Zipping and Unzipping: Protein Modifications Regulating Synaptonemal Complex Dynamics

The ATPase activity of yeast chromosome axis protein Hop1 affects the frequency of meiotic crossovers

Saccharomyces cerevisiae meiosis-specific Hop1, a structural constituent of the synaptonemal complex, also facilitates the formation of programmed DNA double-strand breaks and the pairing of homologous chromosomes. Here, we reveal a serendipitous discovery that Hop1 possesses robust DNA-independent ATPase activity, although it lacks recognizable sequence motifs required for ATP binding and hydrolysis. By leveraging molecular docking combined with molecular dynamics simulations and biochemical assays, we identified an ensemble of five amino acid residues in Hop1 that could potentially participate in ATP-binding and hydrolysis. Consistent with this premise, we found that Hop1 binds to ATP and that substitution of amino acid residues in the putative ATP-binding site significantly impaired its ATPase activity, suggesting that this activity is intrinsic to Hop1. Notably, K65A and N67Q substitutions in the Hop1 N-terminal HORMA domain synergistically abolished its ATPase activity, noticeably impaired its DNA-binding affinity and reduced its association with meiotic chromosomes, while enhancing the frequency of meiotic crossovers (COs). Overall, our study establishes Hop1 as a DNA-independent ATPase and reveals a potential biological function for its ATPase activity in the regulation of meiotic CO frequency.

Histone H3 tail modifications required for meiosis in Saccharomyces cerevisiae

Histone tail phosphorylation has diverse effects on a myriad of cellular processes, including cell division, and is highly conserved throughout eukaryotes. Histone H3 phosphorylation at threonine 3 (H3T3) during mitosis occurs at the inner centromeres and is required for proper biorientation of chromosomes on the mitotic spindle. While H3T3 is also phosphorylated during meiosis, a possible role for this modification has not been tested. Here, we asked if H3T3 phosphorylation (H3T3ph) is important for meiotic division by quantifying sporulation efficiency and spore viability in Saccharomyces cerevisiae mutants with a T3A amino acid substitution. The T3A substitution resulted in greatly reduced sporulation efficiency and reduced spore viability. Analysis of two other H3 tail mutants, K4A and S10A, revealed different effects on sporulation efficiency and spore viability compared to the T3A mutant, suggesting that these phenotypes are due to failures in distinct functions. To determine if the spindle checkpoint promotes spore viability of the T3A mutant, the MAD2 gene required for the spindle assembly checkpoint was deleted to abolish spindle assembly checkpoint function. This resulted in a severe reduction in spore viability following meiosis. Altogether, the data reveal a critical function for histone H3 threonine 3 that requires monitoring by the spindle checkpoint to ensure successful completion of meiosis.

Diffusion within the synaptonemal complex can account for signal transduction along meiotic chromosomes

Meiotic chromosomes efficiently transduce information along their length to regulate the distribution of genetic exchanges (crossovers). However, the mode of signal transduction remains unknown. A conserved protein interface called the synaptonemal complex forms between the parental chromosomes. The synaptonemal complex exhibits liquid-like behaviors, suggesting that the diffusion of signaling molecules along its length could coordinate crossover formation. Here, we directly test the feasibility of such a mechanism by tracking a component of the synaptonemal complex (SYP-3) and a conserved regulator of exchanges (ZHP-3) in live Caenorhabditis elegans gonads. While we find that both proteins diffuse within the synaptonemal complex, ZHP-3 diffuses 4- and 9-fold faster than SYP-3 before and after crossover designation, respectively. We use these measurements to parameterize a physical model for signal transduction. We find that ZHP-3, but not SYP-3, can explore the lengths of chromosomes on the time scale of crossover designation, consistent with a role in the spatial regulation of exchanges. Given the conservation of ZHP-3 paralogues across eukaryotes, we propose that diffusion along the synaptonemal complex may be a conserved mechanism of meiotic regulation. More broadly, our work explores how diffusion compartmentalized by condensates could regulate crucial chromosomal functions.

BCAS2 and hnRNPH1 orchestrate alternative splicing for DNA double-strand break repair and synapsis in meiotic prophase I

Understanding the intricacies of homologous recombination during meiosis is crucial for reproductive biology. However, the role of alternative splicing (AS) in DNA double-strand breaks (DSBs) repair and synapsis remains elusive. In this study, we investigated the impact of conditional knockout (cKO) of the splicing factor gene Bcas2 in mouse germ cells, revealing impaired DSBs repair and synapsis, resulting in non-obstructive azoospermia (NOA). Employing crosslinking immunoprecipitation and sequencing (CLIP-seq), we globally mapped BCAS2 binding sites in the testis, uncovering its predominant association with 5' splice sites (5'SS) of introns and a preference for GA-rich regions. Notably, BCAS2 exhibited direct binding and regulatory influence on Trp53bp1 (codes for 53BP1) and Six6os1 through AS, unveiling novel insights into DSBs repair and synapsis during meiotic prophase I. Furthermore, the interaction between BCAS2, hnRNPH1, and SRSF3 was discovered to orchestrate Trp53bp1 expression via AS, underscoring its role in meiotic prophase I DSBs repair. In summary, our findings delineate the indispensable role of BCAS2-mediated post-transcriptional regulation in DSBs repair and synapsis during male meiosis. This study provides a comprehensive framework for unraveling the molecular mechanisms governing the post-transcriptional network in male meiosis, contributing to the broader understanding of reproductive biology.

