Zipping and Unzipping: Protein Modifications Regulating Synaptonemal Complex Dynamics

Multivalent weak interactions between assembly units drive synaptonemal complex formation

The synaptonemal complex (SC) is an ordered but highly dynamic structure assembled between homologous chromosomes to control interhomologous crossover formation, ensuring accurate meiotic chromosome segregation. However, the mechanisms regulating SC assembly and dynamics remain unclear. Here, we identified two new SC components, SYP-5 and SYP-6, in Caenorhabditis elegans that have distinct expression patterns and form distinct SC assembly units with other SYPs through stable interactions. SYP-5 and SYP-6 exhibit diverse in vivo SC regulatory functions and distinct phase separation properties in cells. Charge-interacting elements (CIEs) are enriched in SC intrinsically disordered regions (IDRs), and IDR deletion or CIE removal confirmed a requirement for these elements in SC regulation. Our data support the theory that multivalent weak interactions between the SC units drive SC formation and that CIEs confer multivalency to the assembly units.

Diter von Wettstein and The Meiotic Program of Pairing and Recombination

Recombination and pairing are prominent features of meiosis where they play an important role in increasing genetic diversity. In most organisms recombination also plays mechanical roles in mediating pairing of homologous chromosomes during prophase and in ensuring regular segregation of homologous pairs at the first meiotic division. The laboratory directed by D. von Wettstein identified six key steps in the meiotic process: (1) Recombination mediated processes occur in physical and functional linkage with the synaptonemal complex (SC), a highly conserved, meiosis-specific structure that links homologous axes along their lengths. (2) The pairing process involves formation and resolution of chromosomal entanglements/interlockings. (3) The SC normally forms specifically between homologous chromosomes, but in unusual situations can form between nonhomologous chromosomes or regions resulting in two-phase SC formation. (4) In hexaploid common wheat, extensive multivalents form with multiple, pairing partner shifts, indicating homology recognition and SC formation among homoeologs as well as homologs. (5) Linkage between recombination and the SC is revealed by crossover-correlated nodules localized in the SC central region. (6) Modified SCs sometimes play a direct role in homolog segregation, providing the required connection between homologs in absence of crossovers/chiasmata.

Homozygous mutations in C14orf39/SIX6OS1 cause non-obstructive azoospermia and premature ovarian insufficiency in humans

Human infertility is a multifactorial disease that affects 8%-12% of reproductive-aged couples worldwide. However, the genetic causes of human infertility are still poorly understood. Synaptonemal complex (SC) is a conserved tripartite structure that holds homologous chromosomes together and plays an indispensable role in the meiotic progression. Here, we identified three homozygous mutations in the SC coding gene C14orf39/SIX6OS1 in infertile individuals from different ethnic populations by whole-exome sequencing (WES). These mutations include a frameshift mutation (c.204_205del [p.His68Glnfs∗2]) from a consanguineous Pakistani family with two males suffering from non-obstructive azoospermia (NOA) and one female diagnosed with premature ovarian insufficiency (POI) as well as a nonsense mutation (c.958G>T [p.Glu320∗]) and a splicing mutation (c.1180-3C>G) in two unrelated Chinese men (individual P3907 and individual P6032, respectively) with meiotic arrest. Mutations in C14orf39 resulted in truncated proteins that retained SYCE1 binding but exhibited impaired polycomplex formation between C14ORF39 and SYCE1. Further cytological analyses of meiosis in germ cells revealed that the affected familial males with the C14orf39 frameshift mutation displayed complete asynapsis between homologous chromosomes, while the affected Chinese men carrying the nonsense or splicing mutation showed incomplete synapsis. The phenotypes of NOA and POI in affected individuals were well recapitulated by Six6os1 mutant mice carrying an analogous mutation. Collectively, our findings in humans and mice highlight the conserved role of C14ORF39/SIX6OS1 in SC assembly and indicate that the homozygous mutations in C14orf39/SIX6OS1 described here are responsible for infertility of these affected individuals, thus expanding our understanding of the genetic basis of human infertility.

SUMO is a pervasive regulator of meiosis

Protein modification by SUMO helps orchestrate the elaborate events of meiosis to faithfully produce haploid gametes. To date, only a handful of meiotic SUMO targets have been identified. Here, we delineate a multidimensional SUMO-modified meiotic proteome in budding yeast, identifying 2747 conjugation sites in 775 targets, and defining their relative levels and dynamics. Modified sites cluster in disordered regions and only a minority match consensus motifs. Target identities and modification dynamics imply that SUMOylation regulates all levels of chromosome organization and each step of meiotic prophase I. Execution-point analysis confirms these inferences, revealing functions for SUMO in S-phase, the initiation of recombination, chromosome synapsis and crossing over. K15-linked SUMO chains become prominent as chromosomes synapse and recombine, consistent with roles in these processes. SUMO also modifies ubiquitin, forming hybrid oligomers with potential to modulate ubiquitin signaling. We conclude that SUMO plays diverse and unanticipated roles in regulating meiotic chromosome metabolism.

