of this activity in SYCP3- or SYCP2-deficient oocytes may underlie the aneuploidy in derivative mouse embryos and spontaneous abortions in women.

The cohesin subunit RAD21L functions in meiotic synapsis and exhibits sexual dimorphism in fertility

The cohesin complex is a ring-shaped proteinaceous structure that entraps the two sister chromatids after replication until the onset of anaphase when the ring is opened by proteolytic cleavage of its α-kleisin subunit (RAD21 at mitosis and REC8 at meiosis) by separase. RAD21L is a recently identified α-kleisin that is present from fish to mammals and biochemically interacts with the cohesin subunits SMC1, SMC3 and STAG3. RAD21L localizes along the axial elements of the synaptonemal complex of mouse meiocytes. However, its existence as a bona fide cohesin and its functional role awaits in vivo validation. Here, we show that male mice lacking RAD21L are defective in full synapsis of homologous chromosomes at meiotic prophase I, which provokes an arrest at zygotene and leads to total azoospermia and consequently infertility. In contrast, RAD21L-deficient females are fertile but develop an age-dependent sterility. Thus, our results provide in vivo evidence that RAD21L is essential for male fertility and in females for the maintenance of fertility during natural aging.

Can corruption of chromosome cohesion create a conduit to cancer?

Cohesin is a conserved multisubunit protein complex with diverse cellular roles, making key contributions to the coordination of chromosome segregation, the DNA damage response and chromatin regulation by epigenetic mechanisms. Much has been learned in recent years about the roles of cohesin in a physiological context, whereas its potential and emerging role in tumour initiation and/or progression has received relatively little attention. In this Opinion article we examine how cohesin deregulation could contribute to cancer development on the basis of its physiological roles.

A new meiosis-specific cohesin complex implicated in the cohesin code for homologous pairing

We identify a new mammalian cohesin subunit, RAD21-like protein (RAD21L), with sequence similarity to RAD21 and REC8. RAD21L localizes along axial elements in early meiotic prophase, in a manner that is spatiotemporally different to either REC8 or RAD21. Remarkably, RAD21L and REC8 have symmetrical, mutually exclusive localization on the not-yet-synapsed homologues, implying that the cohesin patterning could provide a code for homologue recognition. RAD21 transiently localizes to axial elements after the dissociation of RAD21L and REC8 in late pachytene, a period of recombination repair. Further, we show that the removal of cohesins and synaptonemal complex during late meiotic prophase is promoted by Polo-like kinase 1, which is similar to the mitotic prophase pathway.

Cohesin proteins load sequentially during prophase I in tomato primary microsporocytes

Proteins of the cohesin complex are essential for sister chromatid cohesion and proper chromosome segregation during both mitosis and meiosis. Cohesin proteins are also components of axial elements/lateral elements (AE/LEs) of synaptonemal complexes (SCs) during meiosis, and cohesins are thought to play an important role in meiotic chromosome morphogenesis and recombination. Here, we have examined the cytological behavior of four cohesin proteins (SMC1, SMC3, SCC3, and REC8/SYN1) during early prophase I in tomato microsporocytes using immunolabeling. All four cohesins are discontinuously distributed along the length of AE/LEs from leptotene through early diplotene. Based on current models for the cohesin complex, the four cohesin proteins should be present at the same time and place in equivalent amounts. However, we observed that cohesins often do not colocalize at the same AE/LE positions, and cohesins differ in when they load onto and dissociate from AE/LEs of early prophase I chromosomes. Cohesin labeling of LEs from pachytene nuclei is similar through euchromatin, pericentric heterochromatin, and kinetochores but is distinctly reduced through the nucleolar organizer region of chromosome 2. These results indicate that the four cohesin proteins may form different complexes and/or perform additional functions during meiosis in plants, which are distinct from their essential function in sister chromatid cohesion.

RAD21L, a novel cohesin subunit implicated in linking homologous chromosomes in mammalian meiosis

Cohesins are multi-subunit protein complexes that regulate sister chromatid cohesion during mitosis and meiosis. Here we identified a novel kleisin subunit of cohesins, RAD21L, which is conserved among vertebrates. In mice, RAD21L is expressed exclusively in early meiosis: it apparently replaces RAD21 in premeiotic S phase, becomes detectable on the axial elements in leptotene, and stays on the axial/lateral elements until mid pachytene. RAD21L then disappears, and is replaced with RAD21. This behavior of RAD21L is unique and distinct from that of REC8, another meiosis-specific kleisin subunit. Remarkably, the disappearance of RAD21L at mid pachytene correlates with the completion of DNA double-strand break repair and the formation of crossovers as judged by colabeling with molecular markers, γ-H2AX, MSH4, and MLH1. RAD21L associates with SMC3, STAG3, and either SMC1α or SMC1β. Our results suggest that cohesin complexes containing RAD21L may be involved in synapsis initiation and crossover recombination between homologous chromosomes.