First investigation into the genetic control of meiosis in sugarcane

The sugarcane (Saccharum spp.) genome is one of the most complex of all. Modern varieties are highly polyploid and aneuploid as a result of hybridization between Saccharum officinarum and S. spontaneum. Little research has been done on meiotic control in polyploid species, with the exception of the wheat Ph1 locus harboring the ZIP4 gene (TaZIP4-B2) which promotes pairing between homologous chromosomes while suppressing crossover between homeologs. In sugarcane, despite its interspecific origin, bivalent association is favored, and multivalents, if any, are resolved at the end of prophase I. Thus, our aim herein was to investigate the purported genetic control of meiosis in the parental species and in sugarcane itself. We investigated the ZIP4 gene and immunolocalized meiotic proteins, namely synaptonemal complex proteins Zyp1 and Asy1. The sugarcane ZIP4 gene is located on chromosome 2 and expressed more abundantly in flowers, a similar profile to that found for TaZIP4-B2. ZIP4 expression is higher in S. spontaneum a neoautopolyploid, with lower expression in S. officinarum, a stable octoploid species. The sugarcane Zip4 protein contains a TPR domain, essential for scaffolding. Its 3D structure was also predicted, and it was found to be very similar to that of TaZIP4-B2, reflecting their functional relatedness. Immunolocalization of the Asy1 and Zyp1 proteins revealed that S. officinarum completes synapsis. However, in S. spontaneum and SP80-3280 (a modern variety), no nuclei with complete synapsis were observed. Importantly, our results have implications for sugarcane cytogenetics, genetic mapping, and genomics.

Roles of histone post-translational modifications in meiosis†

Histone post-translational modifications, such as phosphorylation, methylation, acetylation, and ubiquitination, play vital roles in various chromatin-based cellular processes. Meiosis is crucial for organisms that depend on sexual reproduction to produce haploid gametes, during which chromatin undergoes intricate conformational changes. An increasing body of evidence is clarifying the essential roles of histone post-translational modifications during meiotic divisions. In this review, we concentrate on the post-translational modifications of H2A, H2B, H3, and H4, as well as the linker histone H1, that are required for meiosis, and summarize recent progress in understanding how these modifications influence diverse meiotic events. Finally, challenges and exciting open questions for future research in this field are discussed. Summary Sentence  Diverse histone post-translational modifications exert important effects on the meiotic cell cycle and these "histone codes" in meiosis might lead to the development of novel therapeutic strategies against reproductive diseases.

A transcriptomics-based RNAi screen for regulators of meiosis and early stages of oocyte development in Drosophila melanogaster

A properly regulated series of developmental and meiotic events must occur to ensure the successful production of gametes. In Drosophila melanogaster ovaries, these early developmental and meiotic events include the production of the 16-cell cyst, meiotic entry, synaptonemal complex (SC) formation, recombination, and oocyte specification. In order to identify additional genes involved in early oocyte development and meiosis, we reanalyzed 3 published single-cell RNA-seq datasets from Drosophila ovaries, using vasa (germline) together with c(3)G, cona, and corolla (SC) as markers. Our analysis generated a list of 2,743 co-expressed genes. Many known SC-related and early oocyte development genes fell within the top 500 genes on this list, as ranked by the abundance and specificity of each gene's expression across individual analyses. We tested 526 available RNAi lines containing shRNA constructs in germline-compatible vectors representing 331 of the top 500 genes. We assessed targeted ovaries for SC formation and maintenance, oocyte specification, cyst development, and double-strand break dynamics. Six uncharacterized genes exhibited early developmental defects. SC and developmental defects were observed for additional genes not well characterized in the early ovary. Interestingly, in some lines with developmental delays, meiotic events could still be completed once oocyte specificity occurred indicating plasticity in meiotic timing. These data indicate that a transcriptomics approach can be used to identify genes involved in functions in a specific cell type in the Drosophila ovary.

The dynamic recruitment of LAB proteins senses meiotic chromosome axis differentiation in C. elegans

During meiosis, cohesin and meiosis-specific proteins organize chromatin into an axis-loop architecture, coordinating homologous synapsis, recombination, and ordered chromosome segregation. However, how the meiotic chromosome axis is assembled and differentiated with meiotic progression remains elusive. Here, we explore the dynamic recruitment of two long arms of the bivalent proteins, LAB-1 and LAB-2, in Caenorhabditis elegans. LAB proteins directly interact with the axis core HORMA complexes and weak interactions contribute to their recruitment. LAB proteins phase separate in vitro, and this capacity is promoted by HORMA complexes. During early prophase, synapsis oppositely regulates the axis enrichment of LAB proteins. After the pachytene exit, LAB proteins switch from a reciprocal localization pattern to a colocalization pattern, and the normal dynamic pattern of LAB proteins is altered in meiotic mutants. We propose that LAB recruitment senses axis differentiation, and phase separation of meiotic structures helps subdomain establishment and accurate segregation of the chromosomes.

Homologous chromosome pairing: The linchpin of accurate segregation in meiosis

Meiosis is a specialized cell division that occurs in sexually reproducing organisms, generating haploid gametes containing half the chromosome number through two rounds of cell division. Homologous chromosomes pair and prepare for their proper segregation in subsequent divisions. How homologous chromosomes recognize each other and achieve pairing is an important question. Early studies showed that in most organisms, homologous pairing relies on homologous recombination. However, pairing mechanisms differ across species. Evidence indicates that chromosomes are dynamic and move during early meiotic stages, facilitating pairing. Recent studies in various model organisms suggest conserved mechanisms and key regulators of homologous chromosome pairing. This review summarizes these findings and compare similarities and differences in homologous chromosome pairing mechanisms across species.

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.

A novel recombination protein C12ORF40/REDIC1 is required for meiotic crossover formation

During meiosis, at least one crossover must occur per homologous chromosome pair to ensure normal progression of meiotic division and accurate chromosome segregation. However, the mechanism of crossover formation is not fully understood. Here, we report a novel recombination protein, C12ORF40/REDIC1, essential for meiotic crossover formation in mammals. A homozygous frameshift mutation in C12orf40 (c.232_233insTT, p.Met78Ilefs*2) was identified in two infertile men with meiotic arrest. Spread mouse spermatocyte fluorescence immunostaining showed that REDIC1 forms discrete foci between the paired regions of homologous chromosomes depending on strand invasion and colocalizes with MSH4 and later with MLH1 at the crossover sites. Redic1 knock-in (KI) mice homozygous for mutation c.232_233insTT are infertile in both sexes due to insufficient crossovers and consequent meiotic arrest, which is also observed in our patients. The foci of MSH4 and TEX11, markers of recombination intermediates, are significantly reduced numerically in the spermatocytes of Redic1 KI mice. More importantly, our biochemical results show that the N-terminus of REDIC1 binds branched DNAs present in recombination intermediates, while the identified mutation impairs this interaction. Thus, our findings reveal a crucial role for C12ORF40/REDIC1 in meiotic crossover formation by stabilizing the recombination intermediates, providing prospective molecular targets for the clinical diagnosis and therapy of infertility.