Most mammalian, yeast and other eukaryote cells have two sets of chromosomes, one from each parent, which contain all the cell’s DNA. Sex cells – like the sperm and egg – however, have half the number of chromosomes and are formed by a specialized type of cell division known as meiosis. At the start of meiosis, each cell replicates its chromosomes so that it has twice the amount of DNA. The cell then undergoes two rounds of division to form sex cells which each contain only one set of chromosomes. Before the cell divides, the two duplicated sets of chromosomes pair up and swap sections of their DNA. This exchange allows each new sex cell to have a unique combination of DNA, resulting in offspring that are genetically distinct from their parents.

This complex series of events is tightly regulated, in part, by a protein called the 'small ubiquitin-like modifier' (or SUMO for short), which attaches itself to other proteins and modifies their behavior. This process, known as SUMOylation, can affect a protein’s stability, where it is located in the cell and how it interacts with other proteins. However, despite SUMO being known as a key regulator of meiosis, only a handful of its protein targets have been identified.

To gain a better understanding of what SUMO does during meiosis, Bhagwat et al. set out to find which proteins are targeted by SUMO in budding yeast and to map the specific sites of modification. The experiments identified 2,747 different sites on 775 different proteins, suggesting that SUMO regulates all aspects of meiosis. Consistently, inactivating SUMOylation at different times revealed SUMO plays a role at every stage of meiosis, including the replication of DNA and the exchanges between chromosomes. In depth analysis of the targeted proteins also revealed that SUMOylation targets different groups of proteins at different stages of meiosis and interacts with other protein modifications, including the ubiquitin system which tags proteins for destruction.

The data gathered by Bhagwat et al. provide a starting point for future research into precisely how SUMO proteins control meiosis in yeast and other organisms. In humans, errors in meiosis are the leading cause of pregnancy loss and congenital diseases. Most of the proteins identified as SUMO targets in budding yeast are also present in humans. So, this research could provide a platform for medical advances in the future. The next step is to study mammalian models, such as mice, to confirm that the regulation of meiosis by SUMO is the same in mammals as in yeast.

Synaptonemal complex proteins modulate the level of genome integrity in cancers

The synaptonemal complex (SC) is a proteinaceous structure that is transiently formed during meiosis to promote homologous recombination between maternal and paternal chromosomes. As this structure is required only for meiotic recombination, the proteins constituting the complex are almost undetectable in normal somatic cells, but they can be expressed under the conditions in which the transcriptional machinery is deregulated. Accumulating evidence indicates that they are epigenetically expressed in cancers of various origin. Not surprisingly, in contrast to their meiotic roles, the somatic roles of the SC proteins remain to be investigated. However, it has recently been reported that SYCP3 and SYCE2 control DNA double‐strand break repair negatively and positively, respectively, suggesting that the ectopic expression of the SC proteins in somatic cells could be associated with the maintenance of genomic instability. Thus, it is highly likely that the investigation of the somatic roles of the SC proteins would improve our understanding of the mechanisms underlying tumor development.

Synaptonemal complex formation produces a particular arrangement of the lateral element-associated DNA

During meiosis, homologous chromosomes exchange genetic material. This exchange or meiotic recombination is mediated by a proteinaceous scaffold known as the Synaptonemal complex (SC). Any defects in its formation produce failures in meiotic recombination, chromosome segregation and meiosis completion. It has been proposed that DNA repair events that will be resolved by crossover between homologous chromosomes are predetermined by the SC. Hence, structural analysis of the organization of the DNA in the SC could shed light on the process of crossover interference. In this work, we employed an ultrastructural DNA staining technique on mouse testis and followed nuclei of pachytene cells. We observed structures organized similarly to the SCs stained with conventional techniques. These structures, presumably the DNA in the SCs, are delineating the edges of both lateral elements and no staining was observed between them. DNA in the LEs resembles two parallel tracks. However, a bubble-like staining pattern in certain regions of the SC was observed. Furthermore, this staining pattern is found in SCs formed between non-homologous chromosomes, in SCs formed between sister chromatids and in SCs without lateral elements, suggesting that this particular organization of the DNA is determined by the synapsis of the chromosomes despite their lack of homology or the presence of partially formed SCs.

The Essential Function of SETDB1 in Homologous Chromosome Pairing and Synapsis during Meiosis

SETDB1 is a histone-lysine N-methyltransferase critical for germline development. However, its function in early meiotic prophase I remains unknown. Here, we report that Setdb1 null spermatocytes display aberrant centromere clustering during leptotene, bouquet formation during zygotene, and subsequent failure in pairing and synapsis of homologous chromosomes, as well as compromised meiotic silencing of unsynapsed chromatin, which leads to meiotic arrest before pachytene and apoptosis of spermatocytes. H3K9me3 is enriched in centromeric or pericentromeric regions and is present in many sites throughout the genome, with a subset changed in the Setdb1 mutant. These observations indicate that SETDB1-mediated H3K9me3 is essential for the bivalent formation in early meiosis. Transcriptome analysis reveals the function of SETDB1 in repressing transposons and transposon-proximal genes and in regulating meiotic and somatic lineage genes. These findings highlight a mechanism in which SETDB1-mediated H3K9me3 during early meiosis ensures the formation of homologous bivalents and survival of spermatocytes.

