Meiotic Knockdown and Complementation Reveals Essential Role of RAD51 in Mouse Spermatogenesis

Divergence and conservation of the meiotic recombination machinery

Sexually reproducing eukaryotes use recombination between homologous chromosomes to promote chromosome segregation during meiosis. Meiotic recombination is almost universally conserved in its broad strokes, but specific molecular details often differ considerably between taxa, and the proteins that constitute the recombination machinery show substantial sequence variability. The extent of this variation is becoming increasingly clear because of recent increases in genomic resources and advances in protein structure prediction. We discuss the tension between functional conservation and rapid evolutionary change with a focus on the proteins that are required for the formation and repair of meiotic DNA double-strand breaks. We highlight phylogenetic relationships on different time scales and propose that this remarkable evolutionary plasticity is a fundamental property of meiotic recombination that shapes our understanding of molecular mechanisms in reproductive biology.

RAD51 and Infertility: A Review and Case-Control Study

RAD51 is a highly conserved recombinase involved in the strand invasion/exchange of double-stranded DNA by homologous single-stranded DNA during homologous recombination repair. Although a majority of existing literature associates RAD51 with the pathogenesis of various types of cancer, recent reports indicate a role of RAD51 in maintenance of fertility. The present study reviews the role of RAD51 and its interacting proteins in spermatogenesis/oogenesis and additionally reports the findings from the molecular genetic screening of RAD51 135 G > C polymorphism in infertile cases and controls. Fifty-nine articles from PubMed and Google Scholar related to the reproductive role of RAD51 were reviewed. For case-control study, the PCR-RFLP method was used to screen the RAD51 135 G > C polymorphism in 201 infertile cases (100 males, 101 females) and 201 age- and gender-matched healthy controls (100 males, 101 females) from Punjab, North-West India. The review of literature shows that RAD51 is indispensable for spermatogenesis and oogenesis in animal models. Reports on the role of RAD51 in human fertility are limited, however it is involved in the pathogenesis of infertility in both males and females. Molecular genetic analyses in the infertile cases and healthy controls showed no statistically significant difference in the genotypic and allelic frequencies for RAD51 135 G > C polymorphism, even after segregation of the cases by type of infertility (primary/secondary). Therefore, the present study concluded that the RAD51 135 G > C polymorphism was neither associated with male nor female infertility in North-West Indians. This is the first report on RAD51 135 G > C polymorphism and infertility.

Novel compound heterozygous variants in FANCI cause premature ovarian insufficiency

Premature ovarian insufficiency (POI) is a common reproductive aging disorder due to a dramatic decline of ovarian function before 40 years of age. Accumulating evidence reveals that genetic defects, particularly those related to DNA damage response, are a crucial contributing factor to POI. We have demonstrated that the functional Fanconi anemia (FA) pathway maintains the rapid proliferation of primordial germ cells to establish a sufficient reproductive reserve by counteracting replication stress, but the clinical implications of this function in human ovarian function remain to be established. Here, we screened the FANCI gene, which encodes a key component for FA pathway activation, in our whole-exome sequencing database of 1030 patients with idiopathic POI, and identified two pairs of novel compound heterozygous variants, c.[97C > T];[1865C > T] and c.[158-2A > G];[c.959A > G], in two POI patients, respectively. The missense variants did not alter FANCI protein expression and nuclear localization, apart from the variant c.158-2A > G causing abnormal splicing and leading to a truncated mutant p.(S54Pfs*5). Furthermore, the four variants all diminished FANCD2 ubiquitination levels and increased DNA damage under replication stress, suggesting that the FANCI variants impaired FA pathway activation and replication stress response. This study first links replication stress response defects with the pathogenesis of human POI, providing a new insight into the essential roles of the FA genes in ovarian function.

Multifaceted Roles of Histone Lysine Lactylation in Meiotic Gene Dynamics and Recombination

Male germ cells, which are responsible for producing millions of genetically diverse sperm through meiosis in the testis, rely on lactate as their central energy metabolite. Recent study has revealed that lactate induces epigenetic modification in cells through histone lactylation, a post-translational modification involving the addition of lactyl groups to lysine residues on histones. Here we report dynamic histone lactylation at histone H4-lysine 5 (K5), -K8, and -K12 during meiosis prophase I in mouse spermatogenesis. By profiling genome-wide occupancy of histone H4-K8 lactylation (H4K8la), which peaks at zygotene, our data show that H4K8la mark is observed at the promoters of genes exhibiting active expression with Gene Ontology (GO) functions enriched for meiosis. Notably, our data also demonstrate that H4K8la is closely associated with recombination hotspots, where machinery involved in the processing DNA double-stranded breaks (DSBs), such as SPO11, DMC1, RAD51, and RPA2, is engaged. In addition, H4K8la was also detected at the meiosis-specific cohesion sites (marked by RAD21L and REC8) flanking the recombination hotspots. Collectively, our findings suggest that histone lactylation serves as a novel mechanism through which lactate regulates germ cell meiosis.

Genome Instability and DNA Repair in Somatic and Reproductive Aging

Genetic material is constantly subjected to genotoxic insults and is critically dependent on DNA repair. Genome maintenance mechanisms differ in somatic and germ cells as the soma only requires maintenance during an individual's lifespan, while the germline indefinitely perpetuates its genetic information. DNA lesions are recognized and repaired by mechanistically highly diverse repair machineries. The DNA damage response impinges on a vast array of homeostatic processes and can ultimately result in cell fate changes such as apoptosis or cellular senescence. DNA damage causally contributes to the aging process and aging-associated diseases, most prominently cancer. By causing mutations, DNA damage in germ cells can lead to genetic diseases and impact the evolutionary trajectory of a species. The mechanisms ensuring tight control of germline DNA repair could be highly instructive in defining strategies for improved somatic DNA repair. They may provide future interventions to maintain health and prevent disease during aging.

Molecular cloning and expression analysis of rad51 gene associated with gametogenesis in Chinese soft-shell turtle (Pelodiscus sinensis)

Rad51 is a recA-like recombinase that plays a crucial role in repairing DNA double-strand breaks through homologous recombination during mitosis and meiosis in mammals and other organisms. However, its role in reptiles remains largely unclear. In this study, we aimed to investigate the physiological role of the rad51 gene in reptiles, particularly in Pelodiscus sinensis. Firstly, the cDNA of rad51 gene was cloned and analyzed in P. sinensis. The cloned cDNA contained an open reading frame (ORF) of 1020 bp and encodeed a peptide of 339 amino acids. The multiple alignments and phylogenetic tree analysis of Rad51 showed that P. sinensis shares the high identity with Chelonia mydas (97.95%) and Mus musculus (95.89%). Secondly, reverse transcription-polymerase chain reaction (RT-PCR) and real-time quantitative polymerase chain reaction (RT-qPCR) analysis showed that rad51 mRNA was highly expressed in both ovary and testis, while being weak in the somatic tissues examined in this study. Furthermore, chemical in situ hybridization (CISH) was performed to examine the expression profile of rad51 mRNA in germ cells at different stages. In the testis, rad51 mRNA expression was found to be stronger in the germ cells at early stages, specifically in spermatogonia and spermatocytes, but it was undetectable in spermatids. In the ovary, rad51 mRNA exhibited a uniform distribution in the cytoplasm of oocytes at early stages. The signal intensity of rad51 mRNA was highest in primary oocytes and gradually declined during oogenesis as the oocytes developed. These results suggest that rad51 plays a vital role in the development of germ cells, particularly during the early stages of gametogenesis in P. sinensis. The dynamic expression pattern of rad51 mRNA provides insights into the mechanisms underlying germ cell development and differentiation into gametes in turtles, even in reptiles.

Caffeine enhances chemosensitivity to irinotecan in the treatment of colorectal cancer

BACKGROUND

Colorectal cancer (CRC) is one of the most common types of cancer. This disease arises from gene mutations and epigenetic alterations that transform colonic epithelial cells into colon adenocarcinoma cells, which display a unique gene expression pattern compared to normal cells. Specifically, CRC cells exhibit significantly higher expression levels of genes involved in DNA repair or replication, which is attributed to the accumulation of DNA breakage resulting from rapid cell cycle progression.

PURPOSE

This study aimed to investigate the in vivo effects of caffeine on CRC cells and evaluate its impact on the sensitivity of these cells to irinotecan, a topoisomerase I inhibitor widely used for CRC treatment.

