Synaptonemal complex components persist at centromeres and are required for homologous centromere pairing in mouse spermatocytes

Heterochromatin in plant meiosis

Heterochromatin is an organizational property of eukaryotic chromosomes, characterized by extensive DNA and histone modifications, that is associated with the silencing of transposable elements and repetitive sequences. Maintaining heterochromatin is crucial for ensuring genomic integrity and stability during the cell cycle. During meiosis, heterochromatin is important for homologous chromosome synapsis, recombination, and segregation, but our understanding of meiotic heterochromatin formation and condensation is limited. In this review, we focus on the dynamics and features of heterochromatin and how it condenses during meiosis in plants. We also discuss how meiotic heterochromatin influences the interaction and recombination of homologous chromosomes during prophase I.

FBXO47 regulates centromere pairing as key component of centromeric SCF E3 ligase in mouse spermatocytes

Centromere pairing is crucial for synapsis in meiosis. This study delves into the Skp1-Cullin1-F-box protein (SCF) E3 ubiquitin ligase complex, specifically focusing on F-box protein 47 (FBXO47), in mouse meiosis. Here, we revealed that FBXO47 is localized at the centromere and it regulates centromere pairing cooperatively with SKP1 to ensure proper synapsis in pachynema. The absence of FBXO47 causes defective centromeres, resulting in incomplete centromere pairing, which leads to corruption of SC at centromeric ends and along chromosome axes, triggering premature dissociation of chromosomes and pachytene arrest. FBXO47 deficient pachytene spermatocytes exhibited drastically reduced SKP1 expression at centromeres and chromosomes. Additionally, FBXO47 stabilizes SKP1 by down-regulating its ubiquitination in HEK293T cells. In essence, we propose that FBXO47 collaborates with SKP1 to facilitate centromeric SCF formation in spermatocytes. In summary, we posit that the centromeric SCF E3 ligase complex regulates centromere pairing for pachynema progression in mice.

TRIP13 localizes to synapsed chromosomes and functions as a dosage-sensitive regulator of meiosis

Meiotic progression requires coordinated assembly and disassembly of protein complexes involved in chromosome synapsis and meiotic recombination. Mouse TRIP13 and its ortholog Pch2 are instrumental in remodeling HORMA domain proteins. HORMAD proteins are associated with unsynapsed chromosome axes but depleted from the synaptonemal complex (SC) of synapsed homologs. Here we report that TRIP13 localizes to the synapsed SC in early pachytene spermatocytes and to telomeres throughout meiotic prophase I. Loss of TRIP13 leads to meiotic arrest and thus sterility in both sexes. Trip13-null meiocytes exhibit abnormal persistence of HORMAD1 and HOMRAD2 on synapsed SC and chromosome asynapsis that preferentially affects XY and centromeric ends. These major phenotypes are consistent with reported phenotypes of Trip13 hypomorph alleles. Trip13 heterozygous mice exhibit meiotic defects that are less severe than the Trip13-null mice, showing that TRIP13 is a dosage-sensitive regulator of meiosis. Localization of TRIP13 to the synapsed SC is independent of SC axial element proteins such as REC8 and SYCP2/SYCP3. Terminal FLAG-tagged TRIP13 proteins are functional and recapitulate the localization of native TRIP13 to SC and telomeres. Therefore, the evolutionarily conserved localization of TRIP13/Pch2 to the synapsed chromosomes provides an explanation for dissociation of HORMA domain proteins upon synapsis in diverse organisms.

Exploration of tissue fixation methods suitable for digital pathological studies of the testis

BACKGROUND

The way of testicular tissue fixation directly affects the correlation and structural integrity between connective tissue and seminiferous tubules, which is essential for the study of male reproductive development. This study aimed to find the optimal fixative and fixation time to produce high-quality testicular histopathological sections, and provided a suitable foundation for in-depth study of male reproductive development with digital pathology technology.

METHODS

Testes were removed from both sides of 25 male C57BL/6 mice. Samples were fixed in three different fixatives, 10% neutral buffered formalin (10% NBF), modified Davidson's fluid (mDF), and Bouin's Fluid (BF), for 8, 12, and 24 h, respectively. Hematoxylin and eosin (H&E) staining, periodic acid Schiff-hematoxylin (PAS-h) staining, and immunohistochemistry (IHC) were used to evaluate the testicle morphology, staging of mouse seminiferous tubules, and protein preservation. Aperio ScanScope CS2 panoramic scanning was used to perform quantitative analyses.

RESULTS

H&E staining showed 10% NBF resulted in an approximately 15-17% reduction in the thickness of seminiferous epithelium. BF and mDF provided excellent results when staining acrosomes with PAS-h. IHC staining of synaptonemal complexes 3 (Sycp3) was superior in mDF compared to BF-fixed samples. Fixation in mDF and BF improved testis tissue morphology compared to 10% NBF.

CONCLUSIONS

Quantitative analysis showed that BF exhibited a very low IHC staining efficiency and revealed that mouse testes fixed for 12 h with mDF, exhibited morphological details, excellent efficiency of PAS-h staining for seminiferous tubule staging, and IHC results. In addition, the morphological damage of testis was prolonged with the duration of fixation time.

Centromere pairing enables correct segregation of meiotic chromosomes

Proper chromosome segregation in meiosis I relies on the formation of connections between homologous chromosomes. Crossovers between homologs provide a connection that allows them to attach correctly to the meiosis I spindle. Tension is transmitted across the crossover when the partners attach to microtubules from opposing poles of the spindle. Tension stabilizes microtubule attachments that will pull the partners toward opposite poles at anaphase. Paradoxically, in many organisms, non-crossover partners segregate correctly. The mechanism by which non-crossover partners become bioriented on the meiotic spindle is unknown. Both crossover and non-crossover partners pair their centromeres early in meiosis (prophase). In budding yeast, centromere pairing is correlated with subsequent correct segregation of the partners. The mechanism by which centromere pairing, in prophase, promotes later correct attachment of the partners to the metaphase spindle is unknown. We used live cell imaging to track the biorientation process of non-crossover chromosomes. We find that centromere pairing allows the establishment of connections between the partners that allows their later interdependent attachment to the meiotic spindle using tension-sensing biorientation machinery. Because all chromosome pairs experience centromere pairing, our findings suggest that crossover chromosomes also utilize this mechanism to achieve maximal segregation fidelity.

synapsis: A Bioconductor Package to Automate the Analysis of Meiotic Double-Strand Break Repair and Crossover Formation

Immunofluorescent staining is commonly used to generate images to characterize cytological phenotypes. The manual quantification of DNA double-strand breaks and their repair intermediates during meiosis using image data requires a series of subjective steps, from image selection to the counting of particular events per nucleus. Here we describe "synapsis," a bioconductor package, which includes a set of functions to automate the process of identifying meiotic nuclei and quantifying key double-strand break formation and repair events in a rapid, scalable, and reproducible workflow, and compare it to manual user quantification. The software can be extended for other applications in meiosis research, such as incorporating machine learning approaches to categorize meiotic substages.