SCFCdc4 ubiquitin ligase regulates synaptonemal complex formation during meiosis

During meiosis, homologous chromosomes pair to form the synaptonemal complex (SC). This study showed that SCF^Cdc4 ubiquitin ligase is required for and works with Pch2 AAA+ ATPase for SC assembly.

Homologous chromosomes pair with each other during meiosis, culminating in the formation of the synaptonemal complex (SC), which is coupled with meiotic recombination. In this study, we showed that a meiosis-specific depletion mutant of a cullin (Cdc53) in the SCF (Skp-Cullin-F-box) ubiquitin ligase, which plays a critical role in cell cycle regulation during mitosis, is deficient in SC formation. However, the mutant is proficient in forming crossovers, indicating the uncoupling of meiotic recombination with SC formation in the mutant. Furthermore, the deletion of the PCH2 gene encoding a meiosis-specific AAA+ ATPase suppresses SC-assembly defects induced by CDC53 depletion. On the other hand, the pch2 cdc53 double mutant is defective in meiotic crossover formation, suggesting the assembly of SC with unrepaired DNA double-strand breaks. A temperature-sensitive mutant of CDC4, which encodes an F-box protein of SCF, shows meiotic defects similar to those of the CDC53-depletion mutant. These results suggest that SCF^Cdc4, probably SCF^Cdc4-dependent protein ubiquitylation, regulates and collaborates with Pch2 in SC assembly and meiotic recombination.

Aurora B and C kinases regulate chromosome desynapsis and segregation during mouse and human spermatogenesis

Precise control of chromosome dynamics during meiosis is critical for fertility. A gametocyte undergoing meiosis coordinates formation of the synaptonemal complex (SC) to promote efficient homologous chromosome recombination. Subsequent disassembly of the SC occurs prior to segregation of homologous chromosomes during meiosis I. We examined the requirements of the mammalian Aurora kinases (AURKA, AURKB and AURKC) during SC disassembly and chromosome segregation using a combination of chemical inhibition and gene deletion approaches. We find that both mouse and human spermatocytes fail to disassemble SC lateral elements when the kinase activity of AURKB and AURKC are chemically inhibited. Interestingly, both Aurkb conditional knockout and Aurkc knockout mouse spermatocytes successfully progress through meiosis, and the mice are fertile. In contrast, Aurkb, Aurkc double knockout spermatocytes fail to coordinate disassembly of SC lateral elements with chromosome condensation and segregation, resulting in delayed meiotic progression. In addition, deletion of Aurkb and Aurkc leads to an accumulation of metaphase spermatocytes, chromosome missegregation and aberrant cytokinesis. Collectively, our data demonstrate that AURKB and AURKC functionally compensate for one another ensuring successful mammalian spermatogenesis.This article has an associated First Person interview with the first author of the paper.

Genetic background impacts the timing of synaptonemal complex breakdown in Drosophila melanogaster

Experiments performed in different genetic backgrounds occasionally exhibit failure in experimental reproducibility. This is a serious issue in Drosophila where there are no standard control stocks. Here, we illustrate the importance of controlling genetic background by showing that the timing of a major meiotic event, the breakdown of the synaptonemal complex (SC), varies in different genetic backgrounds. We assessed SC breakdown in three different control stocks and found that in one control stock, y w; svspa-pol, the SC broke down earlier than in Oregon-R and w1118 stocks. We further examined SC breakdown in these three control backgrounds with flies heterozygous for a null mutation in c(3)G, which encodes a key structural component of the SC. Flies heterozygous for c(3)G displayed differences in the timing of SC breakdown in different control backgrounds, providing evidence of a sensitizing effect of this mutation. These observations suggest that SC maintenance is associated with the dosage of c(3)G in some backgrounds. Lastly, chromosome segregation was not affected by premature SC breakdown in mid-prophase, consistent with previous findings that chromosome segregation is not dependent on full-length SC in mid-prophase. Thus, genetic background is an important variable to consider with respect to SC behavior during Drosophila meiosis.

Micromanipulation of prophase I chromosomes from mouse spermatocytes reveals high stiffness and gel-like chromatin organization

Meiosis produces four haploid cells after two successive divisions in sexually reproducing organisms. A critical event during meiosis is construction of the synaptonemal complex (SC), a large, protein-based bridge that physically links homologous chromosomes. The SC facilitates meiotic recombination, chromosome compaction, and the eventual separation of homologous chromosomes at metaphase I. We present experiments directly measuring physical properties of captured mammalian meiotic prophase I chromosomes. Mouse meiotic chromosomes are about ten-fold stiffer than somatic mitotic chromosomes, even for genetic mutants lacking SYCP1, the central element of the SC. Meiotic chromosomes dissolve when treated with nucleases, but only weaken when treated with proteases, suggesting that the SC is not rigidly connected, and that meiotic prophase I chromosomes are a gel meshwork of chromatin, similar to mitotic chromosomes. These results are consistent with a liquid- or liquid-crystal SC, but with SC-chromatin stiff enough to mechanically drive crossover interference.