METHODS

Two CRC cell lines, HCT116 and HT29, were treated with irinotecan and caffeine. Western blot analysis assessed protein expression levels in caffeine/irinotecan-treated CRC cells. Immunofluorescence staining determined protein localization, measured DNA breaks, and explored the effects of DNA damage reagents during cell cycle progression and flow cytometry analysis was used to measure cell viability. Fiber assays investigated DNA synthesis in DNA-damaged cells during S-phase, while the comet assay assessed DNA fragmentation caused by DNA breaks.

RESULTS

Our findings demonstrated that the combination of irinotecan and caffeine exhibits a synergistic effect in suppressing CRC cell proliferation and inducing cell death. Compared to treatment with only irinotecan or caffeine, the combined irinotecan and caffeine treatment was more effective in inducing DNA lesions by displacing RAD51 from DNA break sites and inhibiting DNA repair progression, leading to cell cycle arrest. This combination also resulted in more severe effects, including DNA fragmentation and mitotic catastrophe.

CONCLUSION

Caffeine could enhance the effectiveness of an existing drug for CRC treatment despite having little impact on the cell survival rate of CRC cells. Our findings suggest that the beneficial adjuvant effects of caffeine may not only be applicable to CRC but also to various other types of cancers at different stages of development.

LncRNA Functional Screening in Organismal Development

Controversy continues over the functional prevalence of long non-coding RNAs (lncRNAs) despite their being widely investigated in all kinds of cells and organisms. In animals, lncRNAs have aroused general interest from exponentially increasing transcriptomic repertoires reporting their highly tissue-specific and developmentally dynamic expression, and more importantly, from growing experimental evidence supporting their functionality in facilitating organogenesis and individual fitness. In mammalian testes, while a great multitude of lncRNA species are identified, only a minority of them have been shown to be useful, and even fewer have been demonstrated as true requirements for male fertility using knockout models to date. This noticeable gap is attributed to the virtual existence of a large number of junk lncRNAs, the lack of an ideal germline culture system, difficulty in loss-of-function interrogation, and limited screening strategies. Facing these challenges, in this review, we discuss lncRNA functionality in organismal development and especially in mouse testis, with a focus on lncRNAs with functional screening.

Transcriptomic analysis of the Non-Obstructive Azoospermia (NOA) to address gene expression regulation in human testis

There is a need to understand the molecular basis of testes under Non-Obstructive Azoospermia (NOA), a state of failed spermatogenesis. There has been a lack of attention to the transcriptome at the level of alternatively spliced mRNAs (iso-mRNAs) and the mechanism of gene expression regulation. Hence, we aimed to establish a reliable iso-mRNA profile of NOA-testes, and explore molecular mechanisms - especially those related to gene expression regulation. We sequenced mRNAs from testicular samples of donors with complete spermatogenesis (control samples) and a failure of spermatogenesis (NOA samples). We identified differentially expressed genes and their iso-mRNAs via standard NGS data analyses. We then listed these iso-mRNAs hierarchically based on the extent of consistency of differential quantities across samples and groups, and validated the lists via RT-qPCRs (for 80 iso-mRNAs). In addition, we performed extensive bioinformatic analysis of the splicing features, domains, interactions, and functions of differentially expressed genes and iso-mRNAs. Many top-ranking down-regulated genes and iso-mRNAs, i.e., those down-regulated more consistently across the NOA samples, are associated with mitosis, replication, meiosis, cilium, RNA regulation, and post-translational modifications such as ubiquitination and phosphorylation. Most down-regulated iso-mRNAs correspond to full-length proteins that include all expected domains. The predominance of alternative promoters and termination sites in these iso-mRNAs indicate their gene expression regulation via promoters and UTRs. We compiled a new, comprehensive list of human transcription factors (TFs) and used it to identify TF-'TF gene' interactions with potential significance in down-regulating genes under the NOA condition. The results indicate that RAD51 suppression by HSF4 prevents SP1-activation, and SP1, in turn, could regulate multiple TF genes. This potential regulatory axis and other TF interactions identified in this study could explain the down-regulation of multiple genes in NOA-testes. Such molecular interactions may also have key regulatory roles during normal human spermatogenesis.

Repair of DNA double-strand breaks in plant meiosis: role of eukaryotic RecA recombinases and their modulators

Homologous recombination during meiosis is crucial for the DNA double-strand breaks (DSBs) repair that promotes the balanced segregation of homologous chromosomes and enhances genetic variation. In most eukaryotes, two recombinases RAD51 and DMC1 form nucleoprotein filaments on single-stranded DNA generated at DSB sites and play a central role in the meiotic DSB repair and genome stability. These nucleoprotein filaments perform homology search and DNA strand exchange to initiate repair using homologous template-directed sequences located elsewhere in the genome. Multiple factors can regulate the assembly, stability, and disassembly of RAD51 and DMC1 nucleoprotein filaments. In this review, we summarize the current understanding of the meiotic functions of RAD51 and DMC1 and the role of their positive and negative modulators. We discuss the current models and regulators of homology searches and strand exchange conserved during plant meiosis. Manipulation of these repair factors during plant meiosis also holds a great potential to accelerate plant breeding for crop improvements and productivity.

A truncating variant of RAD51B associated with primary ovarian insufficiency provides insights into its meiotic and somatic functions

Primary ovarian insufficiency (POI) causes female infertility by abolishing normal ovarian function. Although its genetic etiology has been extensively investigated, most POI cases remain unexplained. Using whole-exome sequencing, we identified a homozygous variant in RAD51B -(c.92delT) in two sisters with POI. In vitro studies revealed that this variant leads to translation reinitiation at methionine 64. Here, we show that this is a pathogenic hypomorphic variant in a mouse model. Rad51bc.92delT/c.92delT mice exhibited meiotic DNA repair defects due to RAD51 and HSF2BP/BMRE1 accumulation in the chromosome axes leading to a reduction in the number of crossovers. Interestingly, the interaction of RAD51B-c.92delT with RAD51C and with its newly identified interactors RAD51 and HELQ was abrogated or diminished. Repair of mitomycin-C-induced chromosomal aberrations was impaired in RAD51B/Rad51b-c.92delT human and mouse somatic cells in vitro and in explanted mouse bone marrow cells. Accordingly, Rad51b-c.92delT variant reduced replication fork progression of patient-derived lymphoblastoid cell lines and pluripotent reprogramming efficiency of primary mouse embryonic fibroblasts. Finally, Rad51bc.92delT/c.92delT mice displayed increased incidence of pituitary gland hyperplasia. These results provide new mechanistic insights into the role of RAD51B not only in meiosis but in the maintenance of somatic genome stability.

Rad51-mediated interhomolog recombination during budding yeast meiosis is promoted by the meiotic recombination checkpoint and the conserved Pif1 helicase

During meiosis, recombination between homologous chromosomes (homologs) generates crossovers that promote proper segregation at the first meiotic division. Recombination is initiated by Spo11-catalyzed DNA double strand breaks (DSBs). 5' end resection of the DSBs creates 3' single strand tails that two recombinases, Rad51 and Dmc1, bind to form presynaptic filaments that search for homology, mediate strand invasion and generate displacement loops (D-loops). D-loop processing then forms crossover and non-crossover recombinants. Meiotic recombination occurs in two temporally distinct phases. During Phase 1, Rad51 is inhibited and Dmc1 mediates the interhomolog recombination that promotes homolog synapsis. In Phase 2, Rad51 becomes active and functions with Rad54 to repair residual DSBs, making increasing use of sister chromatids. The transition from Phase 1 to Phase 2 is controlled by the meiotic recombination checkpoint through the meiosis-specific effector kinase Mek1. This work shows that constitutive activation of Rad51 in Phase 1 results in a subset of DSBs being repaired by a Rad51-mediated interhomolog recombination pathway that is distinct from that of Dmc1. Strand invasion intermediates generated by Rad51 require more time to be processed into recombinants, resulting in a meiotic recombination checkpoint delay in prophase I. Without the checkpoint, Rad51-generated intermediates are more likely to involve a sister chromatid, thereby increasing Meiosis I chromosome nondisjunction. This Rad51 interhomolog recombination pathway is specifically promoted by the conserved 5'-3' helicase PIF1 and its paralog, RRM3 and requires Pif1 helicase activity and its interaction with PCNA. This work demonstrates that (1) inhibition of Rad51 during Phase 1 is important to prevent competition with Dmc1 for DSB repair, (2) Rad51-mediated meiotic recombination intermediates are initially processed differently than those made by Dmc1, and (3) the meiotic recombination checkpoint provides time during prophase 1 for processing of Rad51-generated recombination intermediates.