Sister chromatid cohesion control and aneuploidy

Apart from a personal tragedy, could Down syndrome, cancer and infertility possibly have something in common? Are there links between a syndrome with physical and mental problems, a tumor growing out of control and the incapability to reproduce? These questions can be answered if we look at the biological functions of a protein complex, named cohesin, which is the main protagonist in the regulation of sister chromatid cohesion during chromosome segregation in cell division. The establishment, maintenance and removal of sister chromatid cohesion is one of the most fascinating and dangerous processes in the life of a cell. Errors in the control of sister chromatid cohesion frequently lead to cell death or aneuploidy. Recent results showed that cohesins also have important functions in non-dividing cells, revealing new, unexplored roles for these proteins in human syndromes, currently known as cohesinopathies. In the last 10 years, we have improved our understanding of the molecular mechanisms of the cohesin and cohesin-interacting proteins regulating the different events of sister chromatid cohesion during cell division in mitosis and meiosis.

Synaptonemal complex length variation in wild-type male mice

Meiosis yields haploid gametes following two successive divisions of a germ cell in the absence of intervening DNA replication. Balanced segregation of homologous chromosomes in Meiosis I is aided by a proteinaceous structure, the synaptonemal complex (SC). The objective of this study was to determine total average autosomal SC lengths in spermatocytes in three commonly used mouse strains (129S4/SvJae, C57BL/6J, and BALB/c). Our experiments revealed that the total autosomal SC length in BALB/c spermatocytes is 9% shorter than in the two other strains. Shorter SCs are also observed in spermatocytes of (BALB/c × 129S4/SvJae) and (C57BL/6J × BALB/c) F1 hybrids suggesting a genetic basis of SC length regulation. Along these lines, we studied expression of a selected group of genes implicated in meiotic chromosome architecture. We found that BALB/c testes express up to 6-fold less of Rec8 mRNA and 4-fold less of REC8 protein. These results suggest that the mechanism that defines the SC length operates via a REC8‑dependent process. Finally, our results demonstrate that genetic background can have an effect on meiotic studies in mice.

The Mouse Cohesin-Associated Protein PDS5B Is Expressed in Testicular Cells and Is Associated with the Meiotic Chromosome Axes

During the first meiotic prophase, the cohesin complex is localized to the chromosome axis and contributes to chromosome organization, pairing, synapsis, and recombination. The PDS5 protein, an accessory factor of the cohesin complex, is known to be a component of meiotic chromosome cores in fungi and to be implicated in meiotic chromosome structure and function. We found by immunoblotting experiments that a mammalian PDS5 protein, PDS5B, is abundantly expressed in mouse testis compared to other tissues. Immunofluorescence labeling experiments revealed that PDS5B is highly expressed in spermatogonia and that most PDS5B is depleted from chromatin as cells enter meiosis. During the first meiotic prophase, PDS5B associates with the axial cores of chromosomes. The axial association of PDS5B was observed also in the absence of synaptonemal complex proteins, such as SYCP1 and SYCP3, suggesting that PDS5B is an integral part of the chromosome axis as defined by the cohesin complex. These results suggest that PDS5B modulates cohesin functions in spermatocytes as well as in spermatogonia, contributing to meiotic chromosome structure and function.

Cohesin in oocytes-tough enough for Mammalian meiosis?

Sister chromatid cohesion is essential for cell division. During meiosis, it is also required for proper synapsis of pairs of sister chromatids and for chiasma formation and maintenance. Since mammalian oocytes remain arrested in late prophase for a very long period—up to five decades in humans—the preservation of cohesion throughout this period is a formidable challenge. Mouse models with cohesin deficiencies and aging wild-type mice showed that this challenge is not fully met: cohesion weakens and deteriorates with increasing age. These recent findings have highly significant implications for our comprehension of the genesis of aneuploidies.

Many functions of the meiotic cohesin

Sister chromatids are held together from the time of their formation in S phase until they segregate in anaphase by the cohesin complex. In meiosis of most organisms, the mitotic Mcd1/Scc1/Rad21 subunit of the cohesin complex is largely replaced by its paralog named Rec8. This article reviews the specialized functions of Rec8 that are crucial for diverse aspects of chromosome dynamics in meiosis, and presents some speculations relating to meiotic chromosome organization.