Ronald Biggs et al. report biophysical measurements of intact chromosomes isolated from mouse spermatocytes. They compare chromosomes in meiosis prophase I to mitotic chromosomes and find that meiotic chromosomes are much stiffer, and this stiffness does not depend on the central element of the synaptonemal complex (SYCP1).

Alternative Synaptonemal Complex Structures: Too Much of a Good Thing?

The synaptonemal complex (SC), a highly conserved structure built between homologous meiotic chromosomes, is required for crossover formation and ensuring proper chromosome segregation. In many organisms, SC components can also form alternative structures, including repeating SC structures that are known as polycomplexes (PCs), and extensively modified SC structures that are maintained late in meiosis. PCs display differences in their ability to localize with lateral element proteins, recombination machinery, and DNA. They can be created by defects in post-translational modification, suggesting that these modifications have roles in preventing alternate SC structures. These SC-like structures provide insight into the rules for building and maintaining the SC by offering an 'in vivo laboratory' for models of SC assembly, structure, and disassembly. Here, we discuss what these structures can tell us about the rules for building the SC and the roles of the SC in meiotic processes.

ATR signaling in mammalian meiosis: From upstream scaffolds to downstream signaling

In germ cells undergoing meiosis, the induction of double strand breaks (DSBs) is required for the generation of haploid gametes. Defects in the formation, detection, or recombinational repair of DSBs often result in defective chromosome segregation and aneuploidies. Central to the ability of meiotic cells to properly respond to DSBs are DNA damage response (DDR) pathways mediated by DNA damage sensor kinases. DDR signaling coordinates an extensive network of DDR effectors to induce cell cycle arrest and DNA repair, or trigger apoptosis if the damage is extensive. Despite their importance, the functions of DDR kinases and effector proteins during meiosis remain poorly understood and can often be distinct from their known mitotic roles. A key DDR kinase during meiosis is ataxia telangiectasia and Rad3-related (ATR). ATR mediates key signaling events that control DSB repair, cell cycle progression, and meiotic silencing. These meiotic functions of ATR depend on upstream scaffolds and regulators, including the 9-1-1 complex and TOPBP1, and converge on many downstream effectors such as the checkpoint kinase CHK1. Here, we review the meiotic functions of the 9-1-1/TOPBP1/ATR/CHK1 signaling pathway during mammalian meiosis.

Primary ovarian insufficiency, meiosis and DNA repair

Premature ovarian insufficiency (POI) is a major cause of female infertility. It is a heterogeneous disease that affects about 1% of women under 40 years of age. POI may be due to abnormal follicle stock formation, increased follicular atresia, impaired recruitment of dominant follicles, blocked follicular maturation or rapid depletion of the follicular stock. It remains idiopathic in most cases but the existence of familial cases shows that it can have a genetic origin. Next generation sequencing (NGS) strategies have allowed the identification of new genes involved in the etiology of POI. Here, I briefly describe some studies demonstrating that pathogenic variants in ‘DNA repair and meiotic genes’ underlie POI. Some of the examples show the power of the combination of classical genetics and NGS in the discovery of novel ‘POI genes’.

A Meiotic Checkpoint Alters Repair Partner Bias to Permit Inter-sister Repair of Persistent DSBs

Accurate meiotic chromosome segregation critically depends on the formation of inter-homolog crossovers initiated by double-strand breaks (DSBs). Inaccuracies in this process can drive aneuploidy and developmental defects, but how meiotic cells are protected from unscheduled DNA breaks remains unexplored. Here we define a checkpoint response to persistent meiotic DSBs in C. elegans that phosphorylates the synaptonemal complex (SC) to switch repair partner from the homolog to the sister chromatid. A key target of this response is the core SC component SYP-1, which is phosphorylated in response to ionizing radiation (IR) or unrepaired meiotic DSBs. Failure to phosphorylate (syp-1^6A) or dephosphorylate (syp-1^6D) SYP-1 in response to DNA damage results in chromosome non-dysjunction, hyper-sensitivity to IR-induced DSBs, and synthetic lethality with loss of brc-1^BRCA1. Since BRC-1 is required for inter-sister repair, these observations reveal that checkpoint-dependent SYP-1 phosphorylation safeguards the germline against persistent meiotic DSBs by channelling repair to the sister chromatid.

