Purification, culture, and fractionation of spermatogenic cells

Human-specific epigenomic states in spermatogenesis

Infertility is becoming increasingly common, affecting one in six people globally. Half of these cases can be attributed to male factors, many driven by abnormalities in the process of sperm development. Emerging evidence from genome-wide association studies, genetic screening of patient cohorts, and animal models highlights an important genetic contribution to spermatogenic defects, but comprehensive identification and characterization of the genes critical for male fertility remain lacking. High divergence of gene regulation in spermatogenic cells across species poses challenges for delineating the genetic pathways required for human spermatogenesis using common model organisms. In this study, we leveraged post-translational histone modification and gene transcription data for 15,491 genes in four mammalian species (human, rhesus macaque, mouse, and opossum), to identify human-specific patterns of gene regulation during spermatogenesis. We combined H3K27me3 ChIP-seq, H3K4me3 ChIP-seq, and RNA-seq data to define epigenetic states for each gene at two stages of spermatogenesis, pachytene spermatocytes and round spermatids, in each species. We identified 239 genes that are uniquely active, poised, or dynamically regulated in human spermatogenic cells distinct from the other three species. While some of these genes have been implicated in reproductive functions, many more have not yet been associated with human infertility and may be candidates for further molecular and epidemiologic studies. Our analysis offers an example of the opportunities provided by evolutionary and epigenomic data for broadly screening candidate genes implicated in reproduction, which might lead to discoveries of novel genetic targets for diagnosis and management of male infertility and male contraception.

The landscape of RNA-binding proteins in mammalian spermatogenesis

Despite continuous expansion of the RNA-binding protein (RBP) world, there is a lack of systematic understanding of RBPs in mammalian testis, which harbors one of the most complex tissue transcriptomes. We adapted RNA interactome capture to mouse male germ cells, building an RBP atlas characterized by multiple layers of dynamics along spermatogenesis. Trapping of RNA-crosslinked peptides showed that the glutamic acid-arginine (ER) patch, a residue-coevolved polyampholytic element present in coiled-coils, enhances RNA binding of its host RBPs. Deletion of this element in NONO (non-POU domain-containing octamer-binding protein) led to a defective mitosis-to-meiosis transition due to compromised NONO-RNA interactions. Whole-exome sequencing of over 1000 infertile men revealed a prominent role of RBPs in the human genetic architecture of male infertility and identified risk ER patch variants.

ARID1A governs the silencing of sex-linked transcription during male meiosis in the mouse

We present evidence implicating the BAF (BRG1/BRM Associated Factor) chromatin remodeler in meiotic sex chromosome inactivation (MSCI). By immunofluorescence (IF), the putative BAF DNA binding subunit, ARID1A (AT-rich Interaction Domain 1a), appeared enriched on the male sex chromosomes during diplonema of meiosis I. Those germ cells showing a Cre-induced loss of ARID1A were arrested in pachynema and failed to repress sex-linked genes, indicating a defective MSCI. Consistent with this defect, mutant sex chromosomes displayed an abnormal presence of elongating RNA polymerase II coupled with an overall increase in chromatin accessibility detectable by ATAC-seq. By investigating potential mechanisms underlying these anomalies, we identified a role for ARID1A in promoting the preferential enrichment of the histone variant, H3.3, on the sex chromosomes, a known hallmark of MSCI. Without ARID1A, the sex chromosomes appeared depleted of H3.3 at levels resembling autosomes. Higher resolution analyses by CUT&RUN revealed shifts in sex-linked H3.3 associations from discrete intergenic sites and broader gene-body domains to promoters in response to the loss of ARID1A. Several sex-linked sites displayed ectopic H3.3 occupancy that did not co-localize with DMC1 (DNA Meiotic Recombinase 1). This observation suggests a requirement for ARID1A in DMC1 localization to the asynapsed sex chromatids. We conclude that ARID1A-directed H3.3 localization influences meiotic sex chromosome gene regulation and DNA repair.

Stage-specific regulation of undifferentiated spermatogonia by AKT1S1-mediated AKT-mTORC1 signaling during mouse spermatogenesis

Undifferentiated spermatogonia are composed of a heterogeneous cell population including spermatogonial stem cells (SSCs). Molecular mechanisms underlying the regulation of various spermatogonial cohorts during their self-renewal and differentiation are largely unclear. Here we show that AKT1S1, an AKT substrate and inhibitor of mTORC1, regulates the homeostasis of undifferentiated spermatogonia. Although deletion of Akt1s1 in mouse appears not grossly affecting steady-state spermatogenesis and male mice are fertile, the subset of differentiation-primed OCT4+ spermatogonia decreased significantly, whereas self-renewing GFRα1+ and proliferating PLZF+ spermatogonia were sustained. Both neonatal prospermatogonia and the first wave spermatogenesis were greatly reduced in Akt1s1-/- mice. Further analyses suggest that OCT4+ spermatogonia in Akt1s1-/- mice possess altered PI3K/AKT-mTORC1 signaling, gene expression and carbohydrate metabolism, leading to their functionally compromised developmental potential. Collectively, these results revealed an important role of AKT1S1 in mediating the stage-specific signals that regulate the self-renewal and differentiation of spermatogonia during mouse spermatogenesis.