TRIP13 localizes to synapsed chromosomes and functions as a dosage-sensitive regulator of meiosis

Meiotic progression requires coordinated assembly and disassembly of protein complexes involved in chromosome synapsis and meiotic recombination. The AAA+ ATPase TRIP13 and its orthologue Pch2 are instrumental in remodeling HORMA domain proteins. Meiosis-specific HORMAD proteins are associated with unsynapsed chromosome axes but depleted from the synaptonemal complex (SC) of synapsed chromosome homologues. Here we report that TRIP13 localizes to the synapsed SC in early pachytene spermatocytes and to telomeres throughout meiotic prophase I. Loss of TRIP13 leads to meiotic arrest and thus sterility in both sexes. Trip13-null meiocytes exhibit abnormal persistence of HORMAD1 and HOMRAD2 on synapsed SC and chromosome asynapsis that preferentially affects XY and centromeric ends. These findings confirm the previously reported phenotypes of the Trip13 hypomorph alleles. Trip13 heterozygous (Trip13+/-) mice also exhibit meiotic defects that are less severe than the Trip13-null mice, showing that TRIP13 is a dosage-sensitive regulator of meiosis. Localization of TRIP13 to the synapsed SC is independent of SC axial element proteins such as REC8 and SYCP2/SYCP3. The N- or C-terminal FLAG-tagged TRIP13 proteins are functional and recapitulate the localization of native TRIP13 to SC and telomeres in knockin mice. Therefore, the evolutionarily conserved localization of TRIP13/Pch2 to the synapsed chromosomes provides an explanation for dissociation of HORMA domain proteins upon chromosome synapsis in diverse organisms.

Repositioning of centromere-associated repeats during karyotype evolution in Oryzias fishes

The karyotype, which is the number and shape of chromosomes, is a fundamental characteristic of all eukaryotes. Karyotypic changes play an important role in many aspects of evolutionary processes, including speciation. In organisms with monocentric chromosomes, it was previously thought that chromosome number changes were mainly caused by centric fusions and fissions, whereas chromosome shape changes, that is, changes in arm numbers, were mainly due to pericentric inversions. However, recent genomic and cytogenetic studies have revealed examples of alternative cases, such as tandem fusions and centromere repositioning, found in the karyotypic changes within and between species. Here, we employed comparative genomic approaches to investigate whether centromere repositioning occurred during karyotype evolution in medaka fishes. In the medaka family (Adrianichthyidae), the three phylogenetic groups differed substantially in their karyotypes. The Oryzias latipes species group has larger numbers of chromosome arms than the other groups, with most chromosomes being metacentric. The O. javanicus species group has similar numbers of chromosomes to the O. latipes species group, but smaller arm numbers, with most chromosomes being acrocentric. The O. celebensis species group has fewer chromosomes than the other two groups and several large metacentric chromosomes that were likely formed by chromosomal fusions. By comparing the genome assemblies of O. latipes, O. javanicus, and O. celebensis, we found that repositioning of centromere-associated repeats might be more common than simple pericentric inversion. Our results demonstrated that centromere repositioning may play a more important role in karyotype evolution than previously appreciated.

Meiosis: Dances Between Homologs

The raison d'être of meiosis is shuffling of genetic information via Mendelian segregation and, within individual chromosomes, by DNA crossing-over. These outcomes are enabled by a complex cellular program in which interactions between homologous chromosomes play a central role. We first provide a background regarding the basic principles of this program. We then summarize the current understanding of the DNA events of recombination and of three processes that involve whole chromosomes: homolog pairing, crossover interference, and chiasma maturation. All of these processes are implemented by direct physical interaction of recombination complexes with underlying chromosome structures. Finally, we present convergent lines of evidence that the meiotic program may have evolved by coupling of this interaction to late-stage mitotic chromosome morphogenesis.

The construction of a testis transcriptional cell atlas from embryo to adult reveals various somatic cells and their molecular roles

BACKGROUND

The testis is a complex organ that undergoes extensive developmental changes from the embryonic stage to adulthood. The development of germ cells, which give rise to spermatozoa, is tightly regulated by the surrounding somatic cells.

METHODS

To better understand the dynamics of these changes, we constructed a transcriptional cell atlas of the testis, integrating single-cell RNA sequencing data from over 26,000 cells across five developmental stages: fetal germ cells, infants, childhood, peri-puberty, and adults. We employed various analytical techniques, including clustering, cell type assignments, identification of differentially expressed genes, pseudotime analysis, weighted gene co-expression network analysis, and evaluation of paracrine cell-cell communication, to comprehensively analyze this transcriptional cell atlas of the testis.

RESULTS

Our analysis revealed remarkable heterogeneity in both somatic and germ cell populations, with the highest diversity observed in Sertoli and Myoid somatic cells, as well as in spermatogonia, spermatocyte, and spermatid germ cells. We also identified key somatic cell genes, including RPL39, RPL10, RPL13A, FTH1, RPS2, and RPL18A, which were highly influential in the weighted gene co-expression network of the testis transcriptional cell atlas and have been previously implicated in male infertility. Additionally, our analysis of paracrine cell-cell communication supported specific ligand-receptor interactions involved in neuroactive, cAMP, and estrogen signaling pathways, which support the crucial role of somatic cells in regulating germ cell development.

CONCLUSIONS

Overall, our transcriptional atlas provides a comprehensive view of the cell-to-cell heterogeneity in the testis and identifies key somatic cell genes and pathways that play a central role in male fertility across developmental stages.

Meiotic Chromosome Structure, the Synaptonemal Complex, and Infertility

In meiosis, homologous chromosome synapsis is mediated by a supramolecular protein structure, the synaptonemal complex (SC), that assembles between homologous chromosome axes. The mammalian SC comprises at least eight largely coiled-coil proteins that interact and self-assemble to generate a long, zipper-like structure that holds homologous chromosomes in close proximity and promotes the formation of genetic crossovers and accurate meiotic chromosome segregation. In recent years, numerous mutations in human SC genes have been associated with different types of male and female infertility. Here, we integrate structural information on the human SC with mouse and human genetics to describe the molecular mechanisms by which SC mutations can result in human infertility. We outline certain themes in which different SC proteins are susceptible to different types of disease mutation and how genetic variants with seemingly minor effects on SC proteins may act as dominant-negative mutations in which the heterozygous state is pathogenic.

Loss-of-function variants in KCTD19 cause non-obstructive azoospermia in humans

Azoospermia is a significant cause of male infertility, with non-obstructive azoospermia (NOA) being the most severe type of spermatogenic failure. NOA is mostly caused by congenital factors, but our understanding of its genetic causes is very limited. Here, we identified a frameshift variant (c.201_202insAC, p.Tyr68Thrfs∗17) and two nonsense variants (c.1897C>T, p.Gln633∗; c.2005C>T, p.Gln669∗) in KCTD19 (potassium channel tetramerization domain containing 19) from two unrelated infertile Chinese men and a consanguineous Pakistani family with three infertile brothers. Testicular histological analyses revealed meiotic metaphase I (MMI) arrest in the affected individuals. Mice modeling KCTD19 variants recapitulated the same MMI arrest phenotype due to severe disrupted individualization of MMI chromosomes. Further analysis showed a complete loss of KCTD19 protein in both Kctd19 mutant mouse testes and affected individual testes. Collectively, our findings demonstrate the pathogenicity of the identified KCTD19 variants and highlight an essential role of KCTD19 in MMI chromosome individualization.

The courtship choreography of homologous chromosomes: timing and mechanisms of DSB-independent pairing

Meiosis involves deep changes in the spatial organisation and interactions of chromosomes enabling the two primary functions of this process: increasing genetic diversity and reducing ploidy level. These two functions are ensured by crucial events such as homologous chromosomal pairing, synapsis, recombination and segregation. In most sexually reproducing eukaryotes, homologous chromosome pairing depends on a set of mechanisms, some of them associated with the repair of DNA double-strand breaks (DSBs) induced at the onset of prophase I, and others that operate before DSBs formation. In this article, we will review various strategies utilised by model organisms for DSB-independent pairing. Specifically, we will focus on mechanisms such as chromosome clustering, nuclear and chromosome movements, as well as the involvement of specific proteins, non-coding RNA, and DNA sequences.