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Meiotic DNA damage triggers phosphorylation of the synaptonemal complex (SC)

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ATM-ATR kinases phosphorylate the SC in response to excessive meiotic DSBs

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SC phosphorylation channels DNA repair to the sister chromatid

Meiotic sex chromosome cohesion and autosomal synapsis are supported by Esco2

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

Acute myeloid leukemia immune escape by epigenetic CD48 silencing

Acute myeloid leukemia (AML) is a malignant disorder of hemopoietic stem cells. AML can escape immunosurveillance of natural killer (NK) by gene mutation, fusions and epigenetic modification. The mechanism of AML immune evasion is not clearly understood. Here we show that CD48 high expression is a favorable prognosis factor that is down-regulated in AML patients, which can help AML evade from NK cell recognition and killing. Furthermore, we demonstrate that CD48 expression is regulated by methylation and that a hypomethylating agent can increase the CD48 expression, which increases the NK cells killing in vitro. Finally, we show that CD48 high expression can reverse the AML immune evasion and activate NK cells function in vivo. The present study suggests that a combination the hypomethylating agent and NK cell infusion could be a new strategy to cure AML.

Slx5p-Slx8p Promotes Accurate Chromosome Segregation by Mediating the Degradation of Synaptonemal Complex Components during Meiosis

Meiosis increases genetic diversity, yet the genome complement needs to be stable to ensure offspring viability. Both small ubiquitin-like modifier (SUMO) and ubiquitin have been reported to participate in meiotic regulation, yet functions of the SUMO-ubiquitination crosstalk in meiosis remain unclear. Here, it is reported that a SUMO-targeted ubiquitin ligase, Slx8p, promotes accurate chromosome segregation during meiosis, since the deletion of SLX8 leads to increased aneuploidy due to a defect in synaptonemal complex (SC) component degradation. Both the RING domain and SUMO interacting motifs of Slx8p are essential for meiotic progression and maintaining spore viability, and the expression of tetraubiquitin fused with SUMO partially rescues meiotic defects in the SLX8-deletion strain. Furthermore, Slx5p-Slx8p can directly add ubiquitin to SUMOylated Zip1p and Ecm11p, and forced degradation of Ecm11p partially rescues the sporulation defects of the SLX8 deletion strain. These findings provide a mechanism for SC disassembly and reveal that the crosstalk between SUMOylation and ubiquitination facilitates accurate chromosome segregation by promoting SC component degradation during meiosis.

CENP-A binding domains and recombination patterns in horse spermatocytes

Centromeres exert an inhibitory effect on meiotic recombination, but the possible contribution of satellite DNA to this "centromere effect" is under debate. In the horse, satellite DNA is present at all centromeres with the exception of the one from chromosome 11. This organization of centromeres allowed us to investigate the role of satellite DNA on recombination suppression in horse spermatocytes at the stage of pachytene. To this aim we analysed the distribution of the MLH1 protein, marker of recombination foci, relative to CENP-A, marker of centromeric function. We demonstrated that the satellite-less centromere of chromosome 11 causes crossover suppression, similarly to satellite-based centromeres. These results suggest that the centromere effect does not depend on satellite DNA. During this analysis, we observed a peculiar phenomenon: while, as expected, the centromere of the majority of meiotic bivalent chromosomes was labelled with a single immunofluorescence centromeric signal, double-spotted or extended signals were also detected. Their number varied from 0 to 7 in different cells. This observation can be explained by positional variation of the centromeric domain on the two homologs and/or misalignment of pericentromeric satellite DNA arrays during homolog pairing confirming the great plasticity of equine centromeres.

The architecture of the Plasmodiophora brassicae nuclear and mitochondrial genomes

Plasmodiophora brassicae is a soil-borne pathogen that attacks roots of cruciferous plants causing clubroot disease. The pathogen belongs to the Plasmodiophorida order in Phytomyxea. Here we used long-read SMRT technology to clarify the P. brassicae e3 genomic constituents along with comparative and phylogenetic analyses. Twenty contigs representing the nuclear genome and one mitochondrial (mt) contig were generated, together comprising 25.1 Mbp. Thirteen of the 20 nuclear contigs represented chromosomes from telomere to telomere characterized by [TTTTAGGG] sequences. Seven active gene candidates encoding synaptonemal complex-associated and meiotic-related protein homologs were identified, a finding that argues for possible genetic recombination events. The circular mt genome is large (114,663 bp), gene dense and intron rich. It shares high synteny with the mt genome of Spongospora subterranea, except in a unique 12 kb region delimited by shifts in GC content and containing tandem minisatellite- and microsatellite repeats with partially palindromic sequences. De novo annotation identified 32 protein-coding genes, 28 structural RNA genes and 19 ORFs. ORFs predicted in the repeat-rich region showed similarities to diverse organisms suggesting possible evolutionary connections. The data generated here form a refined platform for the next step involving functional analysis, all to clarify the complex biology of P. brassicae.