FBXO24 modulates mRNA alternative splicing and MIWI degradation and is required for normal sperm formation and male fertility

Spermiogenesis is a critical, post-meiotic phase of male gametogenesis, in which the proper gene expression is essential for sperm maturation. However, the underFlying molecular mechanism that controls mRNA expression in the round spermatids remains elusive. Here, we identify that FBXO24, an orphan F-box protein, is highly expressed in the testis of humans and mice and interacts with the splicing factors (SRSF2, SRSF3, and SRSF9) to modulate the gene alternative splicing in the round spermatids. Genetic mutation of FBXO24 in mice causes many abnormal splicing events in round spermatids, thus affecting a large number of critical genes related to sperm formation that were dysregulated. Further molecular and phenotypical analyses revealed that FBXO24 deficiency results in aberrant histone retention, incomplete axonemes, oversized chromatoid body, and abnormal mitochondrial coiling along sperm flagella, ultimately leading to male sterility. In addition, we discovered that FBXO24 interacts with MIWI and SCF subunits and mediates the degradation of MIWI via K48-linked polyubiquitination. Furthermore, we show that FBXO24 depletion could lead to aberrant piRNA production in testes, which suggests FBXO24 is required for normal piRNA counts. Collectively, these data demonstrate that FBXO24 is essential for sperm formation by regulating mRNA alternative splicing and MIWI degradation during spermiogenesis.

KDM6A/UTX promotes spermatogenic gene expression across generations and is not required for male fertility†

Paternal chromatin undergoes extensive structural and epigenetic changes during mammalian spermatogenesis, producing sperm with an epigenome optimized for the transition to embryogenesis. Lysine demethylase 6a (KDM6A, also called UTX) promotes gene activation in part via demethylation of H3K27me3, a developmentally important repressive modification abundant throughout the epigenome of spermatogenic cells and sperm. We previously demonstrated increased cancer risk in genetically wild-type mice derived from a paternal germ line lacking Kdm6a (Kdm6a cKO), indicating a role for KDM6A in regulating heritable epigenetic states. However, the regulatory function of KDM6A during spermatogenesis is not known. Here, we show that Kdm6a is transiently expressed in spermatogenesis, with RNA and protein expression largely limited to late spermatogonia and early meiotic prophase. Kdm6a cKO males do not have defects in fertility or the overall progression of spermatogenesis. However, hundreds of genes are deregulated upon loss of Kdm6a in spermatogenic cells, with a strong bias toward downregulation coinciding with the time when Kdm6a is expressed. Misregulated genes encode factors involved in chromatin organization and regulation of repetitive elements, and a subset of these genes was persistently deregulated in the male germ line across two generations of offspring of Kdm6a cKO males. Genome-wide epigenetic profiling revealed broadening of H3K27me3 peaks in differentiating spermatogonia of Kdm6a cKO mice, suggesting that KDM6A demarcates H3K27me3 domains in the male germ line. Our findings highlight KDM6A as a transcriptional activator in the mammalian male germ line that is dispensable for spermatogenesis but important for safeguarding gene regulatory state intergenerationally.

Transcriptome analysis of meiotic and post-meiotic spermatogenic cells reveals the potential hub genes of aging on the decline of male fertility

Genetic and epigenetic changes in sperm caused by male aging may be essential factors affecting semen parameters, but the effects and specific molecular mechanisms of aging on male reproduction have not been fully clarified. In this study, to explore the effect of aging on male fertility and seek the potential molecular etiology, we performed high-throughput RNA-sequencing in isolated spermatogenic cells, including pachytene spermatocytes (marked by the completion of chromosome synapsis) and round spermatids (produced by the separation of sister chromatids) from the elderly and the young men. Functional enrichment analysis of differentially expressed genes (DEGs) in round spermatids between the elderly and young showed that they were significantly enriched in gamete generation, spindle assembly, and cilium movement involved in cell motility. In addition, the expression levels of DEGs in round spermatids (post-meiotic cells) were found to be more susceptible to age. Furthermore, ten genes (AURKA, CCNB1, CDC20, CCNB2, KIF2C, KIAA0101, NR5A1, PLK1, PTTG1, RAD51AP1) were identified to be the hub genes involved in the regulation of sperm quality in the elderly through Protein-Protein Interaction (PPI) network construction and measuring semantic among GO terms and gene products. Our data provide aging-related molecular alterations in meiotic and post-meiotic spermatogenic cells, and the information gained from this study may explain the abnormal aging-related male fertility decline.