Deterioration without replenishment--the misery of oocyte cohesin

Humans suffer a steep increase in aneuploidies when oocytes age, and deterioration of cohesin was suggested recently as a prominent cause. In the November 15, 2010, issue of Genes & Development, Tachibana-Konwalski and colleagues (pp. 2505-2516) answered a question central to this hypothesis: Can cohesin be reloaded onto mouse oocyte chromosomes long after birth? They found that it cannot, or at least not with an efficiency adequate to rescue cohesin deficiency. With no chance for sufficient replenishment, age-related loss of sister chromatid cohesion seems unavoidable.

Rec8-containing cohesin maintains bivalents without turnover during the growing phase of mouse oocytes

During female meiosis, bivalent chromosomes are thought to be held together from birth until ovulation by sister chromatid cohesion mediated by cohesin complexes whose ring structure depends on kleisin subunits, either Rec8 or Scc1. Because cohesion is established at DNA replication in the embryo, its maintenance for such a long time may require cohesin turnover. To address whether Rec8- or Scc1-containing cohesin holds bivalents together and whether it turns over, we created mice whose kleisin subunits can be cleaved by TEV protease. We show by microinjection experiments and confocal live-cell imaging that Rec8 cleavage triggers chiasmata resolution during meiosis I and sister centromere disjunction during meiosis II, while Scc1 cleavage triggers sister chromatid disjunction in the first embryonic mitosis, demonstrating a dramatic transition from Rec8- to Scc1-containing cohesin at fertilization. Crucially, activation of an ectopic Rec8 transgene during the growing phase of Rec8(TEV)(/TEV) oocytes does not prevent TEV-mediated bivalent destruction, implying little or no cohesin turnover for ≥2 wk during oocyte growth. We suggest that the inability of oocytes to regenerate cohesion may contribute to age-related meiosis I errors.

Mouse models in male fertility research

Limited knowledge of the genetic causes of male infertility has resulted in few treatment and targeted therapeutic options. Although the ideal approach to identify infertility causing mutations is to conduct studies in the human population, this approach has progressed slowly due to the limitations described herein. Given the complexity of male fertility, the entire process cannot be modeled in vitro. As such, animal models, in particular mouse models, provide a valuable alternative for gene identification and experimentation. Since the introduction of molecular biology and recent advances in animal model production, there has been a substantial acceleration in the identification and characterization of genes associated with many diseases, including infertility. Three major types of mouse models are commonly used in biomedical research, including knockout/knockin/gene-trapped, transgenic and chemical-induced point mutant mice. Using these mouse models, over 400 genes essential for male fertility have been revealed. It has, however, been estimated that thousands of genes are involved in the regulation of the complex process of male fertility, as many such genes remain to be characterized. The current review is by no means a comprehensive list of these mouse models, rather it contains examples of how mouse models have advanced our knowledge of post-natal germ cell development and male fertility regulation.

Age-related meiotic segregation errors in mammalian oocytes are preceded by depletion of cohesin and Sgo2

BACKGROUND

The growing trend for women to postpone childbearing has resulted in a dramatic increase in the incidence of trisomic pregnancies. Maternal age-related miscarriage and birth defects are predominantly a consequence of chromosome segregation errors during the first meiotic division (MI), which involves the segregation of replicated recombined homologous chromosomes. Despite the importance to human reproductive health, the events precipitating female age-related meiotic errors are poorly understood.

RESULTS

Here we use a long-lived wild-type mouse strain to show that the ability to segregate chromosomes synchronously during anaphase of MI declines dramatically during female aging. This is preceded by depletion of chromosome-associated cohesin in association with destabilization of chiasmata, the physical linkages between homologous chromosomes, and loss of the tight association between sister centromeres. Loss of cohesin is not due to an age-related decline in the ability of the spindle checkpoint to delay separase-mediated cleavage of cohesin until entry into anaphase I. However, we find that reduced cohesin is accompanied by depletion of Sgo2, which protects centromeric cohesin during MI.

CONCLUSIONS

The data indicate that cohesin declines gradually during the long prophase arrest that precedes MI in female mammals. In aged oocytes, cohesin levels fall below the level required to stabilize chiasmata and to hold sister centromeres tightly together, leading to chromosome missegregation during MI. Cohesin loss may be amplified by a concomitant decline in the levels of the centromeric cohesin protector Sgo2. These findings indicate that cohesin is a key molecular link between female aging and chromosome missegregation during MI.

The use of transgenic mouse models in the study of male infertility

Over the past few decades with the rapid advances in embryo and embryonic stem cell manipulation techniques, transgenic mouse models have emerged as a powerful tool for the study of gene function and complex diseases including male infertility. In this review we give a brief history of the development of tools for the production of transgenic mouse models. This spans the advances from early pronuclear injection to the use of targeted embryonic stem cells to produce gene targeted, conditional, and inducible knockout mouse models. Lastly we provide a few examples to illustrate the utility of mouse models in the study of male infertility.