B Chromosomes in Psalidodon scabripinnis (Characiformes, Characidae) Species Complex

Simple Summary

For more than a century, B chromosomes have been investigated in several eukaryotic species. These supernumerary genomic elements behave as parasites or provide fitness benefits to the hosts. They are mostly composed of repetitive DNA, but they also have protein-coding genes. B chromosomes are associated with differential gene expression and phenotypic effects. This makes them one of the most interesting genomic elements to investigate. Fish species of the Psalidodon genus harbor a great diversity of B chromosomes. Recent studies showed they share a common ancestor, persisting in the genus for a long time and enduring speciation processes. In the Psalidodon scabripinnis species complex, B chromosomes express their own genes, mostly related to cell cycle and gonad differentiation. Moreover, these B chromosomes are associated with functional effects, e.g., cell cycle extension. Here, we review the current knowledge regarding these elements in the P. scabripinnis species complex and propose a chromosome speciation model facilitated by the B chromosome manipulation of the cell machinery.

Abstract

B chromosomes are extra-genomic components of cells found in individuals and in populations of some eukaryotic organisms. They have been described since the first observations of chromosomes, but several aspects of their biology remain enigmatic. Despite being present in hundreds of fungi, plants, and animal species, only a small number of B chromosomes have been investigated through high-throughput analyses, revealing the remarkable mechanisms employed by these elements to ensure their maintenance. Populations of the Psalidodon scabripinnis species complex exhibit great B chromosome diversity, making them a useful material for various analyses. In recent years, important aspects of their biology have been revealed. Here, we review these studies presenting a comprehensive view of the B chromosomes in the P. scabripinnis complex and a new hypothesis regarding the role of the B chromosome in the speciation process.

DMC1 attenuates RAD51-mediated recombination in Arabidopsis

Ensuring balanced distribution of chromosomes in gametes, meiotic recombination is essential for fertility in most sexually reproducing organisms. The repair of the programmed DNA double strand breaks that initiate meiotic recombination requires two DNA strand-exchange proteins, RAD51 and DMC1, to search for and invade an intact DNA molecule on the homologous chromosome. DMC1 is meiosis-specific, while RAD51 is essential for both mitotic and meiotic homologous recombination. DMC1 is the main catalytically active strand-exchange protein during meiosis, while this activity of RAD51 is downregulated. RAD51 is however an essential cofactor in meiosis, supporting the function of DMC1. This work presents a study of the mechanism(s) involved in this and our results point to DMC1 being, at least, a major actor in the meiotic suppression of the RAD51 strand-exchange activity in plants. Ectopic expression of DMC1 in somatic cells renders plants hypersensitive to DNA damage and specifically impairs RAD51-dependent homologous recombination. DNA damage-induced RAD51 focus formation in somatic cells is not however suppressed by ectopic expression of DMC1. Interestingly, DMC1 also forms damage-induced foci in these cells and we further show that the ability of DMC1 to prevent RAD51-mediated recombination is associated with local assembly of DMC1 at DNA breaks. In support of our hypothesis, expression of a dominant negative DMC1 protein in meiosis impairs RAD51-mediated DSB repair. We propose that DMC1 acts to prevent RAD51-mediated recombination in Arabidopsis and that this down-regulation requires local assembly of DMC1 nucleofilaments.

Essential for fertility and responsible for a major part of genetic variation in sexually reproducing species, meiotic recombination establishes the physical linkages between homologous chromosomes which ensure their balanced segregation in the production of gametes. These linkages, or chiasmata, result from DNA strand exchange catalyzed by the RAD51 and DMC1 recombinases and their numbers and distribution are tightly regulated. Essential for maintaining chromosomal integrity in mitotic cells, the strand-exchange activity of RAD51 is downregulated in meiosis, where it plays a supporting role to the activity of DMC1. Notwithstanding considerable attention from the genetics community, precisely why this is done and the mechanisms involved are far from being fully understood. We show here in the plant Arabidopsis that DMC1 can downregulate RAD51 strand-exchange activity and propose that this may be a general mechanism for suppression of RAD51-mediated recombination in meiosis.

Multi-color dSTORM microscopy in Hormad1-/- spermatocytes reveals alterations in meiotic recombination intermediates and synaptonemal complex structure

Recombinases RAD51 and its meiosis-specific paralog DMC1 accumulate on single-stranded DNA (ssDNA) of programmed DNA double strand breaks (DSBs) in meiosis. Here we used three-color dSTORM microscopy, and a mouse model with severe defects in meiotic DSB formation and synapsis (Hormad1^-/-) to obtain more insight in the recombinase accumulation patterns in relation to repair progression. First, we used the known reduction in meiotic DSB frequency in Hormad1^-/- spermatocytes to be able to conclude that the RAD51/DMC1 nanofoci that preferentially localize at distances of ~300 nm form within a single DSB site, whereas a second preferred distance of ~900 nm, observed only in wild type, represents inter-DSB distance. Next, we asked whether the proposed role of HORMAD1 in repair inhibition affects the RAD51/DMC1 accumulation patterns. We observed that the two most frequent recombinase configurations (1 DMC1 and 1 RAD51 nanofocus (D1R1), and D2R1) display coupled frequency dynamics over time in wild type, but were constant in the Hormad1^-/- model, indicating that the lifetime of these intermediates was altered. Recombinase nanofoci were also smaller in Hormad1^-/- spermatocytes, consistent with changes in ssDNA length or protein accumulation. Furthermore, we established that upon synapsis, recombinase nanofoci localized closer to the synaptonemal complex (SYCP3), in both wild type and Hormad1^-/- spermatocytes. Finally, the data also revealed a hitherto unknown function of HORMAD1 in inhibiting coil formation in the synaptonemal complex. SPO11 plays a similar but weaker role in coiling and SYCP1 had the opposite effect. Using this large super-resolution dataset, we propose models with the D1R1 configuration representing one DSB end containing recombinases, and the other end bound by other ssDNA binding proteins, or both ends loaded by the two recombinases, but in below-resolution proximity. This may then often evolve into D2R1, then D1R2, and finally back to D1R1, when DNA synthesis has commenced.

In order to correctly pair homologous chromosomes in the first meiotic prophase, repair of programmed double strand breaks (DSBs) is essential. By unravelling molecular details of the protein assemblies at single DSBs, using super-resolution microscopy, we aim to understand the dynamics of repair intermediates and their functions. We investigated the localization of the two recombinases RAD51 and DMC1 in wild type and HORMAD1-deficient cells. HORMAD1 is involved in multiple aspects of homologous chromosome association: it regulates formation and repair of DSBs, and it stimulates formation of the synaptonemal complex (SC), the macromolecular protein assembly that connects paired chromosomes. RAD51 and DMC1 enable chromosome pairing by promoting the invasions of the intact chromatids by single-stranded DNA ends that result from DSBs. We found that in absence of HORMAD1, RAD51 and DMC1 showed small but significant morphological and positional changes, combined with altered kinetics of specific RAD51/DMC1 configurations. We also determined that there is a generally preferred distance of ~900 nm between meiotic DSBs along the SC. Finally, we observed changes in the structure of the SC in Hormad1^-/- spermatocytes. This study contributes to a better understanding of the molecular details of meiotic homologous recombination and the role of HORMAD1 in meiotic prophase.

A Homozygous Loss-of-Function Mutation in MSH5 Abolishes MutSγ Axial Loading and Causes Meiotic Arrest in NOA-Affected Individuals

Non-obstructive azoospermia (NOA), characterized by spermatogenesis failure and the absence of sperm in ejaculation, is the most severe form of male infertility. However, the etiology and pathology between meiosis-associated monogenic alterations and human NOA remain largely unknown. A homozygous MSH5 mutation (c.1126del) was identified from two idiopathic NOA patients in the consanguineous family. This mutation led to the degradation of MSH5 mRNA and abolished chromosome axial localization of MutSγ in spermatocytes from the affected males. Chromosomal spreading analysis of the patient's meiotic prophase I revealed that the meiosis progression was arrested at a zygotene-like stage with extensive failure of homologous synapsis and DSB repair. Therefore, our study demonstrates that the MSH5 c.1126del could cause meiotic recombination failure and lead to human infertility, improving the genetic diagnosis of NOA clinically. Furthermore, the study of human spermatocytes elucidates the meiosis defects caused by MSH5 variant, and reveals a conserved and indispensable role of MutSγ in human synapsis and meiotic recombination, which have not previously been well-described.