Efficient isolation and purification of spermatogonia, spermatocytes, and spermatids from mice, piglets, and adult boars using an optimized STA-PUT method

Spermatogenesis is a delicate and complex biological process in which spermatogonial stem cells continue to proliferate and differentiate into mature spermatozoa, maintaining sperm production in male mammals throughout the lifetime. To study the molecular mechanism of spermatogenesis, researchers had to isolate different germ cell subpopulations for in vitro culture and characterization. However, due to the existence of several stages of germ cells and a variety of populations of somatic cells in the testis of male mammals, it is a challenge for us to obtain high-purity germ cell subpopulations for further research. Here, we optimized the STA-PUT device and successfully applied it to isolate and purify spermatogonia populations in piglets, and multiple germ cell populations at different developmental stages in testes of adult mice and boars. This work provides a simple platform for germ cell fractionation to facilitate the molecular mechanistic study of animal spermatogenesis in vitro.

Dynamic regulatory phosphorylation of mouse CDK2 occurs during meiotic prophase I

During prophase I of meiosis, DNA double-strand breaks form throughout the genome, with a subset repairing as crossover events, enabling the accurate segregation of homologous chromosomes during the first meiotic division. The mechanism by which DSBs become selected to repair as crossovers is unknown, although the crossover positioning and levels in each cell indicate it is a highly regulated process. One of the proteins that localises to crossover sites is the serine/threonine cyclin-dependent kinase CDK2. Regulation of CDK2 occurs via phosphorylation at tyrosine 15 (Y15) and threonine 160 (T160) inhibiting and activating the kinase, respectively. In this study we use a combination of immunofluorescence staining on spread spermatocytes and fixed testis sections, and STA-PUT gravitational sedimentation to isolate cells at different developmental stages to further investigate the temporal phospho regulation of CDK2 during prophase I. Western blotting reveals differential levels of the two CDK2 isoforms (CDK233kDa and CDK239kDa) throughout prophase I, with inhibitory phosphorylation of CDK2 at Y15 occurring early in prophase I, localising to telomeres and diminishing as cells enter pachynema. Conversely, the activatory phosphorylation on T160 occurs later, specifically the CDK233kDa isoform, and T160 signal is detected in spermatogonia and pachytene spermatocytes, where it co-localises with the Class I crossover protein MLH3. Taken together, our data reveals intricate control of CDK2 both with regards to levels of the two CDK2 isoforms, and differential regulation via inhibitory and activatory phosphorylation.

Sequencing of the ZMYND15 gene in a cohort of infertile Chinese men reveals novel mutations in patients with teratozoospermia

BACKGROUND

The information of ZMYND15 in human reproduction is very limited, resulting in the unclear link between ZMYND15 variants and male infertility.

METHODS

Whole exome sequencing and Sanger sequencing to identify the potential pathogenic variation of ZMYND15 in infertile men, Papanicolaou staining and electron microscopy to investigate the spermatozoa morphology, western blotting and immunofluorescence staining to confirm the pathogenicity of the identified variants, and proteomic analysis and coimmunoprecipitation to clarify the potential molecular mechanism.

RESULTS

A total of 31 ZMYND15 variants were identified in 227 infertile patients. Three deleterious biallelic variants, including a novel compound heterozygous variant of c.1105delG (p.A369Qfs*15) and c.1853T>C (p.F618S), a new homozygous splicing mutation of c.1297+5G>A and a reported homozygous nonsense mutation of c.1209T>A (p.Y403*), were detected in three affected individuals with oligoasthenoteratozoospermia, showing a biallelic pathogenic mutation frequency of 1.3% (3/227). No biallelic pathogenic mutation was found in 692 fertile men. Morphology analysis showed abnormalities in sperm morphology in the patients harbouring ZMYND15 mutations. Western blotting and immunofluorescence staining confirmed the nearly absent ZMYND15 expression in the sperm of the patients. Mechanistically, ZMYND15 might regulate spermatogenesis by interacting with key molecules involved in sperm development, such as DPY19L2, AKAP4 and FSIP2, and might also mediate the expression of the autophagy-associated protein SPATA33 to maintain sperm individualisation and unnecessary cytoplasm removal.

CONCLUSION

Our findings broaden the variant and phenotype spectrum of ZMYND15 in male infertility, and reveal the potential signalling pathway of ZMYND15 regulating spermatogenesis, finally confirming the essential role of ZMYND15 in human fertility.

FSIP2 plays a role in the acrosome development during spermiogenesis

BACKGROUND

Loss-of-function mutations in FSIP2 result in multiple morphological abnormalities of the flagella in humans and mice. Intriguingly, a recent study found that FSIP2 might regulate the expression of acrosomal proteins, indicating that Fsip2 might be involved in acrosome development in mice. However, whether FSIP2 also function in acrosome biogenesis in humans is largely unknown, and the underlying mechanism of which is unexplored.

OBJECTIVE

Our objective was to reveal potential function of FSIP2 in regulating sperm acrosome formation.

METHODS

We performed whole exome sequencing on four asthenoteratozoospermic patients. Western blot analysis and immunofluorescence staining were conducted to assess the protein expression of FSIP2. Proteomics approach, liquid chromatography-tandem mass spectrometry and co-immunoprecipitation were implemented to clarify the molecules in acrosome biogenesis regulated by FSIP2.