Genetic control of meiosis surveillance mechanisms in mammals

Meiosis is a specialized cell division that generates haploid gametes and is critical for successful sexual reproduction. During the extended meiotic prophase I, homologous chromosomes progressively pair, synapse and desynapse. These chromosomal dynamics are tightly integrated with meiotic recombination (MR), during which programmed DNA double-strand breaks (DSBs) are formed and subsequently repaired. Consequently, parental chromosome arms reciprocally exchange, ultimately ensuring accurate homolog segregation and genetic diversity in the offspring. Surveillance mechanisms carefully monitor the MR and homologous chromosome synapsis during meiotic prophase I to avoid producing aberrant chromosomes and defective gametes. Errors in these critical processes would lead to aneuploidy and/or genetic instability. Studies of mutation in mouse models, coupled with advances in genomic technologies, lead us to more clearly understand how meiosis is controlled and how meiotic errors are linked to mammalian infertility. Here, we review the genetic regulations of these major meiotic events in mice and highlight our current understanding of their surveillance mechanisms. Furthermore, we summarize meiotic prophase genes, the mutations that activate the surveillance system leading to meiotic prophase arrest in mouse models, and their corresponding genetic variants identified in human infertile patients. Finally, we discuss their value for the diagnosis of causes of meiosis-based infertility in humans.

CLPP Depletion Causes Diplotene Arrest; Underlying Testis Mitochondrial Dysfunction Occurs with Accumulation of Perrault Proteins ERAL1, PEO1, and HARS2

Human Perrault syndrome (PRLTS) is autosomal, recessively inherited, and characterized by ovarian insufficiency with hearing loss. Among the genetic causes are mutations of matrix peptidase CLPP, which trigger additional azoospermia. Here, we analyzed the impact of CLPP deficiency on male mouse meiosis stages. Histology, immunocytology, different OMICS and biochemical approaches, and RT-qPCR were employed in CLPP-null mouse testis. Meiotic chromosome pairing and synapsis proceeded normally. However, the foci number of the crossover marker MLH1 was slightly reduced, and foci persisted in diplotene, most likely due to premature desynapsis, associated with an accumulation of the DNA damage marker γH2AX. No meiotic M-phase cells were detected. Proteome profiles identified strong deficits of proteins involved in male meiotic prophase (HSPA2, SHCBP1L, DMRT7, and HSF5), versus an accumulation of AURKAIP1. Histone H3 cleavage, mtDNA extrusion, and cGAMP increase suggested innate immunity activation. However, the deletion of downstream STING/IFNAR failed to alleviate pathology. As markers of underlying mitochondrial pathology, we observed an accumulation of PRLTS proteins ERAL1, PEO1, and HARS2. We propose that the loss of CLPP leads to the extrusion of mitochondrial nucleotide-binding proteins to cytosol and nucleus, affecting late meiotic prophase progression, and causing cell death prior to M-phase entry. This phenotype is more severe than in mito-mice or mutator-mice.

Achiasmatic meiosis in the unisexual Amazon molly, Poecilia formosa

Unisexual reproduction, which generates clonal offspring, is an alternative strategy to sexual breeding and occurs even in vertebrates. A wide range of non-sexual reproductive modes have been described, and one of the least understood questions is how such pathways emerged and how they mechanistically proceed. The Amazon molly, Poecilia formosa, needs sperm from males of related species to trigger the parthenogenetic development of diploid eggs. However, the mechanism, of how the unreduced female gametes are produced, remains unclear. Cytological analyses revealed that the chromosomes of primary oocytes initiate pachytene but do not proceed to bivalent formation and meiotic crossovers. Comparing ovary transcriptomes of P. formosa and its sexual parental species revealed expression levels of meiosis-specific genes deviating from P. mexicana but not from P. latipinna. Furthermore, several meiosis genes show biased expression towards one of the two alleles from the parental genomes. We infer from our data that in the Amazon molly diploid oocytes are generated by apomixis due to a failure in the synapsis of homologous chromosomes. The fact that this failure is not reflected in the differential expression of known meiosis genes suggests the underlying molecular mechanism may be dysregulation on the protein level or misexpression of a so far unknown meiosis gene, and/or hybrid dysgenesis because of compromised interaction of proteins from diverged genomes.

Syce1 and Syce3 regulate testosterone and dihydrotestosterone synthesis via steroidogenic pathways in mouse Sertoli and Leydig cells

Testosterone (T) and dihydrotestosterone (DHT) are the main hormones regulating reproduction and development of male animals. Although their synthesis and secretion are regulated by the endocrine system [hypothalamic-pituitary-gonadal (adrenal) axis], it is also possible to synthesize T and DHT from the induction of two proteins: Syce1 and Syce3. As central elements of the synaptonemal complex (SC), Syce1 and Syce3 play a key role in the association of homologous chromosomes during meiosis. However, Syce1 and Syce3 also promote the synthesis of T and DHT, although potential mechanisms have yet to be revealed. In this study, Leydig and Sertoli cells, which are responsible for the production and regulation of steroid hormones in testis, were transfected with recombinant Syce1/Syce3 and silence sequence. Our results revealed the highest expression of Syce1 and Syce3 in spermatogenic cells of the testis. Moreover, overexpression or knockdown of Syce1 and Syce3 in Sertoli and Leydig cells resulted in activation or suppression of steroidogenic genes Star and Hsd3b, which are involved in a steroidogenic pathway that upregulates T synthesis. Upregulated expression of Syce1 resulted in a significant increase in Srd5a1, which can promote DHT secretion. Interestingly, Syce1 and Syce3 overexpression synergistically promoted each other's abundance. Our results define a previously unknown mechanism of Syce1 and Syce3 dependent activation of steroidogenic signaling in Sertoli and Leydig cells.

DNA methylome of primary spermatocyte reveals epigenetic dysregulation associated with male sterility of cattleyak

DNA cytosine methylation modification in the germline is of particular importance since it is a highly heritable epigenetic mark. Although cytosine methylation has been analyzed at the genome-scale for several mammalian species, our knowledge of DNA methylation patterns and the mechanisms underlying male hybrid sterility is still limited in domestic animals such as cattleyak. Here we for the first time show the genome-wide and single-base resolution landscape of methylcytosines (mC) in the primary spermatocyte (PSC) genome of yak with normal spermatogenesis and the inter-specific hybrid cattleyak with male infertility. A comparative investigation revealed that widespread differences are observed in the composition and patterning of DNA cytosine methylation between the two methylomes. Global CG or non-CG DNA methylation levels, as well as the number of mC sites, are increased in cattleyak compared to yak. Notably, the DNA methylome in cattleyak PSC exhibits promoter hypermethylation of meiosis-specific genes and piRNA pathway genes with respect to yak. Furthermore, major retrotransposonson classes are predominantly hypermethylated in cattleyak while those are fully hypomethylated in yak. KEGG pathway enrichment indicates Rap1 signaling and MAPK pathways may play potential roles in the spermatogenic arrest of cattleyak. Our present study not only provides valuable insights into distinct features of the cattleyak PSC methylome but also paves the way toward elucidating the complex, yet highly coordinated epigenetic modification during male germline development for inter-specific hybrid animals.