The reduction of oocytes and disruption of the meiotic prophase I in Fanconi Anemia E deficient mice

Fance is an important factor participating in the repair of DNA interstrand cross-links and its defect causes severe follicle depletion in female mice. To explore the underlying mechanisms, we investigated the effects of Fance on ovarian development in embryonic and newborn mice. We found that the number of oocytes was significantly decreased in Fance-/- mice as early as 13.5 days post coitum (dpc). The continuous decrease of oocytes in Fance-/- mice compared with the Fance+/+ mice led to the primordial follicles being almost exhausted at 2 days postpartum (dpp). The mitotic-meiotic transition occurred normally, but the meiotic progression was arrested in pachytene in Fance-/- oocytes. We detected the expressions of RAD51 (homologous recombination repair factor), 53BP1 (non-homologous end-joining repair factor), and γH2AX by immunostaining analysis and chromosome spreads. The expressions of 53BP1 were increased and RAD51 decreased significantly in Fance-/- oocytes compared with Fance+/+ oocytes. Also, the meiotic crossover indicated by MLH1 foci was significantly increased in Fance-/- oocytes. Oocyte proliferation and apoptosis were comparable between Fance-/- and Fance+/+ mice (P>0.05). The aberrant high expression at 17.5dpc and low expressions at 1dpp and 2dpp indicated the expression pattern of pluripotent marker OCT4 was disordered in Fance-/- oocytes. These findings elucidate that Fance mutation leads to a progressive reduction of oocytes and disrupts the progression of meiotic prophase I but not the initiation. And our study reveals that the potential mechanisms involve DNA damage repair, meiotic crossover, and pluripotency of oocytes.

Fancb deficiency causes premature ovarian insufficiency in mice†

FANCB protein is a major component of the Fanconi anemia (FA) core complex and plays important role in hematopoiesis and germ cell development. Deletion of Fancb gene causes the defect of primordial germ cells (PGCs) development and infertility in male mice. However, it remains unknown whether Fancb is required for female germ cell development. In this study, we found that the fertility of Fancb knockout male mice in C57/ICR mixed backgrounds was not affected. Female Fancb-/- mice were obtained by crossing Fancb+/- females with Fancb-/Y males. The number of PGCs was dramatically decreased in Fancb-/- females. Very few oocytes were observed after birth and primordial follicle pool was completely depleted at 6 weeks of age in Fancb-/- females. However, the remained oocytes from Fancb-/- mice were normal in fertilization and embryonic development from 2-cell to blastocyst stage. We also found that Fancb and Fancl double knockout males were also fertile and the number of sperm in epididymis was not reduced comparable to that of Fancb-/- and Fancl-/- single knockout mice. Taken together, these results demonstrated that Fancb is also essential for female germ cell development. Inactivation of Fancb causes massive germ cell loss and infertility in adult females. We also found that Fancb and Fancl do not act synergistically in regulating germ cell development.

BRCA2-DSS1 interaction is dispensable for RAD51 recruitment at replication-induced and meiotic DNA double strand breaks

The interaction between tumor suppressor BRCA2 and DSS1 is essential for RAD51 recruitment and repair of DNA double stand breaks (DSBs) by homologous recombination (HR). We have generated mice with a leucine to proline substitution at position 2431 of BRCA2, which disrupts this interaction. Although a significant number of mutant mice die during embryogenesis, some homozygous and hemizygous mutant mice undergo normal postnatal development. Despite lack of radiation induced RAD51 foci formation and a severe HR defect in somatic cells, mutant mice are fertile and exhibit normal RAD51 recruitment during meiosis. We hypothesize that the presence of homologous chromosomes in close proximity during early prophase I may compensate for the defect in BRCA2-DSS1 interaction. We show the restoration of RAD51 foci in mutant cells when Topoisomerase I inhibitor-induced single strand breaks are converted into DSBs during DNA replication. We also partially rescue the HR defect by tethering the donor DNA to the site of DSBs using streptavidin-fused Cas9. Our findings demonstrate that the BRCA2-DSS1 complex is dispensable for RAD51 loading when the homologous DNA is close to the DSB.

Mishra et al. have generated mice with a single amino acid substitution in BRCA2, which disrupts its interaction with DSS1 resulting in a severe HR defect. They show the interaction to be dispensable for HR at replication induced and meiotic DSBs.

RAD51 is essential for spermatogenesis and male fertility in mice

The recombinase RAD51 catalyzes the DNA strand exchange reaction in homologous recombination (HR) during both mitosis and meiosis. However, the physiological role of RAD51 during spermatogenesis remains unclear since RAD51 null mutation is embryonic lethal in mice. In this study, we generated a conditional knockout mouse model to study the role of RAD51 in spermatogenesis. Conditional disruption of RAD51 in germ cells by Vasa-Cre led to spermatogonial loss and Sertoli cell-only syndrome. Furthermore, tamoxifen-inducible RAD51 knockout by UBC-Cre^ERT2 confirmed that RAD51 deletion led to early spermatogenic cells loss and apoptosis. Notably, inducible knockout of RAD51 in adult mice caused defects in meiosis, with accumulated meiotic double-strand breaks (DSBs), reduced numbers of pachytene spermatocytes and less crossover formation. Our study revealed an essential role for Rad51 in the maintenance of spermatogonia as well as meiotic progression in mice.

BDE-209 induce spermatocytes arrest at early-pachytene stage during meiotic prophase I in mice

Deca-brominated diphenyl ether (BDE-209) is a common flame retardant utilized in electronic products, textiles, furniture, and upholstery materials. Environmental BDE-209 exposure results in spermatogenesis disorder, because of the characteristics of bioaccumulation, persistence, and probably toxicity. Meiotic prophase I is a crucial phase during spermatogenesis which is a key influential factor of normal sperm production. However, the effects of BDE-209 on meiotic prophase I during spermatogenesis are poorly understood. The present study aimed to evaluate whether BDE-209 exposure impairs meiotic prophase I during spermatogenesis of spermatocytes. We validated the effects of BDE-209 on alternations of meiotic prophase I in Balb/c male mice. Firstly, we analyzed sperm quality in cauda epididymis with decreasing sperm count, increasing abnormal sperm, and male reproductive dysfunction after exposure to BDE-209. Then, reactive oxygen species (ROS) and malondialdehyde (MDA) levels in testis and GC-2spd cells were significant increased after treated with BDE-209. Furthermore, we found that meiotic prophase I arrest at early-pachytene stage during spermatogenesis with increasing of DSBs damage and trimethylated histone H3 at lysine-4 (H3K4me3) in spermatocytes exposed to BDE-209. Finally, we conducted homologous recombination (HR) analyses to identify the progression of meiosis. The recombination markers, including DMC1 and RAD51, and crossover marker MLH1 were decreased during spermatogenesis after exposure to BDE-209. Collectively, our data indicated that BDE-209 has detrimental impacts on meiotic prophase I of spermatocytes in mice.

The neglected members of the family: non-BRCA mutations in the Fanconi anemia/BRCA pathway and reproduction

BACKGROUND

BReast CAncer (BRCA) genes are extensively studied in the context of fertility and reproductive aging. BRCA proteins are part of the DNA repair Fanconi anemia (FA)/BRCA pathway, in which more than 20 proteins are implicated. According to which gene is mutated and which interactions are lost owing to the mutation, carriers and patients with monoallelic or biallelic FA/BRCA mutations exhibit very different phenotypes, from overt FA to cancer predisposition or no pathological implications. The effect of the so far neglected non-BRCA FA mutations on fertility also deserves consideration.

OBJECTIVE AND RATIONALE

As improved treatments allow a longer life expectancy in patients with biallelic FA mutations and overt FA, infertility is emerging as a predominant feature. We thus reviewed the mechanisms for such a manifestation, as well as whether they also occur in monoallelic carriers of FA non-BRCA mutations.

SEARCH METHODS

Electronic databases PUBMED, EMBASE and CENTRAL were searched using the following term: 'fanconi' OR 'FANC' OR 'AND' 'fertility' OR 'pregnancy' OR 'ovarian reserve' OR 'spermatogenesis' OR 'hypogonadism'. All pertinent reports in the English-language literature were retrieved until May 2021 and the reference lists were systematically searched in order to identify any potential additional studies.

OUTCOMES

Biallelic FA mutations causing overt FA disease are associated with premature ovarian insufficiency (POI) occurring in the fourth decade in women and with primary non-obstructive azoospermia (NOA) in men. Hypogonadism in FA patients seems mainly associated with a defect in primordial germ cell proliferation in fetal life. In recent small, exploratory whole-exome sequencing studies, biallelic clinically occult mutations in the FA complementation group A (Fanca) and M (Fancm) genes were found in otherwise healthy patients with isolated NOA or POI, and also monoallelic carrier status for a loss-of-function mutation in Fanca has been implicated as a possible cause for POI. In those patients with known monoallelic FA mutations undergoing pre-implantation genetic testing, poor assisted reproduction outcomes are reported. However, the mechanisms underlying the repeated failures and the high miscarriage rates observed are not fully known.