RESULTS

Biallelic FSIP2 variants were identified in four asthenoteratozoospermic individuals. The protein expression of MUT-FSIP2 was sharply decreased or absent in vitro or in vivo. Interestingly, aside from the sperm flagellar defects, the acrosomal hypoplasia was detected in numerous sperm from the four patients. FSIP2 co-localised with peanut agglutinin in the acrosome during spermatogenesis. Moreover, FSIP2 interacted with proteins (DPY19L2, SPACA1, HSP90B1, KIAA1210, HSPA2 and CLTC) involved in acrosome biogenesis. In addition, spermatozoa from patients carrying FSIP2 mutations showed downregulated expression of DPY19L2, ZPBP, SPACA1, CCDC62, CCIN, SPINK2 and CSNK2A2.

CONCLUSION

Our findings unveil that FSIP2 might involve in sperm acrosome development, and consequently, its mutations might contribute to globozoospermia or acrosomal aplasia. We meanwhile first uncover the potential molecular mechanism of FSIP2 regulating acrosome biogenesis.

Decoding the Spermatogenesis Program: New Insights from Transcriptomic Analyses

Spermatogenesis is a complex differentiation process coordinated spatiotemporally across and along seminiferous tubules. Cellular heterogeneity has made it challenging to obtain stage-specific molecular profiles of germ and somatic cells using bulk transcriptomic analyses. This has limited our ability to understand regulation of spermatogenesis and to integrate knowledge from model organisms to humans. The recent advancement of single-cell RNA-sequencing (scRNA-seq) technologies provides insights into the cell type diversity and molecular signatures in the testis. Fine-grained cell atlases of the testis contain both known and novel cell types and define the functional states along the germ cell developmental trajectory in many species. These atlases provide a reference system for integrated interspecies comparisons to discover mechanistic parallels and to enable future studies. Despite recent advances, we currently lack high-resolution data to probe germ cell-somatic cell interactions in the tissue environment, but the use of highly multiplexed spatial analysis technologies has begun to resolve this problem. Taken together, recent single-cell studies provide an improvedunderstanding of gametogenesis to examine underlying causes of infertility and enable the development of new therapeutic interventions.

Widespread association of the Argonaute protein AGO2 with meiotic chromatin suggests a distinct nuclear function in mammalian male reproduction

Argonaute 2 (AGO2) is a ubiquitously expressed protein critical for regulation of mRNA translation and vital to animal development. AGO2 protein is found in both cytoplasmic and nuclear compartments, and although its cytoplasmic role is well studied, the biological relevance of nuclear AGO2 is unclear. Here, we address this problem in vivo using spermatogenic cells as a model. We find that AGO2 transiently binds both chromatin and nucleus-specific mRNA transcripts of hundreds of genes required for sperm production during male meiosis in mice, and that germline conditional knockout (cKO) of Ago2 causes depletion of the encoded proteins. Correspondingly, Ago2 cKO males show abnormal sperm head morphology and reduced sperm count, along with reduced postnatal viability of offspring. Together, our data reveal an unexpected nuclear role for AGO2 in enhancing expression of developmentally important genes during mammalian male reproduction.

Global phosphoproteomic analysis identified key kinases regulating male meiosis in mouse

Meiosis, a highly conserved process in organisms from fungi to mammals, is subjected to protein phosphorylation regulation. Due to the low abundance of phosphorylation, there is a lack of systemic characterization of phosphorylation regulation of meiosis in mammals. Using the phosphoproteomic approach, we profiled large-scale phosphoproteome of purified primary spermatocytes undergoing meiosis I, and identified 14,660 phosphorylation sites in 4419 phosphoproteins. Kinase-substrate phosphorylation network analysis followed by in vitro meiosis study showed that CDK9 was essential for meiosis progression to metaphase I and had enriched substrate phosphorylation sites in proteins involved in meiotic cell cycle. In addition, histones and epigenetic factors were found to be widely phosphorylated. Among those, HASPIN was found to be essential for male fertility. Haspin knockout led to misalignment of chromosomes, apoptosis of metaphase spermatocytes and a decreased number of sperm by deregulation of H3T3ph, chromosomal passenger complex (CPC) and spindle assembly checkpoint (SAC). The complicated protein phosphorylation and its important regulatory functions in meiosis indicated that in-depth studies of phosphorylation-mediated signaling could help us elucidate the mechanisms of meiosis.

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

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

Protocol for isolation and proteostatic analysis of sub-populations of spermatogenic cells in mouse

Spermatogenesis generates heterologous cell populations which, if not distinguished clearly, often hinder mechanistic and etiological studies. Here, we present a protocol to identify and isolate populations of mouse spermatogenic cells, including spermatogonial stem cells (SSCs), spermatocytes, and haploid spermatids. We also describe absolute quantification of mRNA copy numbers in SSCs. The isolated cells can be used for analyzing nascent protein synthesis and protein degradation, two main events that maintain cellular proteostasis important for healthy and long-term production of male gametes. For complete details on the use and execution of this protocol, please refer to Zou et al. (2021).

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.