Building the synaptonemal complex: Molecular interactions between the axis and the central region

The successful delivery of genetic material to gametes requires tightly regulated interactions between the parental chromosomes. Central to this regulation is a conserved chromosomal interface called the synaptonemal complex (SC), which brings the parental chromosomes in close proximity along their length. While many of its components are known, the interfaces that mediate the assembly of the SC remain a mystery. Here, we survey findings from different model systems while focusing on insight gained in the nematode C. elegans. We synthesize our current understanding of the structure, dynamics, and biophysical properties of the SC and propose mechanisms for SC assembly.

Synaptonemal Complex in Human Biology and Disease

The synaptonemal complex (SC) is a meiosis-specific multiprotein complex that forms between homologous chromosomes during prophase of meiosis I. Upon assembly, the SC mediates the synapses of the homologous chromosomes, leading to the formation of bivalents, and physically supports the formation of programmed double-strand breaks (DSBs) and their subsequent repair and maturation into crossovers (COs), which are essential for genome haploidization. Defects in the assembly of the SC or in the function of the associated meiotic recombination machinery can lead to meiotic arrest and human infertility. The majority of proteins and complexes involved in these processes are exclusively expressed during meiosis or harbor meiosis-specific subunits, although some have dual functions in somatic DNA repair and meiosis. Consistent with their functions, aberrant expression and malfunctioning of these genes have been associated with cancer development. In this review, we focus on the significance of the SC and their meiotic-associated proteins in human fertility, as well as how human genetic variants encoding for these proteins affect the meiotic process and contribute to infertility and cancer development.

ASYNAPSIS 1 ensures crossover fidelity in polyploid wheat by promoting homologous recombination and suppressing non-homologous recombination

During meiosis, the chromosome axes and synaptonemal complex mediate chromosome pairing and homologous recombination to maintain genomic stability and accurate chromosome segregation. In plants, ASYNAPSIS 1 (ASY1) is a key component of the chromosome axis that promotes inter-homolog recombination, synapsis and crossover formation. Here, the function of ASY1 has been cytologically characterized in a series of hypomorphic wheat mutants. In tetraploid wheat, asy1 hypomorphic mutants experience a reduction in chiasmata (crossovers) in a dosage-specific manner, resulting in failure to maintain crossover (CO) assurance. In mutants with only one functional copy of ASY1, distal chiasmata are maintained at the expense of proximal and interstitial chiasmata, indicating that ASY1 is required to promote chiasma formation away from the chromosome ends. Meiotic prophase I progression is delayed in asy1 hypomorphic mutants and is arrested in asy1 null mutants. In both tetraploid and hexaploid wheat, single asy1 mutants exhibit a high degree of ectopic recombination between multiple chromosomes at metaphase I. To explore the nature of the ectopic recombination, Triticum turgidum asy1b-2 was crossed with wheat-wild relative Aegilops variabilis. Homoeologous chiasmata increased 3.75-fold in Ttasy1b-2/Ae. variabilis compared to wild type/Ae. variabilis, indicating that ASY1 suppresses chiasma formation between divergent, but related chromosomes. These data suggest that ASY1 promotes recombination along the chromosome arms of homologous chromosomes whilst suppressing recombination between non-homologous chromosomes. Therefore, asy1 mutants could be utilized to increase recombination between wheat wild relatives and elite varieties for expediting introgression of important agronomic traits.

Dual roles of R-loops in the formation and processing of programmed DNA double-strand breaks during meiosis

BACKGROUND

Meiotic recombination is initiated by Spo11-dependent programmed DNA double-strand breaks (DSBs) that are preferentially concentrated within genomic regions called hotspots; however, the factor(s) that specify the positions of meiotic DSB hotspots remain unclear.

RESULTS

Here, we examined the frequency and distribution of R-loops, a type of functional chromatin structure comprising single-stranded DNA and a DNA:RNA hybrid, during budding yeast meiosis and found that the R-loops were changed dramatically throughout meiosis. We detected the formation of multiple de novo R-loops in the pachytene stage and found that these R-loops were associated with meiotic recombination during yeast meiosis. We show that transcription-replication head-on collisions could promote R-loop formation during meiotic DNA replication, and these R-loops are associated with Spo11. Furthermore, meiotic recombination hotspots can be eliminated by reversing the direction of transcription or replication, and reversing both of these directions can reconstitute the hotspots.

CONCLUSIONS

Our study reveals that R-loops may play dual roles in meiotic recombination. In addition to participation in meiotic DSB processing, some meiotic DSB hotspots may be originated from the transcription-replication head-on collisions during meiotic DNA replication.

Postnatal oogenesis leads to an exceptionally large ovarian reserve in naked mole-rats

In the long-lived naked mole-rat (NMR), the entire process of oogenesis occurs postnatally. Germ cell numbers increase significantly in NMRs between postnatal days 5 (P5) and P8, and germs cells positive for proliferation markers (Ki-67, pHH3) are present at least until P90. Using pluripotency markers (SOX2 and OCT4) and the primordial germ cell (PGC) marker BLIMP1, we show that PGCs persist up to P90 alongside germ cells in all stages of female differentiation and undergo mitosis both in vivo and in vitro. We identified VASA+ SOX2+ cells at 6 months and at 3-years in subordinate and reproductively activated females. Reproductive activation was associated with proliferation of VASA+ SOX2+ cells. Collectively, our results suggest that highly desynchronized germ cell development and the maintenance of a small population of PGCs that can expand upon reproductive activation are unique strategies that could help to maintain the NMR's ovarian reserve for its 30-year reproductive lifespan.