WIDER IMPLICATIONS

The so far 'neglected' members of the FA/BRCA family will likely emerge as a relevant focus of investigation in the genetics of reproduction. Several (rather than a single) non-BRCA genes might be implicated. State-of-the-art methods, such as whole-genome/exome sequencing, and further exploratory studies are required to understand the prevalence and mechanisms for occult FA mutations in infertility and recurrent miscarriage.

Microinjection of antisense oligonucleotides into living mouse testis enables lncRNA function study

Background

Long non-coding RNAs (lncRNAs) have been the focus of ongoing research in a diversity of cellular processes. LncRNAs are abundant in mammalian testis, but their biological function remains poorly known.

Results

Here, we established an antisense oligonucleotides (ASOs)-based targeting approach that can efficiently knock down lncRNA in living mouse testis. We cloned the full-length transcript of lncRNA Tsx (testis-specific X-linked) and defined its testicular localization pattern. Microinjection of ASOs through seminiferous tubules in vivo significantly lowered the Tsx levels in both nucleus and cytoplasm. This effect lasted no less than 10 days, conducive to the generation and maintenance of phenotype. Importantly, ASOs performed better in depleting the nuclear Tsx and sustained longer effect than small interfering RNAs (siRNAs). In addition to the observation of an elevated number of apoptotic germ cells upon ASOs injection, which recapitulates the documented description of Tsx knockout, we also found a specific loss of meiotic spermatocytes despite overall no impact on meiosis and male fertility.

Conclusions

Our study detailed the characterization of Tsx and illustrates ASOs as an advantageous tool to functionally interrogate lncRNAs in spermatogenesis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00717-y.

Loss of TDP-43 in male germ cells causes meiotic failure and impairs fertility in mice

Meiotic arrest is a common cause of human male infertility, but the causes of this arrest are poorly understood. Transactive response DNA-binding protein of 43 kDa (TDP-43) is highly expressed in spermatocytes in the preleptotene and pachytene stages of meiosis. TDP-43 is linked to several human neurodegenerative disorders wherein its nuclear clearance accompanied by cytoplasmic aggregates underlies neurodegeneration. Exploring the functional requirement for TDP-43 for spermatogenesis for the first time, we show here that conditional KO (cKO) of the Tardbp gene (encoding TDP-43) in male germ cells of mice leads to reduced testis size, depletion of germ cells, vacuole formation within the seminiferous epithelium, and reduced sperm production. Fertility trials also indicated severe subfertility. Spermatocytes of cKO mice showed failure to complete prophase I of meiosis with arrest at the midpachytene stage. Staining of synaptonemal complex protein 3 and γH2AX, markers of the meiotic synaptonemal complex and DNA damage, respectively, and super illumination microscopy revealed nonhomologous pairing and synapsis defects. Quantitative RT-PCR showed reduction in the expression of genes critical for prophase I of meiosis, including Spo11 (initiator of meiotic double-stranded breaks), Rec8 (meiotic recombination protein), and Rad21L (RAD21-like, cohesin complex component), as well as those involved in the retinoic acid pathway critical for entry into meiosis. RNA-Seq showed 1036 upregulated and 1638 downregulated genes (false discovery rate <0.05) in the Tardbp cKO testis, impacting meiosis pathways. Our work reveals a crucial role for TDP-43 in male meiosis and suggests that some forms of meiotic arrest seen in infertile men may result from the loss of function of TDP-43.

Meiotic self-pairing of the Psalidodon (Characiformes, Characidae) iso-B chromosome: A successful perpetuation mechanism

B chromosomes are non-essential additional genomic elements present in several animal and plant species. In fishes, species of the genus Psalidodon (Characiformes, Characidae) harbor great karyotype diversity, and multiple populations carry different types of non-essential B chromosomes. This study analyzed how the dispensable supernumerary B chromosome of Psalidodon paranae behaves during meiosis to overcome checkpoints and express its own meiosis-specific genes. We visualized the synaptonemal complexes of P. paranae individuals with zero, one, or two B chromosomes using immunodetection with anti-medaka SYCP3 antibody and fluorescence in situ hybridization with a (CA)15 microsatellite probe. Our results showed that B chromosomes self-pair in cells containing only one B chromosome. In cells with two identical B chromosomes, these elements remain as separate synaptonemal complexes or close self-paired elements in the nucleus territory. Overall, we reveal that B chromosomes can escape meiotic silencing of unsynapsed chromatin through a self-pairing process, allowing expression of their own genes to facilitate regular meiosis resulting in fertile individuals. This behavior, also seen in other congeneric species, might be related to their maintenance throughout the evolutionary history of Psalidodon.

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 RAD51 recombinase protects mitotic chromatin in human cells

The RAD51 recombinase plays critical roles in safeguarding genome integrity, which is fundamentally important for all living cells. While interphase functions of RAD51 in maintaining genome stability are well-characterised, its role in mitosis remains contentious. In this study, we show that RAD51 protects under-replicated DNA in mitotic human cells and, in this way, promotes mitotic DNA synthesis (MiDAS) and successful chromosome segregation. In cells experiencing mild replication stress, MiDAS was detected irrespective of mitotically generated DNA damage. MiDAS broadly required de novo RAD51 recruitment to single-stranded DNA, which was supported by the phosphorylation of RAD51 by the key mitotic regulator Polo-like kinase 1. Importantly, acute inhibition of MiDAS delayed anaphase onset and induced centromere fragility, suggesting a mechanism that prevents the satisfaction of the spindle assembly checkpoint while chromosomal replication remains incomplete. This study hence identifies an unexpected function of RAD51 in promoting genomic stability in mitosis.

Expression of Rad51 and the histo-morphological evaluation of testis of the sterile male cattle-yak

Meiotic recombination is key to the repair of DNA double-strand break damage, provide a link between homologs for proper chromosome segregation as well as ensure genetic diversity in organisms. Defects in recombination often lead to sterility. The ubiquitously expressed Rad51 and the meiosis-specific DMC1 are two closely related recombinases that catalyze the key strand invasion and exchange step of meiotic recombination. This study cloned and sequenced the coding region of cattle-yak Rad51 and determined its mRNA and protein expression levels, evaluated its molecular and evolutionary relationship as well as evaluated the histo-morphological structure of testes in the yellow cattle, yak and the sterile cattle-yak hybrid. The Rad51 gene was amplified using PCR, cloned and sequenced using testicular cDNA from yak and cattle-yak. Real-time PCR was used to examine the expression levels of Rad51/DMC1 mRNA in the cattle, yak and cattle-yak testis while western blotting, immunofluorescence and immunohistochemistry were used to assess the protein expression and localization of Rad51/DMC1 protein in the testicular tissue sections. The results revealed that the mRNA and protein expression of Rad51 and DMC1 are extremely low in the male cattle-yak testis with a corresponding higher incidence of germ cell apoptosis. There was also thinning of the germinal epithelium possibly due to the depletion of the germ cells leading to the widening of the lumen area of the cattle-yak seminiferous tubule. Our findings provide support for the hypothesis that the low expression of Rad51 and DMC1 may contribute to the male hybrid sterility in the cattle-yak.

High Resolution View on the Regulation of Recombinase Accumulation in Mammalian Meiosis

A distinguishing feature of meiotic DNA double-strand breaks (DSBs), compared to DSBs in somatic cells, is the fact that they are induced in a programmed and specifically orchestrated manner, which includes chromatin remodeling prior to DSB induction. In addition, the meiotic homologous recombination (HR) repair process that follows, is different from HR repair of accidental DSBs in somatic cells. For instance, meiotic HR involves preferred use of the homolog instead of the sister chromatid as a repair template and subsequent formation of crossovers and non-crossovers in a tightly regulated manner. An important outcome of this distinct repair pathway is the pairing of homologous chromosomes. Central to the initial steps in homology recognition during meiotic HR is the cooperation between the strand exchange proteins (recombinases) RAD51 and its meiosis-specific paralog DMC1. Despite our understanding of their enzymatic activity, details on the regulation of their assembly and subsequent molecular organization at meiotic DSBs in mammals have remained largely enigmatic. In this review, we summarize recent mouse data on recombinase regulation via meiosis-specific factors. Also, we reflect on bulk “omics” studies of initial meiotic DSB processing, compare these with studies using super-resolution microscopy in single cells, at single DSB sites, and explore the implications of these findings for our understanding of the molecular mechanisms underlying meiotic HR regulation.