Citrylglutamate synthase deficient male mice are subfertile with impaired histone and transition protein 2 removal in late spermatids

Chromatin remodelling in spermatids is an essential step in spermiogenesis and involves the exchange of most histones by protamines, which drives chromatin condensation in late spermatids. The gene Rimklb encodes a citrylglutamate synthase highly expressed in testes of vertebrates and the increase of its reaction product, β-citrylglutamate, correlates in time with the appearance of spermatids. Here we show that deficiency in a functional Rimklb gene leads to male subfertility, which could be partially rescued by in vitro fertilization. Rimklb-deficient mice are impaired in a late step of spermiogenesis and produce spermatozoa with abnormally shaped heads and nuclei. Sperm chromatin in Rimklb-deficient mice was less condensed and showed impaired histone to protamine exchange and retained transition protein 2. These observations suggest that citrylglutamate synthase, probably via its reaction product β-citrylglutamate, is essential for efficient chromatin remodelling during spermiogenesis and may be a possible candidate gene for male subfertility or infertility in humans.

CEP128 is involved in spermatogenesis in humans and mice

Centrosomal proteins are necessary components of the centrosome, a conserved eukaryotic organelle essential to the reproductive process. However, few centrosomal proteins have been genetically linked to fertility. Herein we identify a homozygous missense variant of CEP128 (c.665 G > A [p.R222Q]) in two infertile males. Remarkably, male homozygous knock-in mice harboring the orthologous CEP128^R222Q variant show anomalies in sperm morphology, count, and motility. Moreover, Cep128 knock-out mice manifest male infertility associated with disrupted sperm quality. We observe defective sperm flagella in both homozygous Cep128 KO and KI mice; the cilia development in other organs is normal—suggesting that CEP128 variants predominantly affected the ciliogenesis in the testes. Mechanistically, CEP128 is involved in male reproduction via regulating the expression of genes and/or the phosphorylation of TGF-β/BMP-signalling members during spermatogenesis. Altogether, our findings unveil a crucial role for CEP128 in male fertility and provide important insights into the functions of centrosomal proteins in reproductive biology.

CEP128 is a centrosomal protein important for the organization of centriolar microtubules. Here, the authors show that a CEP128 variant observed in human male siblings causes reduced sperm counts and morphologically abnormal sperm when modeled in mice, suggesting a role for CEP128 in male fertility.

Offspring production of haploid spermatid-like cells derived from mouse female germline stem cells with chromatin condensation

Background

During male meiosis, the Y chromosome can form perfect pairing with the X chromosome. However, it is unclear whether mammalian Female germline stem cells (FGSCs) without a Y chromosome can transdifferentiate into functional haploid spermatid-like cells (SLCs).

Results

We found that spermatogenesis was restarted by transplanting FGSCs into Kit^w/wv mutant testes. Complete meiosis and formation of SLCs was induced in vitro by testicular cells of Kit^w/wv mutant mice, cytokines and retinoic acid. Healthy offspring were produced by sperm and SLCs derived from the in vivo and in vitro transdifferentiation of FGSCs, respectively. Furthermore, high-throughput chromosome conformation capture sequencing(Hi-C-seq) and “bivalent” (H3K4me3-H3K27me3) micro chromatin immunoprecipitation sequencing (μChIP-seq) experiments showed that stimulated by retinoic acid gene 8 (STRA8)/protamine 1 (PRM1)-positive transdifferentiated germ cells (tGCs) and male germ cells (mGCs) display similar chromatin dynamics and chromatin condensation during in vitro spermatogenesis.

Conclusion

This study demonstrates that sperm can be produced from FGSCs without a Y chromosome. This suggests a strategy for dairy cattle breeding to produce only female offspring with a high-quality genetic background.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13578-021-00697-z.

Proteostasis regulated by testis-specific ribosomal protein RPL39L maintains mouse spermatogenesis

Maintaining proteostasis is important for animal development. How proteostasis influences spermatogenesis that generates male gametes, spermatozoa, is not clear. We show that testis-specific paralog of ribosomal large subunit protein RPL39, RPL39L, is required for mouse spermatogenesis. Deletion of Rpl39l in mouse caused reduced proliferation of spermatogonial stem cells, malformed sperm mitochondria and flagella, leading to sub-fertility in males. Biochemical analyses revealed that lack of RPL39L deteriorated protein synthesis and protein quality control in spermatogenic cells, partly due to reduced biogenesis of ribosomal subunits and ribosome homeostasis. RPL39/RPL39L is likely assembled into ribosomes via H/ACA domain containing NOP10 complex early in ribosome biogenesis pathway. Furthermore, Rpl39l null mice exhibited compromised regenerative spermatogenesis after chemical insult and early degenerative spermatogenesis in aging mice. These data demonstrate that maintaining proteostasis is important for spermatogenesis, of which ribosome homeostasis maintained by ribosomal proteins coordinates translation machinery to the regulation of cellular growth.