Kinesin-7 CENP-E is essential for chromosome alignment and spindle assembly of mouse spermatocytes

Genome stability depends on chromosome congression and alignment during cell division. Kinesin-7 CENP-E is critical for kinetochore-microtubule attachment and chromosome alignment, which contribute to genome stability in mitosis. However, the functions and mechanisms of CENP-E in the meiotic division of male spermatocytes remain largely unknown. In this study, by combining the use of chemical inhibitors, siRNA-mediated gene knockdown, immunohistochemistry, and high-resolution microscopy, we have found that CENP-E inhibition results in chromosome misalignment and metaphase arrest in dividing spermatocyte during meiosis. Strikingly, we have revealed that CENP-E regulates spindle organization in metaphase I spermatocytes and cultured GC-2 spd cells. CENP-E depletion leads to spindle elongation, chromosome misalignment, and chromosome instability in spermatocytes. Together, these findings indicate that CENP-E mediates the kinetochore recruitment of BubR1, spindle assembly checkpoint and chromosome alignment in dividing spermatocytes, which finally contribute to faithful chromosome segregation and chromosome stability in the male meiotic division.

Dual role of histone variant H3.3B in spermatogenesis: positive regulation of piRNA transcription and implication in X-chromosome inactivation

The histone variant H3.3 is encoded by two distinct genes, H3f3a and H3f3b, exhibiting identical amino-acid sequence. H3.3 is required for spermatogenesis, but the molecular mechanism of its spermatogenic function remains obscure. Here, we have studied the role of each one of H3.3A and H3.3B proteins in spermatogenesis. We have generated transgenic conditional knock-out/knock-in (cKO/KI) epitope-tagged FLAG-FLAG-HA-H3.3B (H3.3B^HA) and FLAG-FLAG-HA-H3.3A (H3.3A^HA) mouse lines. We show that H3.3B, but not H3.3A, is required for spermatogenesis and male fertility. Analysis of the molecular mechanism unveils that the absence of H3.3B led to alterations in the meiotic/post-meiotic transition. Genome-wide RNA-seq reveals that the depletion of H3.3B in meiotic cells is associated with increased expression of the whole sex X and Y chromosomes as well as of both RLTR10B and RLTR10B2 retrotransposons. In contrast, the absence of H3.3B resulted in down-regulation of the expression of piRNA clusters. ChIP-seq experiments uncover that RLTR10B and RLTR10B2 retrotransposons, the whole sex chromosomes and the piRNA clusters are markedly enriched of H3.3. Taken together, our data dissect the molecular mechanism of H3.3B functions during spermatogenesis and demonstrate that H3.3B, depending on its chromatin localization, is involved in either up-regulation or down-regulation of expression of defined large chromatin regions.

FBXW24 controls female meiotic prophase progression by regulating SYCP3 ubiquitination

BACKGROUND

An impeccable female meiotic prophase is critical for producing a high-quality oocyte and, ultimately, a healthy newborn. SYCP3 is a key component of the synaptonemal complex regulating meiotic homologous recombination. However, what regulates SYCP3 stability is unknown.

METHODS

Fertility assays, follicle counting, meiotic prophase stage (leptotene, zygotene, pachytene and diplotene) analysis and live imaging were employed to examine how FBXW24 knockout (KO) affect female fertility, follicle reserve, oocyte quality, meiotic prophase progression of female germ cells, and meiosis of oocytes. Western blot and immunostaining were used to examined the levels & signals (intensity, foci) of SYCP3 and multiple key DSB indicators & repair proteins (γH2AX, RPA2, p-CHK2, RAD51, MLH1, HORMAD1, TRIP13) after FBXW24 KO. Co-IP and immuno-EM were used to examined the interaction between FBXW24 and SYCP3; Mass spec was used to characterize the ubiquitination sites in SYCP3; In vivo & in vitro ubiquitination assays were utilized to determine the key sites in SYCP3 & FBXW24 for ubiquitination.

RESULTS

Fbxw24-knockout (KO) female mice were infertile due to massive oocyte death upon meiosis entry. Fbxw24-KO oocytes were defective due to elevated DNA double-strand breaks (DSBs) and inseparable homologous chromosomes. Fbxw24-KO germ cells showed increased SYCP3 levels, delayed prophase progression, increased DSBs, and decreased crossover foci. Next, we found that FBXW24 directly binds and ubiquitinates SYCP3 to regulate its stability. In addition, several key residues important for SYCP3 ubiquitination and FBXW24 ubiquitinating activity were characterized.

CONCLUSIONS

We proposed that FBXW24 regulates the timely degradation of SYCP3 to ensure normal crossover and DSB repair during pachytene. FBXW24-KO delayed SYCP3 degradation and DSB repair from pachytene until metaphase II (MII), ultimately causing failure in oocyte maturation, oocyte death, and infertility.

Focal Adhesion Protein Vinculin Is Required for Proper Meiotic Progression during Mouse Spermatogenesis

The focal adhesion protein Vinculin (VCL) is ascribed to various cytoplasmic functions; however, its nuclear role has so far been ambiguous. We observed that VCL localizes to the nuclei of mouse primary spermatocytes undergoing first meiotic division. Specifically, VCL localizes along the meiosis-specific structure synaptonemal complex (SC) during prophase I and the centromeric regions, where it remains until metaphase I. To study the role of VCL in meiotic division, we prepared a conditional knock-out mouse (VCL^cKO). We found that the VCL^cKO male mice were semi-fertile, with a decreased number of offspring compared to wild-type animals. This study of events in late prophase I indicated premature splitting of homologous chromosomes, accompanied by an untimely loss of SCP1. This caused erroneous kinetochore formation, followed by failure of the meiotic spindle assembly and metaphase I arrest. To assess the mechanism of VCL involvement in meiosis, we searched for its possible interacting partners. A mass spectrometry approach identified several putative interactors which belong to the ubiquitin–proteasome pathway (UPS). The depletion of VLC leads to the dysregulation of a key subunit of the proteasome complex in the meiotic nuclei and an altered nuclear SUMOylation level. Taken together, we show for the first time the presence of VCL in the nucleus of spermatocytes and its involvement in proper meiotic progress. It also suggests the direction for future studies regarding the role of VCL in spermatogenesis through regulation of UPS.

Insight into 2,3,7,8-tetrachlorodibenzo-p-dioxin-induced disruption of zebrafish spermatogenesis via single cell RNA-seq

2,3,7,8-Tetrachlorodibenzo-p-dioxin (TCDD) is a potent and environmentally persistent endocrine disrupting chemical. Our previous work demonstrated the latent reproductive maladies of early-life TCDD exposure in zebrafish. Zebrafish acutely exposed to low, environmentally relevant levels of TCDD (50 pg/mL) during two windows of sexual differentiation in development (1 hour of exposure at 3 and 7 weeks postfertilization) were later infertile, showed a reduction in sperm, and exhibited gene expression consistent with an altered microenvironment, even months after exposure. Due to the highly heterogeneous cell- type and -stage landscape of the testes, we hypothesized various cell types contribute markedly different profiles toward the pathology of TCDD exposure. To investigate the contributions of the diverse cell types in the adult zebrafish testes to TCDD-induced pathology, we utilized single-cell RNA-seq and the 10x Genomics platform. The method successfully captured every stage of testicular germ cell development. Testes of adult fish exposed during sexual differentiation to TCDD contained sharply decreased populations of late spermatocytes, spermatids, and spermatozoa. Spermatogonia and early spermatocyte populations were, in contrast, enriched following exposure. Pathway analysis of differentially expressed genes supported previous findings that TCDD exposure resulted in male infertility, and suggested this outcome is due to apoptosis of spermatids and spermatozoa, even years after exposure cessation. Increased germ cell apoptosis was confirmed histologically. These results provide support for an environmental exposure explanation of idiopathic male infertility.