Diverse roles for CDK-associated activity during spermatogenesis

The primary function of cyclin-dependent kinases (CDKs) in complex with their activating cyclin partners is to promote mitotic division in somatic cells. This canonical cell cycle-associated activity is also crucial for fertility as it allows the proliferation and differentiation of stem cells within the reproductive organs to generate meiotically competent cells. Intriguingly, several CDKs exhibit meiosis-specific functions and are essential for the completion of the two reductional meiotic divisions required to generate haploid gametes. These meiosis-specific functions are mediated by both known CDK/cyclin complexes and meiosis-specific CDK-regulators and are important for a variety of processes during meiotic prophase. The majority of meiotic defects observed upon deletion of these proteins occur during the extended prophase I of the first meiotic division. Importantly a lack of redundancy is seen within the meiotic arrest phenotypes described for many of these proteins, suggesting intricate layers of cell cycle control are required for normal meiotic progression. Using the process of male germ cell development (spermatogenesis) as a reference, this review seeks to highlight the diverse roles of selected CDKs their activators, and their regulators during gametogenesis.

Molecular architecture of the SYCP3 fibre and its interaction with DNA

The synaptonemal complex (SC) keeps homologous chromosomes in close alignment during meiotic recombination. A hallmark of the SC is the presence of its constituent protein SYCP3 on the chromosome axis. During SC assembly, SYCP3 is deposited on both axes of the homologue pair, forming axial elements that fuse into the lateral element (LE) in the tripartite structure of the mature SC. We have used cryo-electron tomography and atomic force microscopy to study the mechanism of assembly and DNA binding of the SYCP3 fibre. We find that the three-dimensional architecture of the fibre is built on a highly irregular arrangement of SYCP3 molecules displaying very limited local geometry. Interaction between SYCP3 molecules is driven by the intrinsically disordered tails of the protein, with no contact between the helical cores, resulting in a flexible fibre assembly. We demonstrate that the SYCP3 fibre can engage in extensive interactions with DNA, indicative of an efficient mechanism for incorporation of DNA within the fibre. Our findings suggest that SYCP3 deposition on the chromosome axis might take place by polymerization into a fibre that is fastened to the chromosome surface via DNA binding.

Signaling-mediated meiotic recombination in plants

Meiotic recombination provides genetic diversity in populations and ensures accurate homologous chromosome segregation for genome integrity. During meiosis, recombination processes, from DNA double strand breaks (DSBs) to crossover formation are tightly linked to higher order chromosome structure, including chromatid cohesion, axial element formation, homolog pairing and synapsis. The extensive studies on plant meiosis have revealed the important conserved roles for meiotic proteins in homologous recombination. Recent works have focused on elucidating the mechanistic basis of how meiotic proteins regulate recombination events via protein complex formation and modifications such as phosphorylation, ubiquitination, and SUMOylation. Here, we highlight recent advances on the signaling and modifications of meiotic proteins that mediate the formation of DSBs and crossovers in plants.

The pericentromeric heterochromatin of homologous chromosomes remains associated after centromere pairing dissolves in mouse spermatocyte meiosis

In meiosis, crossovers between homologous chromosomes link them together. This enables them to attach to microtubules of the meiotic spindle as a unit, such that the homologs will be pulled away from one another at anaphase I. Homologous pairs can sometimes fail to become linked by crossovers. In some organisms, these non-exchange partners are still able to segregate properly. In several organisms, associations between the centromeres of non-exchange partners occur in meiotic prophase. These associations have been proposed to promote segregation in meiosis I. But it is unclear how centromere pairing could promote subsequent proper segregation. Here we report that meiotic centromere pairing of chromosomes in mouse spermatocytes allows the formation of an association between chromosome pairs. We find that heterochromatin regions of homologous centromeres remain associated even after centromere-pairing dissolves. Our results suggest the model that, in mouse spermatocytes, heterochromatin maintains the association of homologous centromeres in the absence crossing-over.

Meiosis: the chromosomal foundation of reproduction

Meiosis is the chromosomal foundation of reproduction, with errors in this important process leading to aneuploidy and/or infertility. In this review celebrating the 50th anniversary of the founding of the Society for the Study of Reproduction, the important chromosomal structures and dynamics contributing to genomic integrity across generations are highlighted. Critical unsolved biological problems are identified, and the advances that will lead to their ultimate resolution are predicted.

Crossing and zipping: molecular duties of the ZMM proteins in meiosis

Accurate segregation of homologous chromosomes during meiosis depends on the ability of meiotic cells to promote reciprocal exchanges between parental DNA strands, known as crossovers (COs). For most organisms, including budding yeast and other fungi, mammals, nematodes, and plants, the major CO pathway depends on ZMM proteins, a set of molecular actors specifically devoted to recognize and stabilize CO-specific DNA intermediates that are formed during homologous recombination. The progressive implementation of ZMM-dependent COs takes place within the context of the synaptonemal complex (SC), a proteinaceous structure that polymerizes between homologs and participates in close homolog juxtaposition during prophase I of meiosis. While SC polymerization starts from ZMM-bound sites and ZMM proteins are required for SC polymerization in budding yeast and the fungus Sordaria, other organisms differ in their requirement for ZMM in SC elongation. This review provides an overview of ZMM functions and discusses their collaborative tasks for CO formation and SC assembly, based on recent findings and on a comparison of different model organisms.