Panoramic transcriptome analysis and functional screening of long noncoding RNAs in mouse spermatogenesis

Long noncoding RNAs (lncRNAs) have emerged as diverse functional regulators involved in mammalian development; however, large-scale functional investigation of lncRNAs in mammalian spermatogenesis in vivo is lacking. Here, we delineated the global lncRNA expression landscape in mouse spermatogenesis and identified 968 germ cell signature lncRNAs. By combining bioinformatics and functional screening, we identified three functional lncRNAs (Gm4665, 1700027A15Rik, and 1700052I22Rik) that directly influence spermatogenesis in vivo. Knocking down Gm4665 hampered the development of round spermatids into elongating spermatids and disrupted key spermatogenic gene expression. Mechanistically, lncRNA Gm4665 localized in the nucleus of round spermatids and occupied the genomic regulatory region of important spermatogenic genes including Ip6k1 and Akap3. These findings provide a valuable resource and framework for future functional analysis of lncRNAs in spermatogenesis and their potential roles in other biological processes.

The ssDNA-binding protein MEIOB acts as a dosage-sensitive regulator of meiotic recombination

Meiotic recombination enables reciprocal exchange of genetic information between parental chromosomes and is essential for fertility. MEIOB, a meiosis-specific ssDNA-binding protein, regulates early meiotic recombination. Here we report that the human infertility-associated missense mutation (N64I) in MEIOB causes protein degradation and reduced crossover formation in mouse testes. Although the MEIOB N64I substitution is associated with human infertility, the point mutant mice are fertile despite meiotic defects. Meiob mutagenesis identifies serine 67 as a critical residue for MEIOB. Biochemically, these two mutations (N64I and S67 deletion) cause self-aggregation of MEIOB and sharply reduced protein half-life. Molecular genetic analyses of both point mutants reveal an important role for MEIOB in crossover formation in late meiotic recombination. Furthermore, we find that the MEIOB protein levels directly correlate with the severity of meiotic defects. Our results demonstrate that MEIOB regulates meiotic recombination in a dosage-dependent manner.

MEIOK21: a new component of meiotic recombination bridges required for spermatogenesis

Repair of DNA double-strand breaks (DSBs) with homologous chromosomes is a hallmark of meiosis that is mediated by recombination ‘bridges’ between homolog axes. This process requires cooperation of DMC1 and RAD51 to promote homology search and strand exchange. The mechanism(s) regulating DMC1/RAD51-ssDNA nucleoprotein filament and the components of ‘bridges’ remain to be investigated. Here we show that MEIOK21 is a newly identified component of meiotic recombination bridges and is required for efficient formation of DMC1/RAD51 foci. MEIOK21 dynamically localizes on chromosomes from on-axis foci to ‘hanging foci’, then to ‘bridges’, and finally to ‘fused foci’ between homolog axes. Its chromosome localization depends on DSBs. Knockout of Meiok21 decreases the numbers of HSF2BP and DMC1/RAD51 foci, disrupting DSB repair, synapsis and crossover recombination and finally causing male infertility. Therefore, MEIOK21 is a novel recombination factor and probably mediates DMC1/RAD51 recruitment to ssDNA or their stability on chromosomes through physical interaction with HSF2BP.

The Configuration of RPA, RAD51, and DMC1 Binding in Meiosis Reveals the Nature of Critical Recombination Intermediates

Meiotic recombination proceeds via binding of RPA, RAD51, and DMC1 to single-stranded DNA (ssDNA) substrates created after formation of programmed DNA double-strand breaks. Here we report high-resolution in vivo maps of RPA and RAD51 in meiosis, mapping their binding locations and lifespans to individual homologous chromosomes using a genetically engineered hybrid mouse. Together with high-resolution microscopy and DMC1 binding maps, we show that DMC1 and RAD51 have distinct spatial localization on ssDNA: DMC1 binds near the break site, and RAD51 binds away from it. We characterize inter-homolog recombination intermediates bound by RPA in vivo, with properties expected for the critical displacement loop (D-loop) intermediates. These data support the hypothesis that DMC1, not RAD51, performs strand exchange in mammalian meiosis. RPA-bound D-loops can be resolved as crossovers or non-crossovers, but crossover-destined D-loops may have longer lifespans. D-loops resemble crossover gene conversions in size, but their extent is similar in both repair pathways.

The histone modification reader ZCWPW1 links histone methylation to PRDM9-induced double-strand break repair

The histone modification writer Prdm9 has been shown to deposit H3K4me3 and H3K36me3 at future double-strand break (DSB) sites during the very early stages of meiosis, but the reader of these marks remains unclear. Here, we demonstrate that Zcwpw1 is an H3K4me3 reader that is required for DSB repair and synapsis in mouse testes. We generated H3K4me3 reader-dead Zcwpw1 mutant mice and found that their spermatocytes were arrested at the pachytene-like stage, which phenocopies the Zcwpw1 knock-out mice. Based on various ChIP-seq and immunofluorescence analyses using several mutants, we found that Zcwpw1's occupancy on chromatin is strongly promoted by the histone-modification activity of PRDM9. Zcwpw1 localizes to DMC1-labelled hotspots in a largely Prdm9-dependent manner, where it facilitates completion of synapsis by mediating the DSB repair process. In sum, our study demonstrates the function of ZCWPW1 that acts as part of the selection system for epigenetics-based recombination hotspots in mammals.

Dual histone methyl reader ZCWPW1 facilitates repair of meiotic double strand breaks in male mice

Meiotic crossovers result from homology-directed repair of DNA double-strand breaks (DSBs). Unlike yeast and plants, where DSBs are generated near gene promoters, in many vertebrates DSBs are enriched at hotspots determined by the DNA binding activity of the rapidly evolving zinc finger array of PRDM9 (PR domain zinc finger protein 9). PRDM9 subsequently catalyzes tri-methylation of lysine 4 and lysine 36 of Histone H3 in nearby nucleosomes. Here, we identify the dual histone methylation reader ZCWPW1, which is tightly co-expressed during spermatogenesis with Prdm9, as an essential meiotic recombination factor required for efficient repair of PRDM9-dependent DSBs and for pairing of homologous chromosomes in male mice. In sum, our results indicate that the evolution of a dual histone methylation writer/reader (PRDM9/ZCWPW1) system in vertebrates remodeled genetic recombination hotspot selection from an ancestral static pattern near genes towards a flexible pattern controlled by the rapidly evolving DNA binding activity of PRDM9.

Histone Post-Translational Modifications and CircRNAs in Mouse and Human Spermatozoa: Potential Epigenetic Marks to Assess Human Sperm Quality

Spermatozoa (SPZ) are motile cells, characterized by a cargo of epigenetic information including histone post-translational modifications (histone PTMs) and non-coding RNAs. Specific histone PTMs are present in developing germ cells, with a key role in spermatogenic events such as self-renewal and commitment of spermatogonia (SPG), meiotic recombination, nuclear condensation in spermatids (SPT). Nuclear condensation is related to chromatin remodeling events and requires a massive histone-to-protamine exchange. After this event a small percentage of chromatin is condensed by histones and SPZ contain nucleoprotamines and a small fraction of nucleohistone chromatin carrying a landascape of histone PTMs. Circular RNAs (circRNAs), a new class of non-coding RNAs, characterized by a nonlinear back-spliced junction, able to play as microRNA (miRNA) sponges, protein scaffolds and translation templates, have been recently characterized in both human and mouse SPZ. Since their abundance in eukaryote tissues, it is challenging to deepen their biological function, especially in the field of reproduction. Here we review the critical role of histone PTMs in male germ cells and the profile of circRNAs in mouse and human SPZ. Furthermore, we discuss their suggested role as novel epigenetic biomarkers to assess sperm quality and improve artificial insemination procedure.