Spermatogenic differentiation of spermatogonial stem cells on three-dimensional silk nanofiber scaffold

Background

Nano-fibrous scaffolds provide a three-dimensional matrix that guides sufficient orientation of seeded cells similar to a natural niche. In this research, we designed a silk scaffold to improve the differention of mouse spermatogonial stem cells to spermatogenic cell lines. Spermatogonial stem cells were collected from neonatal mouse (2–6 days) testes (n=60) using a two steps mechanical and enzymatic method. Cells were seeded on a silk scaffold and were cultured in Dulbecco’s modified Eagle’s medium, supplemented with 15 % fetal bovine serum and 1000 units/ml leukemia inhibitory factor, and incubated at 32°C in a humidified atmosphere of 5% CO2 in air. SEM technique was done for confirmation of seeding cells.

In this study two major groups (i.e., 2D and 3D culture groups) of 30 mice each. Isolated testicular cells from each group were cultured in the absence of silk scaffold or the presence of silk scaffold.

For induction of differentiation, seeded cells on a scaffold were exposed to 1 μM and 50 ng/ml BMP-4. The specific spermatogenic genes, e.g.; VASA, DAZL, PLZF, and Piwil2, were assessed via real-time PCR and immunocytochemistry techniques. P values less than 0.05 were assumed significant. All experiments were performed at least three times.

Results

SEM analysis confirmed the homogeneity of fabricated silk scaffold and average diameter of 450 nm for nanofibers fibers. Silk scaffold induces attachment of SSCs in comparison to the monolayer group. Spermatogonia stem cell colonies were observed gradually after 1 week of culture. Electrospun scaffold supports the differentiation of SSCs to spermatogenic lines. Dates of real-time PCR showed that the expression of meiotic markers, VASA, DAZL, and Piwil2 as related to specific spermatogenic genes, had a significant upregulation in cell-seeded silk scaffold compared to the control group (P < 0.05).

Immunocytochemistry founding approved the expression of specific spermatogenic markers; DAZL and PLZF were higher in the experiment group compared to the control (P < 0.05).

Conclusion

It is concluded silk scaffold induces spermatogenic differentiation of mouse spermatogonial stem cells in vitro.

PLK1 depletion alters homologous recombination and synaptonemal complex disassembly events during mammalian spermatogenesis

Homologous recombination (HR) is an essential meiotic process that contributes to the genetic variation of offspring and ensures accurate chromosome segregation. Recombination is facilitated by the formation and repair of programmed DNA double-strand breaks. These DNA breaks are repaired via recombination between maternal and paternal homologous chromosomes and a subset result in the formation of crossovers. HR and crossover formation is facilitated by synapsis of homologous chromosomes by a proteinaceous scaffold structure known as the synaptonemal complex (SC). Recent studies in yeast and worms have indicated that polo-like kinases (PLKs) regulate several events during meiosis, including DNA recombination and SC dynamics. Mammals express four active PLKs (PLK1-4), and our previous work assessing localization and kinase function in mouse spermatocytes suggested that PLK1 coordinates nuclear events during meiotic prophase. Therefore, we conditionally mutated Plk1 in early prophase spermatocytes and assessed stages of HR, crossover formation, and SC processes. Plk1 mutation resulted in increased RPA foci and reduced RAD51/DMC1 foci during zygonema, and an increase of both class I and class II crossover events. Furthermore, the disassembly of SC lateral elements was aberrant. Our results highlight the importance of PLK1 in regulating HR and SC disassembly during spermatogenesis.

A rare frameshift mutation in SYCP1 is associated with human male infertility

Proper assembly of the synaptonemal complex is essential for successful meiosis, and impairments in the process lead to infertility. Meiotic transverse filament proteins encoded by the SYCP1 (synaptonemal complex protein 1) gene are one of the main components of the synaptonemal complex and play an important role in correct synapsis and recombination. Family-based whole exome sequencing revealed a rare homozygous SYCP1 frameshift mutation (c.2892delA: p.K967Nfs*1) in two men with severe oligozoospermia, followed by validation and segregation through Sanger sequencing. This single nucleotide deletion not only changes lysine 967 (K) into asparagine (N) but also causes a premature stop codon, which leads to deletion of 968-976 residues from the end of the C-tail region of the SYCP1 protein. Although, sycp1 knockout male mice are reported to be sterile with a complete lack of spermatids and spermatozoa, to date no SYCP1 variant has been associated with human oligozoospermia. HADDOCK analysis indicated that this mutation decreases the ability of the truncated SYCP1 protein to bind DNA. Immunodetection of ϒH2AX signal, in SYCP1 mutant semen cells and a 40% DNA fragmentation index might indicate that a small number of DNA double-strand breaks, which require SYCP1 and/or synapsis to be repaired, are not efficiently repaired, resulting in defects in differentiation of germline cells and appearance of the oligozoospermia phenotype. To our knowledge, this is the first report of homozygous SYCP1 mutation that decreases sperm count. Further studies are required to determine the function of the SYCP1 mutation, which is potentially associated with human oligozoospermia.

Loss, Gain, and Retention: Mechanisms Driving Late Prophase I Chromosome Remodeling for Accurate Meiotic Chromosome Segregation

To generate gametes, sexually reproducing organisms need to achieve a reduction in ploidy, via meiosis. Several mechanisms are set in place to ensure proper reductional chromosome segregation at the first meiotic division (MI), including chromosome remodeling during late prophase I. Chromosome remodeling after crossover formation involves changes in chromosome condensation and restructuring, resulting in a compact bivalent, with sister kinetochores oriented to opposite poles, whose structure is crucial for localized loss of cohesion and accurate chromosome segregation. Here, we review the general processes involved in late prophase I chromosome remodeling, their regulation, and the strategies devised by different organisms to produce bivalents with configurations that promote accurate segregation.

FBXO47 is essential for preventing the synaptonemal complex from premature disassembly in mouse male meiosis

Meiotic prophase I is a prolonged G2 phase that ensures the completion of numerous meiosis-specific chromosome events. During meiotic prophase I, homologous chromosomes undergo synapsis to facilitate meiotic recombination yielding crossovers. It remains largely elusive how homolog synapsis is temporally maintained and destabilized during meiotic prophase I. Here we show that FBXO47 is the stabilizer of the synaptonemal complex during male meiotic prophase I. Disruption of FBXO47 shows severe impact on homologous chromosome synapsis, meiotic recombination, and XY body formation, leading to male infertility. Notably, in the absence of FBXO47, although once homologous chromosomes are synapsed, the synaptonemal complex is precociously disassembled before progressing beyond pachytene. Remarkably, Fbxo47 KO spermatocytes remain in an earlier stage of meiotic prophase I and lack crossovers, despite apparently exhibiting diplotene-like chromosome morphology. We propose that FBXO47 plays a crucial role in preventing the synaptonemal complex from premature disassembly during cell cycle progression of meiotic prophase I.