Kinase PLK1 regulates the disassembly of the lateral elements and the assembly of the inner centromere during the diakinesis/metaphase I transition in male mouse meiosis

PLK1 is a serine/threonine kinase with crucial roles during mitosis. However, its involvement during mammalian male meiosis remains largely unexplored. By inhibiting the kinase activity of PLK1 using BI 2536 on organotypic cultures of seminiferous tubules, we found that the disassembly of SYCP3 and HORMAD1 from the lateral elements of the synaptonemal complex during diakinesis is impeded. We also found that the normal recruitment of SYCP3 and HORMAD1 to the inner centromere in prometaphase I spermatocytes did not occur. Additionally, we analyzed the participation of PLK1 in the assembly of the inner centromere by studying its implication in the Bub1-H2AT120ph-dependent recruitment of shugoshin SGO2, and the Haspin-H3T3ph-dependent recruitment of Aurora B/C and Borealin. Our results indicated that both pathways are regulated by PLK1. Altogether, our results demonstrate that PLK1 is a master regulator of the late prophase I/metaphase I transition in mouse spermatocytes.

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.

ZmSPO11-2 is critical for meiotic recombination in maize

Most plant species have three or more SPO11/TOPOVIA homologs and two TOPOVIB homologs, which associate to trigger meiotic double-strand break (DSB) formation and subsequent meiotic recombination. In Zea mays L. (maize), ZmSPO11-1 and ZmMTOPVIB have been reported to be indispensable for the initiation of meiotic recombination, yet the function of ZmSPO11-2 remains unclear. In this study, we characterized meiotic functions of ZmSPO11-2 during male meiosis in maize. Two independent Zmspo11-1 knock-out mutants exhibited normal vegetative growth but both male and female sterility. The formation of meiotic DSBs of DNA molecules was fully abolished in the Zmspo11-2 plants, leading to the defective homologous chromosome paring, synapsis, recombination, and segregation. However, the bipolar spindle assembly was not noticeably affected in Zmspo11-2 meiocytes. Overall, our results demonstrate that as its partner ZmSPO11-1 and ZmMTOPVIB, ZmSPO11-2 plays essential roles in DSB formation and homologous recombination in maize meiosis.

ATM/ATR kinases link the synaptonemal complex and DNA double-strand break repair pathway choice

DNA double-strand breaks (DSBs) are deleterious lesions, which must be repaired precisely to maintain genomic stability. During meiosis, programmed DSBs are repaired via homologous recombination (HR) while repair using the nonhomologous end joining (NHEJ) pathway is inhibited, thereby ensuring crossover formation and accurate chromosome segregation.1,2 How DSB repair pathway choice is implemented during meiosis is unknown. In C. elegans, meiotic DSB repair takes place in the context of the fully formed, highly dynamic zipper-like structure present between homologous chromosomes called the synaptonemal complex (SC).3,4,5,6,7,8,9 The SC consists of a pair of lateral elements bridged by a central region composed of the SYP proteins in C. elegans. How the structural components of the SC are regulated to maintain the architectural integrity of the assembled SC around DSB repair sites remained unclear. Here, we show that SYP-4, a central region component of the SC, is phosphorylated at Serine 447 in a manner dependent on DSBs and the ATM/ATR DNA damage response kinases. We show that this SYP-4 phosphorylation is critical for preserving the SC structure following exogenous (γ-IR-induced) DSB formation and for promoting normal DSB repair progression and crossover patterning following SPO-11-dependent and exogenous DSBs. We propose a model in which ATM/ATR-dependent phosphorylation of SYP-4 at the S447 site plays important roles both in maintaining the architectural integrity of the SC following DSB formation and in warding off repair via the NHEJ repair pathway, thereby preventing aneuploidy.

Beyond Editing, CRISPR/Cas9 For Protein Localization: An Educational Primer for Use with "A dCas9-Based System Identifies a Central Role for Ctf19 in Kinetochore-Derived Suppression of Meiotic Recombination"

CRISPR/Cas9 has dramatically changed how we conduct genetic research, providing a tool for precise sequence editing. However, new applications of CRISPR/Cas9 have emerged that do not involve nuclease activity. In the accompanying article "A dCas9-Based System Identifies a Central Role for Ctf19 in Kinetochore-Derived Suppression of Meiotic Recombination", Kuhl and colleagues utilize a catalytically dead Cas9 to localize proteins at specific genomic locations. The authors seek to understand the role of kinetochore proteins in the suppression of meiotic recombination, a phenomenon that has been observed in centromere regions. By harnessing the power of CRISPR/Cas9 to bind specific genomic sequences, Kuhl et al. localized individual kinetochore proteins to areas of high meiotic recombination and assessed their role in suppression. This primer article provides undergraduate students with background information on chromosomes, meiosis, recombination and CRISPR/Cas9 to support their reading of the Kuhl et al. study. This primer is intended to help students and instructors navigate the study's experimental design, interpret the results, and appreciate the broader scope of meiotic recombination and CRISPR/Cas9. Questions are included to facilitate discussion of the study.

Arabidopsis HEAT SHOCK FACTOR BINDING PROTEIN is required to limit meiotic crossovers and HEI10 transcription

The number of meiotic crossovers is tightly controlled and most depend on pro-crossover ZMM proteins, such as the E3 ligase HEI10. Despite the importance of HEI10 dosage for crossover formation, how HEI10 transcription is controlled remains unexplored. In a forward genetic screen using a fluorescent crossover reporter in Arabidopsis thaliana, we identify heat shock factor binding protein (HSBP) as a repressor of HEI10 transcription and crossover numbers. Using genome-wide crossover mapping and cytogenetics, we show that hsbp mutations or meiotic HSBP knockdowns increase ZMM-dependent crossovers toward the telomeres, mirroring the effects of HEI10 overexpression. Through RNA sequencing, DNA methylome, and chromatin immunoprecipitation analysis, we reveal that HSBP is required to repress HEI10 transcription by binding with heat shock factors (HSFs) at the HEI10 promoter and maintaining DNA methylation over the HEI10 5' untranslated region. Our findings provide insights into how the temperature response regulator HSBP restricts meiotic HEI10 transcription and crossover number by attenuating HSF activity.

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.