Building bridges to move recombination complexes

A central feature of meiosis is pairing of homologous chromosomes, which occurs in two stages: coalignment of axes followed by installation of the synaptonemal complex (SC). Concomitantly, recombination complexes reposition from on-axis association to the SC central region. We show here that, in the fungus Sordaria macrospora, this critical transition is mediated by robust interaxis bridges that contain an axis component (Spo76/Pds5), DNA, plus colocalizing Mer3/Msh4 recombination proteins and the Zip2-Zip4 mediator complex. Mer3-Msh4-Zip2-Zip4 colocalizing foci are first released from their tight axis association, dependent on the SC transverse-filament protein Sme4/Zip1, before moving to bridges and thus to a between-axis position. Ensuing shortening of bridges and accompanying juxtaposition of axes to 100 nm enables installation of SC central elements at sites of between-axis Mer3-Msh4-Zip2-Zip4 complexes. We show also that the Zip2-Zip4 complex has an intrinsic affinity for chromosome axes at early leptotene, where it localizes independently of recombination, but is dependent on Mer3. Then, later, Zip2-Zip4 has an intrinsic affinity for the SC central element, where it ultimately localizes to sites of crossover complexes at the end of pachytene. These and other findings suggest that the fundamental role of Zip2-Zip4 is to mediate the recombination/structure interface at all post-double-strand break stages. We propose that Zip2-Zip4 directly mediates a molecular handoff of Mer3-Msh4 complexes, from association with axis components to association with SC central components, at the bridge stage, and then directly mediates central region installation during SC nucleation.

The E3 ubiquitin ligase Sina regulates the assembly and disassembly of the synaptonemal complex in Drosophila females

During early meiotic prophase, homologous chromosomes are connected along their entire lengths by a proteinaceous tripartite structure known as the synaptonemal complex (SC). Although the components that comprise the SC are predominantly studied in this canonical ribbon-like structure, they can also polymerize into repeated structures known as polycomplexes. We find that in Drosophila oocytes, the ability of SC components to assemble into canonical tripartite SC requires the E3 ubiquitin ligase Seven in absentia (Sina). In sina mutant oocytes, SC components assemble into large rod-like polycomplexes instead of proper SC. Thus, the wild-type Sina protein inhibits the polymerization of SC components, including those of the lateral element, into polycomplexes. These polycomplexes persist into meiotic stages when canonical SC has been disassembled, indicating that Sina also plays a role in controlling SC disassembly. Polycomplexes induced by loss-of-function sina mutations associate with centromeres, sites of double-strand breaks, and cohesins. Perhaps as a consequence of these associations, centromere clustering is defective and crossing over is reduced. These results suggest that while features of the polycomplexes can be recognized as SC by other components of the meiotic nucleus, polycomplexes nonetheless fail to execute core functions of canonical SC.

Meiotic behavior of a complex hexavalent in heterozygous mice for Robertsonian translocations: insights for synapsis dynamics

Natural populations of the house mouse Mus musculus domesticus show great diversity in chromosomal number due to the presence of chromosomal rearrangements, mainly Robertsonian translocations. Breeding between two populations with different chromosomal configurations generates subfertile or sterile hybrid individuals due to impaired meiotic development. In this study, we have analyzed prophase-I spermatocytes of hybrids formed by crossing mice from Vulcano and Lipari island populations. Both populations have a 2n = 26 karyotype but different combinations of Robertsonian translocations. We studied the progress of synapsis, recombination, and meiotic silencing of unsynapsed chromosomes during prophase-I through the immunolocalization of the proteins SYCP3, SYCP1, γH2AX, RAD51, and MLH1. In these hybrids, a hexavalent is formed that, depending on the degree of synapsis between chromosomes, can adopt an open chain, a ring, or a closed configuration. The frequency of these configurations varies throughout meiosis, with the maximum degree of synapsis occurring at mid pachytene. In addition, we observed the appearance of heterologous synapsis between telocentric and metacentric chromosomes; however, this synapsis seems to be transient and unstable and unsynapsed regions are frequently observed in mid-late pachytene. Interestingly, we found that chiasmata are frequently located at the boundaries of unsynapsed chromosomal regions in the hexavalent during late pachytene. These results provide new clues about synapsis dynamics during meiosis. We propose that mechanical forces generated along chromosomes may induce premature desynapsis, which, in turn, might be counteracted by the location of chiasmata. Despite these and additional meiotic features, such as the accumulation of γH2AX on unsynapsed chromosome regions, we observed a large number of cells that progressed to late stages of prophase-I, indicating that synapsis defects may not trigger a meiotic crisis in these hybrids.