Sycp2 is essential for synaptonemal complex assembly, early meiotic recombination and homologous pairing in zebrafish spermatocytes

Meiotic recombination is essential for faithful segregation of homologous chromosomes during gametogenesis. The progression of recombination is associated with dynamic changes in meiotic chromatin structures. However, whether Sycp2, a key structural component of meiotic chromatin, is required for the initiation of meiotic recombination is still unclear in vertebrates. Here, we describe that Sycp2 is required for assembly of the synaptonemal complex and early meiotic events in zebrafish spermatocytes. Our genetic screening by N-ethyl-N-nitrosourea mutagenesis revealed that ietsugu (its), a mutant zebrafish line with an aberrant splice site in the sycp2 gene, showed a defect during meiotic prophase I. The its mutation appeared to be a hypomorphic mutation compared to sycp2 knockout mutations generated by TALEN mutagenesis. Taking advantage of these sycp2 hypomorphic and knockout mutant lines, we demonstrated that Sycp2 is required for the assembly of the synaptonemal complex that is initiated in the vicinity of telomeres in wild-type zebrafish spermatocytes. Accordingly, homologous pairing, the foci of the meiotic recombinases Dmc1/Rad51 and RPA, and γH2AX signals were largely diminished in sycp2 knockout spermatocytes. Taken together, our data indicate that Sycp2 plays a critical role in not only the assembly of the synaptonemal complex, but also early meiotic recombination and homologous pairing, in vertebrates.

Global Transcriptome and Co-Expression Network Analysis Reveal Contrasting Response of Japonica and Indica Rice Cultivar to γ Radiation

Japonica and indica are two important subspecies in cultivated Asian rice. Irradiation is a classical approach to induce mutations and create novel germplasm. However, little is known about the differential response between japonica and indica rice after γ radiation. Here, we utilized the RNA sequencing and Weighted Gene Co-expression Network Analysis (WGCNA) to compare the transcriptome differences between japonica Nipponbare (NPB) and indica Yangdao6 (YD6) in response to irradiation. Japonica subspecies are more sensitive to irradiation than the indica subspecies. Indica showed a higher seedling survival rate than japonica. Irradiation caused more extensive DNA damage in shoots than in roots, and the severity was higher in NPB than in YD6. GO and KEGG pathway analyses indicate that the core genes related to DNA repair and replication and cell proliferation are similarly regulated between the varieties, however the universal stress responsive genes show contrasting differential response patterns in japonica and indica. WGCNA identifies 37 co-expressing gene modules and ten candidate hub genes for each module. This provides novel evidence indicating that certain peripheral pathways may dominate the molecular networks in irradiation survival and suggests more potential target genes in breeding for universal stress tolerance in rice.

The PSMA8 subunit of the spermatoproteasome is essential for proper meiotic exit and mouse fertility

The ubiquitin proteasome system regulates meiotic recombination in yeast through its association with the synaptonemal complex, a ‘zipper’-like structure that holds homologous chromosome pairs in synapsis during meiotic prophase I. In mammals, the proteasome activator subunit PA200 targets acetylated histones for degradation during somatic DNA double strand break repair and during histone replacement during spermiogenesis. We investigated the role of the testis-specific proteasomal subunit α4s (PSMA8) during spermatogenesis, and found that PSMA8 was localized to and dependent on the central region of the synaptonemal complex. Accordingly, synapsis-deficient mice show delocalization of PSMA8. Moreover, though Psma8-deficient mice are proficient in meiotic homologous recombination, there are alterations in the proteostasis of several key meiotic players that, in addition to the known substrate acetylated histones, have been shown by a proteomic approach to interact with PSMA8, such as SYCP3, SYCP1, CDK1 and TRIP13. These alterations lead to an accumulation of spermatocytes in metaphase I and II which either enter massively into apoptosis or give rise to a low number of aberrant round spermatids that apoptose before histone replacement takes place.

Proteins within the cells that are unnecessary or damaged are degraded by a large protein complex named the proteasome. The proteins to be degraded are marked by a small protein called ubiquitin. The addition of a small modification (acetyl group) to some proteins also promotes their degradation by the proteasome. Proteasomal degradation of proteins is an essential mechanism for many developmental programs including gametogenesis, a process whereby a diploid cell produces a haploid cell or gamete (sperm or egg). The mechanism by which this genome reduction occurs is called meiosis. Here, we report the study of a protein, named PSMA8 that is specific for the testis proteasome in vertebrates. Using the mouse as a model, we show that loss of PSMA8 leads to infertility in males. By co-immunoprecipitation-coupled mass spectroscopy we identified a large list of novel PSMA8 interacting proteins. We focused our functional analysis on several key meiotic proteins which were accumulated such as SYCP3, SYCP1, CDK1 and TRIP13 in addition to the known substrate of the spermatoproteasome, the acetylated histones. We suggest that the altered accumulation of these important proteins causes a disequilibrium of the meiotic division that produces apoptotic spermatocytes in metaphase I and II and also early spermatids that die soon after reaching this stage.

Comprehensive Analysis of Mouse Cancer/Testis Antigen Functions in Cancer Cells and Roles of TEKT5 in Cancer Cells and Testicular Germ Cells

The cancer/testis antigen (CTA) genes were identified as human genes preferentially expressed in cancer cells and testis, but the contribution of CTAs to cancer and male germ cell development is unclear. In this study, we comprehensively examined mouse CTA functions and found that the majority of CTAs are involved in growth and/or survival of cancer cells. We focused on one mouse CTA gene, Tekt5, for its detailed functional analysis. Tekt5 knockdown (KD) in ovarian cancer cells caused G₁ arrest and apoptosis, and p27^kip1 was concomitantly upregulated. Tekt5 KD also resulted in decreased levels of acetylated α-tubulin and subsequent fragmentation of β-III-tubulin, upregulation of HDAC6 that deacetylates α-tubulin, and nuclear accumulation of SMAD3 that induces p27^kip1 expression. Because depolymerization of tubulin is known to cause translocation of SMAD3 to the nucleus, these results together suggested that TEKT5 negatively regulates Hdac6 expression and consequently maintains cell cycle via stabilization of tubulin. We also found that the number of spermatids was significantly decreased and acetylated α-tubulin levels were decreased in vivo by KD of Tekt5 in testis. Because acetylated α-tubulin is required for sperm morphogenesis, these results suggest that TEKT5 is necessary for spermiogenesis via maintenance of acetylated α-tubulin levels.

Unified single-cell analysis of testis gene regulation and pathology in five mouse strains

To fully exploit the potential of single-cell functional genomics in the study of development and disease, robust methods are needed to simplify the analysis of data across samples, time-points and individuals. Here we introduce a model-based factor analysis method, SDA, to analyze a novel 57,600 cell dataset from the testes of wild-type mice and mice with gonadal defects due to disruption of the genes Mlh3, Hormad1, Cul4a or Cnp. By jointly analyzing mutant and wild-type cells we decomposed our data into 46 components that identify novel meiotic gene-regulatory programs, mutant-specific pathological processes, and technical effects, and provide a framework for imputation. We identify, de novo, DNA sequence motifs associated with individual components that define temporally varying modes of gene expression control. Analysis of SDA components also led us to identify a rare population of macrophages within the seminiferous tubules of Mlh3-/- and Hormad1-/- mice, an area typically associated with immune privilege.

Overexpression of MicroRNA-10a in Germ Cells Causes Male Infertility by Targeting Rad51 in Mouse and Human

Spermatogenesis is a complicated process including spermatogonial stem cells self-renewal and differentiates into mature spermatozoa. MicroRNAs (miRNAs) as a class of small non-coding RNAs play a crucial role during the process of spermatogenesis. However, the function of a plenty of miRNAs on spermatogenesis and the potential mechanisms remain largely unknown. Here, we show that genetically conditional overexpressed miR-10a in germ cells caused complete male sterility, characterized by meiotic arrested in germ cells. Analysis of miR-10a overexpression mouse testes reveals that failure of double strand break (DSB) repairs and aberrant spermatogonial differentiation. Furthermore, we identified Rad51 as a key target of miR-10a in germ cell by bioinformatics prediction and luciferase assay, which may be responsible for the infertility of the miR-10a overexpressed mice and germ cell arrested patients. Our data show that miR-10a dependent genetic regulation of meiotic process is crucial for male germ cell development and spermatogenesis in both mouse and human. These findings facilitate our understanding of the roles of miRNA-10a in spermatogenesis and male fertility.

A meiosis-specific BRCA2 binding protein recruits recombinases to DNA double-strand breaks to ensure homologous recombination

Homologous recombination (HR) repairs DNA double-strand breaks (DSBs) to maintain genomic integrity. Recombinase recruited to the DSBs by the mediator protein BRCA2 catalyzes the homology-directed repair. During meiotic HR, programmed DSBs are introduced genome-wide but their repair mechanisms, including the regulation of BRCA2, have remained largely elusive. Here we identify a meiotic localizer of BRCA2, MEILB2/HSF2BP, that localizes to the site of meiotic DSBs in mice. Disruption of Meilb2 abolishes the localization of RAD51 and DMC1 recombinases in spermatocytes, leading to errors in DSB repair and male sterility. MEILB2 directly binds to BRCA2 and regulates its association to meiotic DSBs. We map the MEILB2-binding domain within BRCA2 that is distinct from the canonical DNA-binding domain but is sufficient to localize to meiotic DSBs in a MEILB2-dependent manner. We conclude that localization of BRCA2 to meiotic DSBs is mediated by MEILB2, which is an integral mechanism to repair abundant meiotic DSBs.