Dynamic Interactions between Tumor Cells and Brain Microvascular Endothelial Cells in Glioblastoma

GBM is the most aggressive brain tumor among adults. It is characterized by extensive vascularization, and its further growth and recurrence depend on the formation of new blood vessels. In GBM, tumor angiogenesis is a multi-step process involving the proliferation, migration and differentiation of BMECs under the stimulation of specific signals derived from the cancer cells through a wide variety of communication routes. In this review, we discuss the dynamic interaction between BMECs and tumor cells by providing evidence of how tumor cells hijack the BMECs for the formation of new vessels. Tumor cell-BMECs interplay involves multiple routes of communication, including soluble factors, such as chemokines and cytokines, direct cell-cell contact and extracellular vesicles that participate in and fuel this cooperation. We also describe how this interaction is able to modify the BMECs structure, metabolism and physiology in a way that favors tumor growth and invasiveness. Finally, we briefly reviewed the recent advances and the potential future implications of some high-throughput 3D models to better understanding the complexity of BMECs-tumor cell interaction.

Phase separation in controlling meiotic chromosome dynamics

Sexually reproducing organisms produce haploid gametes through meiotic cell division, during which a single round of DNA replication is followed by two consecutive chromosome segregation. A series of meiosis-specific events take place during the meiotic prophase to ensure successful chromosome segregation. These events include programmed DNA double-strand break formation, chromosome movement driven by cytoplasmic forces, homologous pairing, synaptonemal complex installation, and inter-homolog crossover formation. Phase separation has emerged as a key principle controlling cellular biomolecular material organization and biological processes. Recent studies have revealed the involvements of phase separation in assembling meiotic chromosome-associated structures. Here we review and discuss how phase separation may participate in meiotic chromosome dynamics and propose that it may provide opportunities to understand the mysteries in meiotic regulations.

Understanding the Underlying Molecular Mechanisms of Meiotic Arrest during In Vitro Spermatogenesis in Rat Prepubertal Testicular Tissue

In vitro spermatogenesis appears to be a promising approach to restore the fertility of childhood cancer survivors. The rat model has proven to be challenging, since germ cell maturation is arrested in organotypic cultures. Here, we report that, despite a meiotic entry, abnormal synaptonemal complexes were found in spermatocytes, and in vitro matured rat prepubertal testicular tissues displayed an immature phenotype. RNA-sequencing analyses highlighted up to 600 differentially expressed genes between in vitro and in vivo conditions, including genes involved in blood-testis barrier (BTB) formation and steroidogenesis. BTB integrity, the expression of two steroidogenic enzymes, and androgen receptors were indeed altered in vitro. Moreover, most of the top 10 predicted upstream regulators of deregulated genes were involved in inflammatory processes or immune cell recruitment. However, none of the three anti-inflammatory molecules tested in this study promoted meiotic progression. By analysing for the first time in vitro matured rat prepubertal testicular tissues at the molecular level, we uncovered the deregulation of several genes and revealed that defective BTB function, altered steroidogenic pathway, and probably inflammation, could be at the origin of meiotic arrest.

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.

In silico analysis of a novel pathogenic variant c.7G > A in C14orf39 gene identified by WES in a Pakistani family with azoospermia

Infertility is a multifactorial disorder that affects approximately 12% of couples of childbearing ages worldwide. Few studies have been conducted to understand the genetic causes of infertility in depth. The synaptonemal complex (SC), which is essential for the progression of meiosis, is a conserved tripartite structure that binds homologous chromosomes together and is thus required for fertility. This study investigated genetic causes of infertility in a Pakistani consanguineous family containing two patients suffering from non-obstructive azoospermia (NOA). We performed whole-exome sequencing, followed by Sanger sequencing, and identified a novel pathogenic variant (c.7G > A [p.D3N]) in the SC coding gene C14orf39, which was recessively co-segregated with NOA. In silico analysis revealed that charges on wild-type residues were lost, which may result in loss of interactions with other molecules and residues, and a reduction in protein stability occurred, which was caused by the p.D3N mutation. The novel variant generated the mutant protein C14ORF39^D3N, and homozygous mutations in C14orf39 resulted in NOA. The transcriptome profile of C14ORF39 shows that it is specifically expressed in early brain development, which suggests that research in this area is required to study other functions of C14ORF39 in addition to its role in the germline. This research highlights the conserved role of C14orf39/SIX6OS1 in assembly of the SC and its indispensable role in facilitating genetic diagnosis in patients with infertility, which may enable the development of future treatments.

Bi-allelic variants in SHOC1 cause non-obstructive azoospermia with meiosis arrest in humans and mice

Meiosis is pivotal to gametogenesis and fertility. Meiotic recombination is a mandatory process that ensures faithful chromosome segregation and generates genetic diversity in gametes. Non-obstructive azoospermia (NOA) caused by meiotic arrest is a common cause of male infertility and has many genetic origins, including chromosome abnormalities, Y chromosome microdeletion and monogenic mutations. However, the genetic causes of the majority of NOA cases remain to be elucidated. Here, we report our findings of three Shortage in chiasmata 1 (SHOC1) bi-allelic variants in three NOA patients, of which two are homozygous for the same loss-of-function variant (c.231_232del: p. L78Sfs*9), and one is heterozygous for two different missense variants (c.1978G>A: p.A660T; c.4274G>A: p.R1425H). Testicular biopsy of one patient revealed impairment of spermatocyte maturation. Both germ-cell-specific and general Shoc1-knockout mice exhibited similar male infertility phenotypes. Subsequent analysis revealed comprehensive defects in homologous pairing and synapsis along with abnormal expression of DMC1, RAD51 and RPA2 in Shoc1-defective spermatocyte spreads. These findings imply that SHOC1 may have a presynaptic function during meiotic recombination apart from its previously identified role in crossover formation. Overall, our results provide strong evidence for the clinical relevance of SHOC1 mutations in patients with NOA and contribute to a deeper mechanistic understanding of the role of SHOC1 during meiotic recombination.

Does the Pachytene Checkpoint, a Feature of Meiosis, Filter Out Mistakes in Double-Strand DNA Break Repair and as a side-Effect Strongly Promote Adaptive Speciation?