•

Rpl39l deletion causes reduced spermatogenesis and subfertility in male mice

•

SSC proliferation, mitochondria and sperm flagella compromised in Rpl39l^–/– mice

•

Rpl39l deletion affects ribosomal LSU formation and protein quality control

•

Aberrant proteostasis affects spermatogenesis and regeneration

YTHDC2 is essential for pachytene progression and prevents aberrant microtubule-driven telomere clustering in male meiosis

Mechanisms driving the prolonged meiotic prophase I in mammals are poorly understood. RNA helicase YTHDC2 is critical for mitosis to meiosis transition. However, YTHDC2 is highly expressed in pachytene cells. Here we identify an essential role for YTHDC2 in meiotic progression. Specifically, YTHDC2 deficiency causes microtubule-dependent telomere clustering and apoptosis at the pachytene stage of prophase I. Depletion of YTHDC2 results in a massively dysregulated transcriptome in pachytene cells, with a tendency toward upregulation of genes normally expressed in mitotic germ cells and downregulation of meiotic transcripts. Dysregulation does not correlate with m⁶A status, and YTHDC2-bound mRNAs are enriched in genes upregulated in mutant germ cells, revealing that YTHDC2 primarily targets mRNAs for degradation. Furthermore, altered transcripts in mutant pachytene cells encode microtubule network proteins. Our results demonstrate that YTHDC2 regulates the pachytene stage by perpetuating a meiotic transcriptome and preventing microtubule network changes that could lead to telomere clustering.

Neuron-specific chromosomal megadomain organization is adaptive to recent retrotransposon expansions

Regulatory mechanisms associated with repeat-rich sequences and chromosomal conformations in mature neurons remain unexplored. Here, we map cell-type specific chromatin domain organization in adult mouse cerebral cortex and report strong enrichment of Endogenous Retrovirus 2 (ERV2) repeat sequences in the neuron-specific heterochromatic B₂^NeuN+ megabase-scaling subcompartment. Single molecule long-read sequencing and comparative Hi-C chromosomal contact mapping in wild-derived SPRET/EiJ (Mus spretus) and laboratory inbred C57BL/6J (Mus musculus) reveal neuronal reconfigurations tracking recent ERV2 expansions in the murine germline, with significantly higher B₂^NeuN+ contact frequencies at sites with ongoing insertions in Mus musculus. Neuronal ablation of the retrotransposon silencer Kmt1e/Setdb1 triggers B₂^NeuN+ disintegration and rewiring with open chromatin domains enriched for cellular stress response genes, along with severe neuroinflammation and proviral assembly with infiltration of dendrites . We conclude that neuronal megabase-scale chromosomal architectures include an evolutionarily adaptive heterochromatic organization which, upon perturbation, results in transcriptional dysregulation and unleashes ERV2 proviruses with strong neuronal tropism.

ZDHHC19 localizes to the cell membrane of spermatids and is involved in spermatogenesis

Sperm is the ultimate executor of male reproductive function. Normal morphology, quantity, and motility of sperm ensure the normal reproductive process. Palmitoylation is a posttranslational modification mediated by palmitoyltransferases whereby palmitoyl is added to proteins. Seven palmitoyltransferases have been identified in Saccharomyces cerevisiae and 23 in humans (including ZDHHC1-9 and ZDHHC11-24), with corresponding homologs in mice. We identified two testis-specific palmitoyltransferases ZDHHC11 and ZDHHC19 in mice. The Zdhhc11 and Zdhhc19-knockout mouse models were constructed, and it was found that the Zdhhc11 knockout males were fertile, while Zdhhc19 knockout males were sterile. ZDHHC19 is located in the cell membrane of step 4-9 spermatids in the mouse testis, and phenotypic analysis showed that the testicular weight ratio in the Zdhhc19-/- mice decreased along with the number and motility of the sperm decreased, while sperm abnormalities increased, mainly due to the "folded" abnormal sperm caused by sperm membrane fusion, suggesting the involvement of ZDHHC19 in maintaining membrane stability in the male reproductive system. In addition, Zdhhc19-/- mice showed abnormal sperm morphologies and apoptosis during spermatogenesis, suggesting that spermatogenesis in the Zdhhc19-/- mice was abnormal. These results indicate that ZDHHC19 promotes membrane stability in male germ cells. Summary sentence: ZDHHC19 is located in the cell membrane of Step4-9 spermatids in mouse testis; Zdhhc19 knockout mice showed male infertility, abnormal spermatogenesis, sperm morphology and motility.

A Novel Meiosis-Related lncRNA, Rbakdn, Contributes to Spermatogenesis by Stabilizing Ptbp2

Spermatocyte meiosis is the cornerstone of mammalian production. Thousands of long noncoding RNAs (lncRNAs) have been reported to be functional in various cellular processes, but the function of lncRNAs in meiosis remains largely unknown. Here, we profiled lncRNAs in spermatocytes at stage I of meiosis and identified a testis-specific lncRNA, Rbakdn, as a vital regulator of meiosis. Rbakdn is dynamically expressed during meiosis I, and Rbakdn knockdown inhibits meiosis in vitro. Furthermore, Rbakdn knockdown in testes in mice by intratesticular injection disturbs meiosis, reduces testicular volume, and increases apoptosis of spermatocytes, resulting in vacuolation of the seminiferous tubules. Rbakdn can bind to Ptbp2, an RNA-binding protein that is important in the regulation of the alternative splicing of many genes in spermatogenesis. Rbakdn knockdown leads to a decrease in Ptbp2 through the ubiquitination degradation pathway, indicating that Rbakdn maintains the stability of Ptbp2. In conclusion, our study identified an lncRNA, Rbakdn, with a crucial role in meiosis.