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FBXO47 is a stabilizer of the synaptonemal complex during male meiotic prophase

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FBXO47 KO shows precocious disassembly of the synaptonemal complex

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FBXO47 may function independently of SCF E3 ligase to maintain homolog synapsis

Meiotic Chromosome Dynamics in Zebrafish

Recent studies in zebrafish have revealed key features of meiotic chromosome dynamics, including clustering of telomeres in the bouquet configuration, biogenesis of chromosome axis structures, and the assembly and disassembly of the synaptonemal complex that aligns homologs end-to-end. The telomere bouquet stage is especially pronounced in zebrafish meiosis and sub-telomeric regions play key roles in mediating pairing and homologous recombination. In this review, we discuss the temporal progression of these events in meiosis prophase I and highlight the roles of proteins associated with meiotic chromosome architecture in homologous recombination. Finally, we discuss the interplay between meiotic mutants and gonadal sex differentiation and future research directions to study meiosis in living cells, including cell culture.

Integration and gene co-expression network analysis of scRNA-seq transcriptomes reveal heterogeneity and key functional genes in human spermatogenesis

Spermatogenesis is a complex process of cellular division and differentiation that begins with spermatogonia stem cells and leads to functional spermatozoa production. However, many of the molecular mechanisms underlying this process remain unclear. Single-cell RNA sequencing (scRNA-seq) is used to sequence the entire transcriptome at the single-cell level to assess cell-to-cell variability. In this study, more than 33,000 testicular cells from different scRNA-seq datasets with normal spermatogenesis were integrated to identify single-cell heterogeneity on a more comprehensive scale. Clustering, cell type assignments, differential expressed genes and pseudotime analysis characterized 5 spermatogonia, 4 spermatocyte, and 4 spermatid cell types during the spermatogenesis process. The UTF1 and ID4 genes were introduced as the most specific markers that can differentiate two undifferentiated spermatogonia stem cell sub-cellules. The C7orf61 and TNP can differentiate two round spermatid sub-cellules. The topological analysis of the weighted gene co-expression network along with the integrated scRNA-seq data revealed some bridge genes between spermatogenesis's main stages such as DNAJC5B, C1orf194, HSP90AB1, BST2, EEF1A1, CRISP2, PTMS, NFKBIA, CDKN3, and HLA-DRA. The importance of these key genes is confirmed by their role in male infertility in previous studies. It can be stated that, this integrated scRNA-seq of spermatogenic cells offers novel insights into cell-to-cell heterogeneity and suggests a list of key players with a pivotal role in male infertility from the fertile spermatogenesis datasets. These key functional genes can be introduced as candidates for filtering and prioritizing genotype-to-phenotype association in male infertility.

The organization, regulation, and biological functions of the synaptonemal complex

The synaptonemal complex (SC) is a meiosis-specific proteinaceous macromolecular structure that assembles between paired homologous chromosomes during meiosis in various eukaryotes. The SC has a highly conserved ultrastructure and plays critical roles in controlling multiple steps in meiotic recombination and crossover formation, ensuring accurate meiotic chromosome segregation. Recent studies in different organisms, facilitated by advances in super-resolution microscopy, have provided insights into the macromolecular structure of the SC, including the internal organization of the meiotic chromosome axis and SC central region, the regulatory pathways that control SC assembly and dynamics, and the biological functions exerted by the SC and its substructures. This review summarizes recent discoveries about how the SC is organized and regulated that help to explain the biological functions associated with this meiosis-specific structure.

In Vitro Meiosis of Male Germline Stem Cells

In vitro spermatogenesis has been achieved by culturing mouse embryonic stem cells (ESCs) together with a cell suspension of male juvenile gonad. However, for human fertility treatment or preservation, patient-specific ESCs or juvenile gonad is not available. We therefore aim to achieve in vitro spermatogenesis using male germline stem cells (GSCs) without the use of juvenile gonad. GSCs, when cultured on immortalized Sertoli cells, were able to enter meiosis, reach the meiotic metaphase stages, and sporadically form spermatid-like cells. However, the in vitro-formed pachytene-like spermatocytes did not display full chromosome synapsis and did not form meiotic crossovers. Despite this, the meiotic checkpoints that usually eliminate such cells to prevent genomic instabilities from being transmitted to the offspring were not activated, allowing the cells to proceed to the meiotic metaphase stages. In vitro-generated spermatid-like cells should thus be thoroughly investigated before being considered for clinical use.

Oligopaint DNA FISH reveals telomere-based meiotic pairing dynamics in the silkworm, Bombyx mori

Accurate chromosome segregation during meiosis is essential for reproductive success. Yet, many fundamental aspects of meiosis remain unclear, including the mechanisms regulating homolog pairing across species. This gap is partially due to our inability to visualize individual chromosomes during meiosis. Here, we employ Oligopaint FISH to investigate homolog pairing and compaction of meiotic chromosomes and resurrect a classical model system, the silkworm Bombyx mori. Our Oligopaint design combines multiplexed barcoding with secondary oligo labeling for high flexibility and low cost. These studies illustrate that Oligopaints are highly specific in whole-mount gonads and on meiotic squashes. We show that meiotic pairing is robust in both males and females and that pairing can occur through numerous partially paired intermediate structures. We also show that pairing in male meiosis occurs asynchronously and seemingly in a transcription-biased manner. Further, we reveal that meiotic bivalent formation in B. mori males is highly similar to bivalent formation in C. elegans, with both of these pathways ultimately resulting in the pairing of chromosome ends with non-paired ends facing the spindle pole. Additionally, microtubule recruitment in both C. elegans and B. mori is likely dependent on kinetochore proteins but independent of the centromere-specifying histone CENP-A. Finally, using super-resolution microscopy in the female germline, we show that homologous chromosomes remain associated at telomere domains in the absence of chiasma and after breakdown and modification to the synaptonemal complex in pachytene. These studies reveal novel insights into mechanisms of meiotic homolog pairing both with or without recombination.

Chromosome Organization in Early Meiotic Prophase

One of the most fascinating aspects of meiosis is the extensive reorganization of the genome at the prophase of the first meiotic division (prophase I). The first steps of this reorganization are observed with the establishment of an axis structure, that connects sister chromatids, from which emanate arrays of chromatin loops. This axis structure, called the axial element, consists of various proteins, such as cohesins, HORMA-domain proteins, and axial element proteins. In many organisms, axial elements are required to set the stage for efficient sister chromatid cohesion and meiotic recombination, necessary for the recognition of the homologous chromosomes. Here, we review the different actors involved in axial element formation in Saccharomyces cerevisiae and in mouse. We describe the current knowledge of their localization pattern during prophase I, their functional interdependence, their role in sister chromatid cohesion, loop axis formation, homolog pairing before meiotic recombination, and recombination. We also address further challenges that need to be resolved, to fully understand the interplay between the chromosome structure and the different molecular steps that take place in early prophase I, which lead to the successful outcome of meiosis I.