This essay aims to explain two biological puzzles: why eukaryotic transcription units are composed of short segments of coding DNA interspersed with long stretches of non-coding (intron) DNA, and the near ubiquity of sexual reproduction. As is well known, alternative splicing of its coding sequences enables one transcription unit to produce multiple variants of each encoded protein. Additionally, padding transcription units with non-coding DNA (often many thousands of base pairs long) provides a readily evolvable way to set how soon in a cell cycle the various mRNAs will begin being expressed and the total amount of mRNA that each transcription unit can make during a cell cycle. This regulation complements control via the transcriptional promoter and facilitates the creation of complex eukaryotic cell types, tissues, and organisms. However, it also makes eukaryotes exceedingly vulnerable to double-strand DNA breaks, which end-joining break repair pathways can repair incorrectly. Transcription units cover such a large fraction of the genome that any mis-repair producing a reorganized chromosome has a high probability of destroying a gene. During meiosis, the synaptonemal complex aligns homologous chromosome pairs and the pachytene checkpoint detects, selectively arrests, and in many organisms actively destroys gamete-producing cells with chromosomes that cannot adequately synapse; this creates a filter favoring transmission to the next generation of chromosomes that retain the parental organization, while selectively culling those with interrupted transcription units. This same meiotic checkpoint, reacting to accidental chromosomal reorganizations inflicted by error-prone break repair, can, as a side effect, provide a mechanism for the formation of new species in sympatry. It has been a long-standing puzzle how something as seemingly maladaptive as hybrid sterility between such new species can arise. I suggest that this paradox is resolved by understanding the adaptive importance of the pachytene checkpoint, as outlined above.

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.

The proteome of developing barley anthers during meiotic prophase I

Flowering plants reproduce sexually by combining a haploid male and female gametophyte during fertilization. Male gametophytes are localized in the anthers, each containing reproductive (meiocyte) and non-reproductive tissue necessary for anther development and maturation. Meiosis, where chromosomes pair and exchange their genetic material during a process called recombination, is one of the most important and sensitive stages in breeding, ensuring genetic diversity. Most anther development studies have focused on transcript variation, but very few have been correlated with protein abundance. Taking advantage of a recently published barley anther transcriptomic (BAnTr) dataset and a newly developed sensitive mass spectrometry-based approach to analyse the barley anther proteome, we conducted high-resolution mass spectrometry analysis of barley anthers, collected at six time points and representing their development from pre-meiosis to metaphase. Each time point was carefully staged using immunocytology, providing a robust and accurate staging mirroring our previous BAnTr dataset. We identified >6100 non-redundant proteins including 82 known and putative meiotic proteins. Although the protein abundance was relatively stable throughout prophase I, we were able to quantify the dynamic variation of 336 proteins. We present the first quantitative comparative proteomics study of barley anther development during meiotic prophase I when the important process of homologous recombination is taking place.

The Zip4 protein directly couples meiotic crossover formation to synaptonemal complex assembly

Meiotic recombination is triggered by programmed double-strand breaks (DSBs), a subset of these being repaired as crossovers, promoted by eight evolutionarily conserved proteins, named ZMM. Crossover formation is functionally linked to synaptonemal complex (SC) assembly between homologous chromosomes, but the underlying mechanism is unknown. Here we show that Ecm11, a SC central element protein, localizes on both DSB sites and sites that attach chromatin loops to the chromosome axis, which are the starting points of SC formation, in a way that strictly requires the ZMM protein Zip4. Furthermore, Zip4 directly interacts with Ecm11, and point mutants that specifically abolish this interaction lose Ecm11 binding to chromosomes and exhibit defective SC assembly. This can be partially rescued by artificially tethering interaction-defective Ecm11 to Zip4. Mechanistically, this direct connection ensuring SC assembly from CO sites could be a way for the meiotic cell to shut down further DSB formation once enough recombination sites have been selected for crossovers, thereby preventing excess crossovers. Finally, the mammalian ortholog of Zip4, TEX11, also interacts with the SC central element TEX12, suggesting a general mechanism.

SYP-5 regulates meiotic thermotolerance in Caenorhabditis elegans

Meiosis produces the haploid gametes required by all sexually reproducing organisms, occurring in specific temperature ranges in different organisms. However, how meiotic thermotolerance is regulated remains largely unknown. Using the model organism Caenorhabditis elegans, here, we identified the synaptonemal complex (SC) protein SYP-5 as a critical regulator of meiotic thermotolerance. syp-5-null mutants maintained a high percentage of viable progeny at 20°C but produced significantly fewer viable progeny at 25°C, a permissive temperature in wild-type worms. Cytological analysis of meiotic events in the mutants revealed that while SC assembly and disassembly, as well as DNA double-strand break repair kinetics, were not affected by the elevated temperature, crossover designation, and bivalent formation were significantly affected. More severe homolog segregation errors were also observed at elevated temperature. A temperature switching assay revealed that late meiotic prophase events were not temperature-sensitive and that meiotic defects during pachytene stage were responsible for the reduced viability of syp-5 mutants at the elevated temperature. Moreover, SC polycomplex formation and hexanediol sensitivity analysis suggested that SYP-5 was required for the normal properties of the SC, and charge-interacting elements in SC components were involved in regulating meiotic thermotolerance. Together, these findings provide a novel molecular mechanism for meiotic thermotolerance regulation.

The E3 ubiquitin ligase DESYNAPSIS1 regulates synapsis and recombination in rice meiosis

Synaptonemal complex (SC) assembly and homologous recombination, the most critical events during prophase I, are the prerequisite for faithful meiotic chromosome segregation. However, the underlying regulatory mechanism remains largely unknown. Here, we reveal that a functional RING finger E3 ubiquitin ligase, DESYNAPSIS1 (DSNP1), plays significant roles in SC assembly and homologous recombination during rice meiosis. In the dsnp1 mutant, homologous synapsis is discontinuous and aberrant SC-like polycomplexes occur independent of coaligned homologous chromosomes. Accompanying the decreased foci of HEI10, ZIP4, and MER3 on meiotic chromosomes, the number of crossovers (COs) decreases dramatically in dsnp1 meiocytes. Furthermore, the absence of central elements largely restores the localization of non-ZEP1 ZMM proteins and the number of COs in the dsnp1 background. Collectively, DSNP1 stabilizes the canonical tripartite SC structure along paired homologous chromosomes and further promotes the formation of COs.