A novel CCDC39 mutation causes multiple morphological abnormalities of the flagella in a primary ciliary dyskinesia patient

RESEARCH QUESTION

Male infertility is a widespread symptom in patients with primary ciliary dyskinesia (PCD). PCD-related male infertility is often caused by asthenozoospermia, with barely normal sperm morphology. Multiple morphological abnormalities of the sperm flagella (MMAF) are a major cause of asthenozoospermia, characterized by various malformed morphologies of sperm flagella. To date, a limited number of genes have been suggested to be involved in the pathogenesis of both PCD and MMAF. What other genes associated with both PCD and MMAF are waiting to be discovered?

DESIGN

Whole-exome sequencing (WES) was performed to identify the pathogenic mutation associated with MMAF in a PCD patient. Peripheral venous blood and semen samples were collected from the PCD patient. Transmission electron microscopy (TEM), immunofluorescence staining and western blotting were conducted to confirm the pathogenicity of the identified mutation.

RESULTS

A novel homozygous mutation in CCDC39, c.983 T>C (p. Leu328Pro), was identified in two PCD-affected siblings of a consanguineous family showing a typical PCD phenotype, while the proband was infertile, which is associated with characterized MMAF. Furthermore, TEM revealed the abnormal ultrastructure of the patient's sperm flagella. Moreover, immunofluorescence staining revealed that CCDC39 was almost undetectable in the spermatozoa, which was further confirmed by western blotting. The outcome of intracytoplasmic sperm injection (ICSI) in the patient with the CCDC39 mutation was also favourable.

CONCLUSION

This study demonstrates that a novel loss-of-function mutation of CCDC39 is involved in the pathogenesis of PCD and MMAF and initially reported that ICSI treatment has a good outcome. Therefore, the novel variant of CCDC39 contributes to the genetic diagnosis, counselling and treatment of male infertility in PCD patients with MMAF phenotype.

Current scenario and challenges ahead in application of spermatogonial stem cell technology in livestock

PURPOSE

Spermatogonial stem cells (SSCs) are the source for the mature male gamete. SSC technology in humans is mainly focusing on preserving fertility in cancer patients. Whereas in livestock, it is used for mining the factors associated with male fertility. The review discusses the present status of SSC biology, methodologies developed for in vitro culture, and challenges ahead in establishing SSC technology for the propagation of superior germplasm with special reference to livestock.

METHOD

Published literatures from PubMed and Google Scholar on topics of SSCs isolation, purification, characterization, short and long-term culture of SSCs, stemness maintenance, epigenetic modifications of SSCs, growth factors, and SSC cryopreservation and transplantation were used for the study.

RESULT

The fine-tuning of SSC isolation and culture conditions with special reference to feeder cells, growth factors, and additives need to be refined for livestock. An insight into the molecular mechanisms involved in maintaining stemness and proliferation of SSCs could facilitate the dissemination of superior germplasm through transplantation and transgenesis. The epigenetic influence on the composition and expression of the biomolecules during in vitro differentiation of cultured cells is essential for sustaining fertility. The development of surrogate males through gene-editing will be historic achievement for the foothold of the SSCs technology.

CONCLUSION

Detailed studies on the species-specific factors regulating the stemness and differentiation of the SSCs are required for the development of a long-term culture system and in vitro spermatogenesis in livestock. Epigenetic changes in the SSCs during in vitro culture have to be elucidated for the successful application of SSCs for improving the productivity of the animals.

Autophagic elimination of ribosomes during spermiogenesis provides energy for flagellar motility

How autophagy initiation is regulated and what the functional significance of this regulation is are unknown. Here, we characterized the role of yeast Vac8 in autophagy initiation through recruitment of PIK3C3-C1 to the phagophore assembly site (PAS). This recruitment is dependent on the palmitoylation of Vac8 and on its middle ARM domains for binding PIK3C3-C1. Vac8-mediated anchoring of PIK3C3-C1 promotes PtdIns3P generation at the PAS and recruitment of the PtdIns3P binding protein Atg18-Atg2. The mouse homolog of Vac8, ARMC3, is conserved and functions in autophagy in mouse testes. Mice lacking ARMC3 have normal viability but show complete male infertility. Proteomic analysis indicated that the autophagic degradation of cytosolic ribosomes was blocked in ARMC3-deficient spermatids, which caused low energy levels of mitochondria and motionless flagella. These studies uncovered a function of Vac8/ARMC3 in PtdIns3-kinase anchoring at the PAS and its physical significance in mammalian spermatogenesis with a germ tissue-specific autophagic function.