Disassembly of the synaptonemal complex in chicken oocytes analyzed by super-resolution microscopy

The synaptonemal complex is an evolutionarily conserved, supramolecular structure that holds the homologous chromosomes together during the pachytene stage of the first meiotic prophase. Among vertebrates, synaptonemal complex dynamics has been analyzed in mouse spermatocytes following the assembly of its components from leptotene to pachytene stages. With few exceptions, a detailed study of the disassembly of SCs and the behavior of SC components at recombination sites at the onset of diplotene has not been accomplished. Here, we describe for the first time the progressive disassembly of the SC in chicken oocytes during the initial steps of desynapsis using immunolocalization of specific SC proteins and super-resolution microscopy. We found that transverse filament protein SYCP1 and central element component SYCE3 remain associated with the lateral elements at the beginning of chromosomal axis separation. As the separation between lateral elements widens, these proteins eventually disappear, without any evidence of subsequent association. Our observations support the idea that post-translational modifications of the central region components have a role at the initial phases of the SC disassembly. At the crossover sites, signaled by persistent MLH1 foci, the central region proteins are no longer detected when the SYCP3-positive lateral elements are widely separated. These findings are indicative that SC disassembly follows a general pattern along the desynaptic bivalents. The present work shows that the use of avian oocytes at prophase I provides a valuable model to explore the time course and chromosomal localization of SC proteins and its relationship with local changes along meiotic bivalents.

Sohlh1 is required for synaptonemal complex formation by transcriptionally regulating meiotic genes during spermatogenesis in mice

Gonad-specific transcription factor spermatogenesis- and oogenesis-specific helix-loop-helix transcription factor 1 (SOHLH1) plays a key role in the transcriptional regulation of the expression of differentiating spermatogonial genes. However, its role in spermatocytes (meiotic male germ cells) remains largely unknown. In this study, Sohlh1 knockout (KO) male mice displayed meiotic defects at the zygotene stage during spermatogenesis. Microarray analyses identified 66 upregulated genes and 139 downregulated genes in Sohlh1 KO testes compared with those in wild-type testes at postnatal Day 7.5. Among many of the downregulated genes, Sycp1 and Sycp3, which encode synaptonemal complex proteins 1 and 3 (SYCP1 and SYCP3), respectively, were significantly reduced in Sohlh1 knockout mice. Transmission electron microscopy revealed no formation of the synaptonemal complex in Sohlh1 KO spermatocytes. Luciferase reporter and chromatin-immunoprecipitation assays demonstrated that SOHLH1 enhanced the expression of the Sycp1 and Sycp3 genes by binding the -1276, -708, and -94 basepairs (bp) E-boxes upstream of the Sycp1 promoter and the -64 and -43 bp E-boxes upstream of the Sycp3 promoter. Our data suggest that SOHLH1 transcriptionally regulates the expression of many target genes critical for the meiotic phase of spermatogenesis.

The telomere bouquet is a hub where meiotic double-strand breaks, synapsis, and stable homolog juxtaposition are coordinated in the zebrafish, Danio rerio

Meiosis is a cellular program that generates haploid gametes for sexual reproduction. While chromosome events that contribute to reducing ploidy (homologous chromosome pairing, synapsis, and recombination) are well conserved, their execution varies across species and even between sexes of the same species. The telomere bouquet is a conserved feature of meiosis that was first described nearly a century ago, yet its role is still debated. Here we took advantage of the prominent telomere bouquet in zebrafish, Danio rerio, and super-resolution microscopy to show that axis morphogenesis, synapsis, and the formation of double-strand breaks (DSBs) all take place within the immediate vicinity of telomeres. We established a coherent timeline of events and tested the dependence of each event on the formation of Spo11-induced DSBs. First, we found that the axis protein Sycp3 loads adjacent to telomeres and extends inward, suggesting a specific feature common to all telomeres seeds the development of the axis. Second, we found that newly formed axes near telomeres engage in presynaptic co-alignment by a mechanism that depends on DSBs, even when stable juxtaposition of homologous chromosomes at interstitial regions is not yet evident. Third, we were surprised to discover that ~30% of telomeres in early prophase I engage in associations between two or more chromosome ends and these interactions decrease in later stages. Finally, while pairing and synapsis were disrupted in both spo11 males and females, their reproductive phenotypes were starkly different; spo11 mutant males failed to produce sperm while females produced offspring with severe developmental defects. Our results support zebrafish as an important vertebrate model for meiosis with implications for differences in fertility and genetically derived birth defects in males and females.

Inherent to reproduction is the transmission of genetic information from one generation to the next. In sexually reproducing organisms, each parent contributes an equal amount of genetic information, packaged in chromosomes, to the offspring. Diploid organisms, like humans, have two copies of every chromosome, while their haploid gametes (e.g. eggs and sperm) have only one. This reduction in ploidy depends on the segregation of chromosomes during meiosis, resulting in gametes with one copy of each chromosome. Missegregation of the chromosomes in the parents leads to abnormal chromosome numbers in the offspring, which is usually lethal or has detrimental developmental effects. While it has been known for over a century that homologous chromosomes pair and recombine to facilitate proper segregation, how homologs find their partners has remained elusive. A structure that has been central to the discussion of homolog pairing is the bouquet, or the dynamic clustering of telomeres during early stages of meiosis. Here we use zebrafish to show that the telomere bouquet is the site where key events leading to homologous chromosome pairing are coordinated. Furthermore, we show that deletion of spo11, a gene required for proper recombination in most studied organisms, resulted in very different effects in males and females where males were sterile while females produced deformed progeny.