Concurrent Disruption of Genetic Interference and Increase of Genetic Recombination Frequency in Hybrid Rice Using CRISPR/Cas9

Manipulation of the distribution and frequency of meiotic recombination events to increase genetic diversity and disrupting genetic interference are long-standing goals in crop breeding. However, attenuation of genetic interference is usually accompanied by a reduction in recombination frequency and subsequent loss of plant fertility. In the present study, we generated null mutants of the ZEP1 gene, which encodes the central component of the meiotic synaptonemal complex (SC), in a hybrid rice using CRISPR/Cas9. The null mutants exhibited absolute male sterility but maintained nearly unaffected female fertility. By pollinating the zep1 null mutants with pollen from indica rice variety 93-11, we successfully conducted genetic analysis and found that genetic recombination frequency was greatly increased and genetic interference was completely eliminated in the absence of ZEP1. The findings provided direct evidence to support the controversial hypothesis that SC is involved in mediating interference. Additionally, the remained female fertility of the null mutants makes it possible to break linkage drag. Our study provides a potential approach to increase genetic diversity and fully eliminate genetic interference in rice breeding.

ESA1 regulates meiotic chromosome axis and crossover frequency via acetylating histone H4

Meiotic recombination is integrated into and regulated by meiotic chromosomes, which is organized as loop/axis architecture. However, the regulation of chromosome organization is poorly understood. Here, we show Esa1, the NuA4 complex catalytic subunit, is constitutively expressed and localizes on chromatin loops during meiosis. Esa1 plays multiple roles including homolog synapsis, sporulation efficiency, spore viability, and chromosome segregation in meiosis. Detailed analyses show the meiosis-specific depletion of Esa1 results in decreased chromosome axis length independent of another axis length regulator Pds5, which further leads to a decreased number of Mer2 foci, and consequently a decreased number of DNA double-strand breaks, recombination intermediates, and crossover frequency. However, Esa1 depletion does not impair the occurrence of the obligatory crossover required for faithful chromosome segregation, or the strength of crossover interference. Further investigations demonstrate Esa1 regulates chromosome axis length via acetylating the N-terminal tail of histone H4 but not altering transcription program. Therefore, we firstly show a non-chromosome axis component, Esa1, acetylates histone H4 on chromatin loops to regulate chromosome axis length and consequently recombination frequency but does not affect the basic meiotic recombination process. Additionally, Esa1 depletion downregulates middle induced meiotic genes, which probably causing defects in sporulation and chromosome segregation.

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.

Architecture and Dynamics of Meiotic Chromosomes

The specialized two-stage meiotic cell division program halves a cell's chromosome complement in preparation for sexual reproduction. This reduction in ploidy requires that in meiotic prophase, each pair of homologous chromosomes (homologs) identify one another and form physical links through DNA recombination. Here, we review recent advances in understanding the complex morphological changes that chromosomes undergo during meiotic prophase to promote homolog identification and crossing over. We focus on the structural maintenance of chromosomes (SMC) family cohesin complexes and the meiotic chromosome axis, which together organize chromosomes and promote recombination. We then discuss the architecture and dynamics of the conserved synaptonemal complex (SC), which assembles between homologs and mediates local and global feedback to ensure high fidelity in meiotic recombination. Finally, we discuss exciting new advances, including mechanisms for boosting recombination on particular chromosomes or chromosomal domains and the implications of a new liquid crystal model for SC assembly and structure. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Epigenetic Dysregulation of Mammalian Male Meiosis Caused by Interference of Recombination and Synapsis

Meiosis involves a series of specific chromosome events, namely homologous synapsis, recombination, and segregation. Disruption of either recombination or synapsis in mammals results in the interruption of meiosis progression during the first meiotic prophase. This is usually accompanied by a defective transcriptional inactivation of the X and Y chromosomes, which triggers a meiosis breakdown in many mutant models. However, epigenetic changes and transcriptional regulation are also expected to affect autosomes. In this work, we studied the dynamics of epigenetic markers related to chromatin silencing, transcriptional regulation, and meiotic sex chromosome inactivation throughout meiosis in knockout mice for genes encoding for recombination proteins SPO11, DMC1, HOP2 and MLH1, and the synaptonemal complex proteins SYCP1 and SYCP3. These models are defective in recombination and/or synapsis and promote apoptosis at different stages of progression. Our results indicate that impairment of recombination and synapsis alter the dynamics and localization pattern of epigenetic marks, as well as the transcriptional regulation of both autosomes and sex chromosomes throughout prophase-I progression. We also observed that the morphological progression of spermatocytes throughout meiosis and the dynamics of epigenetic marks are processes that can be desynchronized upon synapsis or recombination alteration. Moreover, we detected an overlap of early and late epigenetic signatures in most mutants, indicating that the normal epigenetic transitions are disrupted. This can alter the transcriptional shift that occurs in spermatocytes in mid prophase-I and suggest that the epigenetic regulation of sex chromosomes, but also of autosomes, is an important factor in the impairment of meiosis progression in mammals.