Sycp1 Is Not Required for Subtelomeric DNA Double-Strand Breaks but Is Required for Homologous Alignment in Zebrafish Spermatocytes

In meiotic prophase I, homologous chromosomes are bound together by the synaptonemal complex, in which two axial elements are connected by transverse filaments and central element proteins. In human and zebrafish spermatocytes, homologous recombination and assembly of the synaptonemal complex initiate predominantly near telomeres. In mice, synapsis is not required for meiotic double-strand breaks (DSBs) and homolog alignment but is required for DSB repair; however, the interplay of these meiotic events in the context of peritelomeric bias remains unclear. In this study, we identified a premature stop mutation in the zebrafish gene encoding the transverse filament protein Sycp1. In sycp1 mutant zebrafish spermatocytes, axial elements were formed and paired at chromosome ends between homologs during early to mid-zygonema. However, they did not synapse, and their associations were mostly lost in late zygotene- or pachytene-like stages. In sycp1 mutant spermatocytes, γH2AX signals were observed, and Dmc1/Rad51 and RPA signals appeared predominantly near telomeres, resembling wild-type phenotypes. We observed persistent localization of Hormad1 along the axis in sycp1 mutant spermatocytes, while the majority of Iho1 signals appeared and disappeared with kinetics similar to those in wild-type spermatocytes. Notably, persistent Iho1 foci were observed in spo11 mutant spermatocytes, suggesting that Iho1 dissociation from axes occurs in a DSB-dependent manner. Our results demonstrated that Sycp1 is not required for peritelomeric DSB formation but is necessary for complete pairing of homologs in zebrafish meiosis.

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.

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.

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.

Phosphoregulation of HORMA domain protein HIM-3 promotes asymmetric synaptonemal complex disassembly in meiotic prophase in Caenorhabditis elegans

In the two cell divisions of meiosis, diploid genomes are reduced into complementary haploid sets through the discrete, two-step removal of chromosome cohesion, a task carried out in most eukaryotes by protecting cohesion at the centromere until the second division. In eukaryotes without defined centromeres, however, alternative strategies have been innovated. The best-understood of these is found in the nematode Caenorhabditis elegans: after the single off-center crossover divides the chromosome into two segments, or arms, several chromosome-associated proteins or post-translational modifications become specifically partitioned to either the shorter or longer arm, where they promote the correct timing of cohesion loss through as-yet unknown mechanisms. Here, we investigate the meiotic axis HORMA-domain protein HIM-3 and show that it becomes phosphorylated at its C-terminus, within the conserved “closure motif” region bound by the related HORMA-domain proteins HTP-1 and HTP-2. Binding of HTP-2 is abrogated by phosphorylation of the closure motif in in vitro assays, strongly suggesting that in vivo phosphorylation of HIM-3 likely modulates the hierarchical structure of the chromosome axis. Phosphorylation of HIM-3 only occurs on synapsed chromosomes, and similarly to other previously-described phosphorylated proteins of the synaptonemal complex, becomes restricted to the short arm after designation of crossover sites. Regulation of HIM-3 phosphorylation status is required for timely disassembly of synaptonemal complex central elements from the long arm, and is also required for proper timing of HTP-1 and HTP-2 dissociation from the short arm. Phosphorylation of HIM-3 thus plays a role in establishing the identity of short and long arms, thereby contributing to the robustness of the two-step chromosome segregation.

To segregate properly in meiosis, cohesion between replicated chromosomes must remain after the first meiotic cell division, so chromosomes can be held together until they finally separate in the second division. While the majority of organisms use centromeres to protect chromosome cohesion in the first division, the nematode worm C. elegans, which lacks single centromeres, instead protects cohesion only on a segment of the chromosome known as the “long arm”. The long arm (and its complement, the short arm) are known to accumulate specific proteins and protein modifications, but it is not known how the short and long arms are first distinguished, nor how their separate functions are carried out. We report here that the chromosome axis protein HIM-3 and its modification by phosphorylation is important for ensuring the robust establishment of short and long arm functions. We show that phosphorylated HIM-3 partitions to the short arms after crossover recombination sites are designated, and HIM-3 mutants that mimic constitutive phosphorylation delay the normal establishment of the two complementary arm domains. Our findings reveal another layer of regulation to an outstanding mystery in chromosome biology.

Molecular basis of reproductive senescence: insights from model organisms

PURPOSE

Reproductive decline due to parental age has become a major barrier to fertility as couples have delayed having offspring into their thirties and forties. Advanced parental age is also associated with increased incidence of neurological and cardiovascular disease in offspring. Thus, elucidating the etiology of reproductive decline is of clinical importance.

METHODS

Deciphering the underlying processes that drive reproductive decline is particularly challenging in women in whom a discrete oocyte pool is established during embryogenesis and may remain dormant for tens of years. Instead, our understanding of the processes that drive reproductive senescence has emerged from studies in model organisms, both vertebrate and invertebrate, that are the focus of this literature review.

CONCLUSIONS

Studies of reproductive aging in model organisms not only have revealed the detrimental cellular changes that occur with age but also are helping identify major regulator proteins controlling them. Here, we discuss what we have learned from model organisms with respect to the molecular mechanisms that maintain both genome integrity and oocyte quality.

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.

FBXO47 regulates telomere-inner nuclear envelope integration by stabilizing TRF2 during meiosis

During meiosis, telomere attachment to the inner nuclear envelope is required for proper pairing of homologous chromosomes and recombination. Here, we identified F-box protein 47 (FBXO47) as a regulator of the telomeric shelterin complex that is specifically expressed during meiotic prophase I. Knockout of Fbxo47 in mice leads to infertility in males. We found that the Fbxo47 deficient spermatocytes are unable to form a complete synaptonemal complex. FBXO47 interacts with TRF1/2, and the disruption of Fbxo47 destabilizes TRF2, leading to unstable telomere attachment and slow traversing through the bouquet stage. Our findings uncover a novel mechanism of FBXO47 in telomeric shelterin subunit stabilization during meiosis.

Chromosome-Level Genome Reveals the Origin of Neo-Y Chromosome in the Male Barred Knifejaw Oplegnathus fasciatus

The barred knifejaw, Oplegnathus fasciatus, is characterized by an X₁X₂Y system with a neo-Y chromosome for males. Here, a chromosome-level genome was assembled to investigate the origin of neo-Y chromosome to the male O. fasciatus. Twenty-three chromosomes corresponding to the male karyotypes were scaffolded to 762-Mb genome with a contig N50 length of 2.18 Mb. A large neo-Y chromosome (Ch9) in the male O. fasciatus genome was also assembled and exhibited high identity to those of the female chromosomes Ch8 and Ch10. Chromosome rearrangements events were detected in the neo-chromosome Ch9. Our results suggested that a centric fusion of acrocentric chromosomes Ch8 and Ch10 should be responsible for the formation of the X₁X₂Y system. The high-quality genome will not only provide a solid foundation for further sex-determining mechanism research in the X₁X₂Y system but also facilitate the artificial breeding aiming to improve the yield and disease resistance for Oplegnathus.

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Construction of a chromosome-level reference genome for the male O. fasciatus

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Identification of the origin of neo-Y chromosome to the X₁X₂Y system

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Accurate comparisons of sequences and genes between female X₁X₁X₂X₂ and male X₁X₂Y

Crossover Position Drives Chromosome Remodeling for Accurate Meiotic Chromosome Segregation

Interhomolog crossovers (COs) are a prerequisite for achieving accurate chromosome segregation during meiosis [1, 2]. COs are not randomly positioned, occurring at distinct genomic intervals during meiosis in all species examined [3-10]. The role of CO position as a major determinant of accurate chromosome segregation has not been previously directly analyzed in a metazoan. Here, we use spo-11 mutants, which lack endogenous DNA double-strand breaks (DSBs), to induce a single DSB by Mos1 transposon excision at defined chromosomal locations in the C. elegans germline and show that the position of the resulting CO directly affects the formation of distinct chromosome subdomains during meiotic chromosome remodeling. CO formation in the typically CO-deprived center region of autosomes leads to premature loss of sister chromatid cohesion and chromosome missegregation, whereas COs at an off-centered position, as in wild type, can result in normal remodeling and accurate segregation. Ionizing radiation (IR)-induced DSBs lead to the same outcomes, and modeling of IR dose-response reveals that the CO-unfavorable center region encompasses up to 6% of the total chromosome length. DSBs proximal to telomeres rarely form COs, likely because of formation of unstable recombination intermediates that cannot be sustained as chiasmata until late prophase. Our work supports a model in which regulation of CO position early in meiotic prophase is required for proper designation of chromosome subdomains and normal chromosome remodeling in late meiotic prophase I, resulting in accurate chromosome segregation and providing a mechanism to prevent aneuploid gamete formation.