Homology directed repair of meiotic double-strand breaks functions via recruitment and assembly of strand-exchange proteins called recombinases. Here the authors reveal and characterize a BRCA2 interactor regulating meiotic recombinases that localizes to chromosomal axes and facilitates the repair of meiotic DSBs.

Dual functions for the ssDNA-binding protein RPA in meiotic recombination

Meiotic recombination permits exchange of genetic material between homologous chromosomes. The replication protein A (RPA) complex, the predominant ssDNA-binding complex, is required for nearly all aspects of DNA metabolism, but its role in mammalian meiotic recombination remains unknown due to the embryonic lethality of RPA mutant mice. RPA is a heterotrimer of RPA1, RPA2, and RPA3. We find that loss of RPA1, the largest subunit, leads to disappearance of RPA2 and RPA3, resulting in the absence of the RPA complex. Using an inducible germline-specific inactivation strategy, we find that loss of RPA completely abrogates loading of RAD51/DMC1 recombinases to programmed meiotic DNA double strand breaks, thus blocking strand invasion required for chromosome pairing and synapsis. Surprisingly, loading of MEIOB, SPATA22, and ATR to DNA double strand breaks is RPA-independent and does not promote RAD51/DMC1 recruitment in the absence of RPA. Finally, inactivation of RPA reduces crossover formation. Our results demonstrate that RPA plays two distinct roles in meiotic recombination: an essential role in recombinase recruitment at early stages and an important role in promoting crossover formation at later stages.

Ablation of Ggnbp2 impairs meiotic DNA double-strand break repair during spermatogenesis in mice

Gametogenetin (GGN) binding protein 2 (GGNBP2) is a zinc finger protein expressed abundantly in spermatocytes and spermatids. We previously discovered that Ggnbp2 resection caused metamorphotic defects during spermatid differentiation and resulted in an absence of mature spermatozoa in mice. However, whether GGNBP2 affects meiotic progression of spermatocytes remains to be established. In this study, flow cytometric analyses showed a decrease in haploid, while an increase in tetraploid spermatogenic cells in both 30‐ and 60‐day‐old Ggnbp2 knockout testes. In spread spermatocyte nuclei, Ggnbp2 loss increased DNA double‐strand breaks (DSB), compromised DSB repair and reduced crossovers. Further investigations demonstrated that GGNBP2 co‐immunoprecipitated with a testis‐enriched protein GGN1. Immunofluorescent staining revealed that both GGNBP2 and GGN1 had the same subcellular localizations in spermatocyte, spermatid and spermatozoa. Ggnbp2 loss suppressed Ggn expression and nuclear accumulation. Furthermore, deletion of either Ggnbp2 or Ggn in GC‐2spd cells inhibited their differentiation into haploid cells in vitro. Overexpression of Ggnbp2 in Ggnbp2 null but not in Ggn null GC‐2spd cells partially rescued the defect coinciding with a restoration of Ggn expression. Together, these data suggest that GGNBP2, likely mediated by its interaction with GGN1, plays a role in DSB repair during meiotic progression of spermatocytes.

XRCC2 mutation causes meiotic arrest, azoospermia and infertility

Background

Meiotic homologous recombination (HR) plays an essential role in gametogenesis. In most eukaryotes, meiotic HR is mediated by two recombinase systems: ubiquitous RAD51 and meiosis-specific DMC1. In the RAD51-mediated HR system, RAD51 and five RAD51 paralogues are essential for normal RAD51 function, but the role of RAD51 in human meiosis is unclear. The knockout of Rad51 or any Rad51 paralogue in mice exhibits embryonic lethality. We investigated a family with meiotic arrest, azoospermia and infertility but without other abnormalities.

Methods

Homozygosity mapping and whole-exome sequencing were performed in a consanguineous family. An animal model carrying a related mutation was created by using a CRISPR/Cas9 system.

Results

We identified a 1 bp homozygous substitution (c.41T>C/p.Leu14Pro) on a RAD51 paralogue, namely, XRCC2, in the consanguineous family. We did not detect any XRCC2 recessive mutation in a cohort of 127 males with non-obstructive-azoospermia. Knockin mice with Xrcc2-c.T41C/p.Leu14Pro mutation were generated successfully by the CRISPR/Cas9 method. The homozygotes survived and exhibited meiotic arrest, azoospermia, premature ovarian failure and infertility.

Conclusion

A XRCC2 recessive mutation causing meiotic arrest and infertility in humans was duplicated with knockin mice. Our results revealed a new Mendelian hereditary entity and provided an experimental model of RAD51-HR gene defect in mammalian meiosis.

ATR is required to complete meiotic recombination in mice

Precise execution of recombination during meiosis is essential for forming chromosomally-balanced gametes. Meiotic recombination initiates with the formation and resection of DNA double-strand breaks (DSBs). Cellular responses to meiotic DSBs are critical for efficient repair and quality control, but molecular features of these remain poorly understood, particularly in mammals. Here we report that the DNA damage response protein kinase ATR is crucial for meiotic recombination and completion of meiotic prophase in mice. Using a hypomorphic Atr mutation and pharmacological inhibition of ATR in vivo and in cultured spermatocytes, we show that ATR, through its effector kinase CHK1, promotes efficient RAD51 and DMC1 assembly at RPA-coated resected DSB sites and establishment of interhomolog connections during meiosis. Furthermore, our findings suggest that ATR promotes local accumulation of recombination markers on unsynapsed axes during meiotic prophase to favor homologous chromosome synapsis. These data reveal that ATR plays multiple roles in mammalian meiotic recombination.

BRDT is an essential epigenetic regulator for proper chromatin organization, silencing of sex chromosomes and crossover formation in male meiosis

The double bromodomain and extra-terminal domain (BET) proteins are critical epigenetic readers that bind to acetylated histones in chromatin and regulate transcriptional activity and modulate changes in chromatin structure and organization. The testis-specific BET member, BRDT, is essential for the normal progression of spermatogenesis as mutations in the Brdt gene result in complete male sterility. Although BRDT is expressed in both spermatocytes and spermatids, loss of the first bromodomain of BRDT leads to severe defects in spermiogenesis without overtly compromising meiosis. In contrast, complete loss of BRDT blocks the progression of spermatocytes into the first meiotic division, resulting in a complete absence of post-meiotic cells. Although BRDT has been implicated in chromatin remodeling and mRNA processing during spermiogenesis, little is known about its role in meiotic processes. Here we report that BRDT is an essential regulator of chromatin organization and reprograming during prophase I of meiosis. Loss of BRDT function disrupts the epigenetic state of the meiotic sex chromosome inactivation in spermatocytes, affecting the synapsis and silencing of the X and Y chromosomes. We also found that BRDT controls the global chromatin organization and histone modifications of the chromatin attached to the synaptonemal complex. Furthermore, the homeostasis of crossover formation and localization during pachynema was altered, underlining a possible epigenetic mechanism by which crossovers are regulated and differentially established in mammalian male genomes. Our observations reveal novel findings about the function of BRDT in meiosis and provide insight into how epigenetic regulators modulate the progression of male mammalian meiosis and the formation of haploid gametes.

BRDT, a testis-specific member of the bromodomain and extra-terminal (BET) subfamily of epigenetic reader proteins, is essential for the generation of male gametes. In post-meiotic cells, BRDT is involved in chromatin organization and transcriptional regulation through its first bromodomain motif, as loss of the BD1 results in a truncated BRDT protein that fully interrupts the differentiation of the germ cells during the process of spermiogenesis. Complete loss of BRDT function results in an arrest during meiotic prophase with no cells progressing into post-meiotic stages. However, neither the specific role of BRDT in meiosis nor the pathways affected by its depletion are known. We investigated how BRDT controls meiosis by examining its subcellular localization during prophase I as well as the meiotic consequences observed with the loss of BRDT function. BRDT localizes throughout the chromatin of autosomes and sex chromosomes in a dynamic pattern during pachynema and diplonema. Loss of BRDT severely disrupts the epigenetic reprograming and silencing of transcription of the sex chromosomes, the global and regional chromatin configuration, and the formation and localization of crossovers in spermatocytes. Thus, BRDT regulates key meiotic processes that determine the genetic and epigenetic homeostasis of the male gamete.