The Zebrafish Meiotic Cohesin Complex Protein Smc1b Is Required for Key Events in Meiotic Prophase I

The eukaryotic structural maintenance of chromosomes (SMC) proteins are involved in key processes of chromosome structure and dynamics. SMC1β was identified as a component of the meiotic cohesin complex in vertebrates, which aids in keeping sister chromatids together prior to segregation in meiosis II and is involved in association of homologous chromosomes in meiosis I. The role of SMC1β in meiosis has primarily been studied in mice, where mutant male and female mice are infertile due to germ cell arrest at pachytene and metaphase II stages, respectively. Here, we investigate the function of zebrafish Smc1b to understand the role of this protein more broadly in vertebrates. We found that zebrafish smc1b is necessary for fertility and has important roles in meiosis, yet has no other apparent roles in development. Therefore, smc1b functions primarily in meiosis in both fish and mammals. In zebrafish, we showed that smc1b mutant spermatocytes initiated telomere clustering in leptotene, but failed to complete this process and progress into zygotene. Furthermore, mutant spermatocytes displayed a complete failure of synapsis between homologous chromosomes and homolog pairing only occurred at chromosome ends. Interestingly, meiotic DNA double strand breaks occurred in the absence of Smc1b despite failed pairing and synapsis. Overall, our findings point to an essential role of Smc1b in the leptotene to zygotene transition during zebrafish spermatogenesis. In addition, ovarian follicles failed to form in smc1b mutants, suggesting an essential role in female meiosis as well. Our results indicate that there are some key differences in Smc1b requirement in meiosis among vertebrates: while Smc1b is not required for homolog pairing and synapsis in mice, it is essential for these processes in zebrafish.

Meiotic Cohesin and Variants Associated With Human Reproductive Aging and Disease

Successful human reproduction relies on the well-orchestrated development of competent gametes through the process of meiosis. The loading of cohesin, a multi-protein complex, is a key event in the initiation of mammalian meiosis. Establishment of sister chromatid cohesion via cohesin rings is essential for ensuring homologous recombination-mediated DNA repair and future proper chromosome segregation. Cohesin proteins loaded during female fetal life are not replenished over time, and therefore are a potential etiology of age-related aneuploidy in oocytes resulting in decreased fecundity and increased infertility and miscarriage rates with advancing maternal age. Herein, we provide a brief overview of meiotic cohesin and summarize the human genetic studies which have identified genetic variants of cohesin proteins and the associated reproductive phenotypes including primary ovarian insufficiency, trisomy in offspring, and non-obstructive azoospermia. The association of cohesion defects with cancer predisposition and potential impact on aging are also described. Expansion of genetic testing within clinical medicine, with a focus on cohesin protein-related genes, may provide additional insight to previously unknown etiologies of disorders contributing to gamete exhaustion in females, and infertility and reproductive aging in both men and women.

PPP2R1B is modulated by ubiquitination and is essential for spermatogenesis

The serine-threonine protein phosphatase 2A (PP2A) is a heterotrimeric enzyme complex that regulates many fundamental cellular processes. PP2A is involved in tumorigenesis because mutations in the scaffold subunit, PPP2R1B, were found in several types of cancers. However, the biological function of PPP2R1B remains largely unknown. We report here that homozygous deletion of Ppp2r1b in Mus musculus impairs meiotic recombination and causes meiotic arrest in spermatocytes. Consistently, male mice lacking Ppp2r1b are characterized with infertility. Furthermore, heterozygous missense mutations in the Homo sapiens PPP2R1B gene, which encodes PPP2R1B, are identified in azoospermia patients with meiotic arrest. We found that PPP2R1B mutants are susceptible to degradation by an E3 ligase CRL4A^DCAF6 , and resistant to de-polyubiquitylation by ubiquitin-specific protease 5 (USP5). In addition, heterozygous mutations in PPP2R1B reduce stability of the wild-type PPP2R1B. Our results demonstrate an essential role of PPP2R1B in spermatogenesis and identify upstream regulators of PPP2R1B.

Let's get physical - mechanisms of crossover interference

The formation of crossovers between homologous chromosomes is key to sexual reproduction. In most species, crossovers are spaced further apart than would be expected if they formed independently, a phenomenon termed crossover interference. Despite more than a century of study, the molecular mechanisms implementing crossover interference remain a subject of active debate. Recent findings of how signaling proteins control the formation of crossovers and about the interchromosomal interface in which crossovers form offer new insights into this process. In this Review, we present a cell biological and biophysical perspective on crossover interference, summarizing the evidence that links interference to the spatial, dynamic, mechanical and molecular properties of meiotic chromosomes. We synthesize this physical understanding in the context of prevailing mechanistic models that aim to explain how crossover interference is implemented.

Regulation of Meiotic Prophase One in Mammalian Oocytes

In female mammals, meiotic prophase one begins during fetal development. Oocytes transition through the prophase one substages consisting of leptotene, zygotene, and pachytene, and are finally arrested at the diplotene substage, for months in mice and years in humans. After puberty, luteinizing hormone induces ovulation and meiotic resumption in a cohort of oocytes, driving the progression from meiotic prophase one to metaphase two. If fertilization occurs, the oocyte completes meiosis two followed by fusion with the sperm nucleus and preparation for zygotic divisions; otherwise, it is passed into the uterus and degenerates. Specifically in the mouse, oocytes enter meiosis at 13.5 days post coitum. As meiotic prophase one proceeds, chromosomes find their homologous partner, synapse, exchange genetic material between homologs and then begin to separate, remaining connected at recombination sites. At postnatal day 5, most of the oocytes have reached the late diplotene (or dictyate) substage of prophase one where they remain arrested until ovulation. This review focuses on events and mechanisms controlling the progression through meiotic prophase one, which include recombination, synapsis and control by signaling pathways. These events are prerequisites for proper chromosome segregation in meiotic divisions; and if they go awry, chromosomes mis-segregate resulting in aneuploidy. Therefore, elucidating the mechanisms regulating meiotic progression is important to provide a foundation for developing improved treatments of female infertility.

Phospho-Regulation of Meiotic Prophase

Germ cells undergoing meiosis rely on an intricate network of surveillance mechanisms that govern the production of euploid gametes for successful sexual reproduction. These surveillance mechanisms are particularly crucial during meiotic prophase, when cells execute a highly orchestrated program of chromosome morphogenesis and recombination, which must be integrated with the meiotic cell division machinery to ensure the safe execution of meiosis. Dynamic protein phosphorylation, controlled by kinases and phosphatases, has emerged as one of the main signaling routes for providing readout and regulation of chromosomal and cellular behavior throughout meiotic prophase. In this review, we discuss common principles and provide detailed examples of how these phosphorylation events are employed to ensure faithful passage of chromosomes from one generation to the next.

Synaptonemal complex

Cathleen Lake and Scott Hawley discuss the components, assembly and functional importance of the synaptonemal complex.