E2f5 is a versatile transcriptional activator required for spermatogenesis and multiciliated cell differentiation in zebrafish

E2f5 is a member of the E2f family of transcription factors that play essential roles during many cellular processes. E2f5 was initially characterized as a transcriptional repressor in cell proliferation studies through its interaction with the Retinoblastoma (Rb) protein for inhibition of target gene transcription. However, the precise roles of E2f5 during embryonic and post-embryonic development remain incompletely investigated. Here, we report that zebrafish E2f5 plays critical roles during spermatogenesis and multiciliated cell (MCC) differentiation. Zebrafish e2f5 mutants develop exclusively as infertile males. In the mutants, spermatogenesis is arrested at the zygotene stage due to homologous recombination (HR) defects, which finally leads to germ cell apoptosis. Inhibition of cell apoptosis in e2f5;tp53 double mutants rescued ovarian development, although oocytes generated from the double mutants were still abnormal, characterized by aberrant distribution of nucleoli. Using transcriptome analysis, we identified dmc1, which encodes an essential meiotic recombination protein, as the major target gene of E2f5 during spermatogenesis. E2f5 can bind to the promoter of dmc1 to promote HR, and overexpression of dmc1 significantly increased the fertilization rate of e2f5 mutant males. Besides gametogenesis defects, e2f5 mutants failed to develop MCCs in the nose and pronephric ducts during early embryonic stages, but these cells recovered later due to redundancy with E2f4. Moreover, we demonstrate that ion transporting principal cells in the pronephric ducts, which remain intercalated with the MCCs, do not contain motile cilia in wild-type embryos, while they generate single motile cilia in the absence of E2f5 activity. In line with this, we further show that E2f5 activates the Notch pathway gene jagged2b (jag2b) to inhibit the acquisition of MCC fate as well as motile cilia differentiation by the neighboring principal cells. Taken together, our data suggest that E2f5 can function as a versatile transcriptional activator and identify novel roles of the protein in spermatogenesis as well as MCC differentiation during zebrafish development.

Mixing and Matching Chromosomes during Female Meiosis

Meiosis is a key event in the manufacturing of an oocyte. During this process, the oocyte creates a set of unique chromosomes by recombining paternal and maternal copies of homologous chromosomes, and by eliminating one set of chromosomes to become haploid. While meiosis is conserved among sexually reproducing eukaryotes, there is a bewildering diversity of strategies among species, and sometimes within sexes of the same species, to achieve proper segregation of chromosomes. Here, we review the very first steps of meiosis in females, when the maternal and paternal copies of each homologous chromosomes have to move, find each other and pair. We explore the similarities and differences observed in C. elegans, Drosophila, zebrafish and mouse females.

Differentiation of neonate mouse spermatogonial stem cells on three-dimensional agar/polyvinyl alcohol nanofiber scaffold

Electrospun nanofiber matrices sufficiently mimic the structural morphology of natural extracellular matrix. In this study, we aimed to examine the effects of agar/polyvinyl alcohol nanofiber (PVA) scaffold on the proliferation efficiency and differentiation potential of neonate mouse spermatogonial stem cells (SCCs). Testicular cells were isolated from testes of 40 mouse pups and were seeded in: 1) 2D cell culture plates in the absence (2D/-GF) or presence (2D/+GF) of growth factors and 2) onto agar/PVA scaffold in the absence (3D/-GF) or presence (3D/+GF) of growth factors. The cells were subsequently cultured for 4 weeks. First 2 weeks were dedicated to proliferative phase, whereas the next 2 weeks emphasized the differentiation phase. The identity of the SCCs was investigated at different time-points by flow cytometry and quantitative reverse transcription PCR (qRT-PCR) analyses against the germ cell markers, including PLZF, Id-4, Gfrα-1, Tekt-1, and Sycp-3. After 2 weeks of culture, the 3D/+GF group showed the highest percentage of PLZF-positive cells among culture systems (P < 0.05). The expression levels of pre-meiotic markers (Id-4 and Gfrα-1) decreased significantly in all groups, particularly in 3D/+GF group after 28 days of culture. Additionally, the cells in the 3D/+GF group displayed the highest expression of meiotic (Sycp-3) and post-meiotic markers (Tekt-1) 14 days after differentiation induction. Seemingly, the combination of the agar/PVA scaffold and growth factor-supplemented medium synergistically increased the differentiation rate of mouse SSCs into meiotic and post-meiotic cells. Thus, agar/PVA nanofiber scaffolds may have the potential for applications in the restoration of infertility, especially in azoospermic males.

ABBREVIATIONS

2D: two dimentional; 3D: three dimentional; bFGF: basic fibroblast growth factor; BMP-4: bone morphogenetic protein 4; DMEM: Dulbecco's modified Eagle's medium; ECM: extracellular matrix; FCS: fetal calf serum; FTIR: Fourier-transform infrared spectroscopy; GDNF: glial cell line-derived neurotrophic factor; GF: growth factors; Gfrα-1, GDNF family co-receptor α1; Id-4, Inhibitor of DNA Binding 4; MTT: methylthiazoltetrazolium; PLZF: promyelocytic leukemia zinc finger; PVA: polyvinyl alcohol; qRT-PCR: quantitative reverse transcription PCR; RA: retinoic acid; SACS: soft agar culture system; SD: standard deviation; SEM: scanning electron microscope; SSCs: spermatogonial stem cells; Sycp-3, Synaptonemal complex protein 3; Tekt-1, Tektin 1.

Molecular evolution of the meiotic recombination pathway in mammals

Meiotic recombination shapes evolution and helps to ensure proper chromosome segregation in most species that reproduce sexually. Recombination itself evolves, with species showing considerable divergence in the rate of crossing-over. However, the genetic basis of this divergence is poorly understood. Recombination events are produced via a complicated, but increasingly well-described, cellular pathway. We apply a phylogenetic comparative approach to a carefully selected panel of genes involved in the processes leading to crossovers-spanning double-strand break formation, strand invasion, the crossover/non-crossover decision, and resolution-to reconstruct the evolution of the recombination pathway in eutherian mammals and identify components of the pathway likely to contribute to divergence between species. Eleven recombination genes, predominantly involved in the stabilization of homologous pairing and the crossover/non-crossover decision, show evidence of rapid evolution and positive selection across mammals. We highlight TEX11 and associated genes involved in the synaptonemal complex and the early stages of the crossover/non-crossover decision as candidates for the evolution of recombination rate. Evolutionary comparisons to MLH1 count, a surrogate for the number of crossovers, reveal a positive correlation between genome-wide recombination rate and the rate of evolution at TEX11 across the mammalian phylogeny. Our results illustrate the power of viewing the evolution of recombination from a pathway perspective.

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.