HDAC3 controls male fertility through enzyme-independent transcriptional regulation at the meiotic exit of spermatogenesis

The transition from meiotic spermatocytes to postmeiotic haploid germ cells constitutes an essential step in spermatogenesis. The epigenomic regulatory mechanisms underlying this transition remain unclear. Here, we find a prominent transcriptomic switch from the late spermatocytes to the early round spermatids during the meiotic-to-postmeiotic transition, which is associated with robust histone acetylation changes across the genome. Among histone deacetylases (HDACs) and acetyltransferases, we find that HDAC3 is selectively expressed in the late meiotic and early haploid stages. Three independent mouse lines with the testis-specific knockout of HDAC3 show infertility and defects in meiotic exit with an arrest at the late stage of meiosis or early stage of round spermatids. Stage-specific RNA-seq and histone acetylation ChIP-seq analyses reveal that HDAC3 represses meiotic/spermatogonial genes and activates postmeiotic haploid gene programs during meiotic exit, with associated histone acetylation alterations. Unexpectedly, abolishing HDAC3 catalytic activity by missense mutations in the nuclear receptor corepressor (NCOR or SMRT) does not cause infertility, despite causing histone hyperacetylation as HDAC3 knockout, demonstrating that HDAC3 enzyme activity is not required for spermatogenesis. Motif analysis of the HDAC3 cistrome in the testes identified SOX30, which has a similar spatiotemporal expression pattern as HDAC3 during spermatogenesis. Depletion of SOX30 in the testes abolishes the genomic recruitment of the HDAC3 to the binding sites. Collectively, these results establish the SOX30/HDAC3 signaling as a key regulator of the transcriptional program in a deacetylase-independent manner during the meiotic-to-postmeiotic transition in spermatogenesis.

Cyclin N-Terminal Domain-Containing-1 Coordinates Meiotic Crossover Formation with Cell-Cycle Progression in a Cyclin-Independent Manner

During mammalian meiotic prophase I, programmed DNA double-strand breaks are repaired by non-crossover or crossover events, the latter predominantly occurring via the class I crossover pathway and requiring the cyclin N-terminal domain-containing 1(CNTD1) protein. Using an epitope-tagged Cntd1 allele, we detect a short isoform of CNTD1 in vivo that lacks a predicted N-terminal cyclin domain and does not bind cyclin-dependent kinases. Instead, we find that the short-form CNTD1 variant associates with components of the replication factor C (RFC) machinery to facilitate crossover formation, and with the E2 ubiquitin conjugating enzyme, CDC34, to regulate ubiquitylation and subsequent degradation of the WEE1 kinase, thereby modulating cell-cycle progression. We propose that these interactions facilitate a role for CNTD1 as a stop-go regulator during prophase I, ensuring accurate and complete crossover formation before allowing metaphase progression and the first meiotic division.

•

CNTD1 associates with sites of crossing over in meiosis, co-localizing with MutLγ

•

In the testis, CNTD1 does not interact with CDKs or with known crossover regulators

•

CNTD1 regulates crossing over via interactions with the replication factor C complex

•

CNTD1 regulates cell-cycle progression via interactions with the SCF complex

MFN2 interacts with nuage-associated proteins and is essential for male germ cell development by controlling mRNA fate during spermatogenesis

Mitochondria play a crucial role in spermatogenesis and are regulated by several mitochondrial fusion proteins. However, their functional importance associated with their structure formation and mRNA fate regulation during spermatogenesis remains unclear. Here, we show that mitofusin 2 (MFN2), a mitochondrial fusion protein, interacts with nuage-associated proteins (including MIWI, DDX4, TDRKH and GASZ) in mice. Conditional mutation of Mfn2 in postnatal germ cells results in male sterility due to germ cell developmental defects. Moreover, MFN2 interacts with MFN1, another mitochondrial fusion protein with a high-sequence similarity to MFN2, in testes to facilitate spermatogenesis. Simultaneous mutation of Mfn1 and Mfn2 in testes causes very severe infertile phenotypes. Importantly, we show that MFN2 is enriched in polysome fractions of testes and interacts with MSY2, a germ cell-specific DNA/RNA-binding protein, to control gamete-specific mRNA (such as Spata19) translational activity during spermatogenesis. Collectively, our findings demonstrate that MFN2 interacts with nuage-associated proteins and MSY2 to regulate male germ cell development by controlling several gamete-specific mRNA fates.

Transcriptomics of Meiosis in the Male Mouse

Molecular studies of meiosis in mammals have been long relegated due to some intrinsic obstacles, namely the impossibility to reproduce the process in vitro, and the difficulty to obtain highly pure isolated cells of the different meiotic stages. In the recent years, some technical advances, from the improvement of flow cytometry sorting protocols to single-cell RNAseq, are enabling to profile the transcriptome and its fluctuations along the meiotic process. In this mini-review we will outline the diverse methodological approaches that have been employed, and some of the main findings that have started to arise from these studies. As for practical reasons most studies have been carried out in males, and mostly using mouse as a model, our focus will be on murine male meiosis, although also including specific comments about humans. Particularly, we will center on the controversy about gene expression during early meiotic prophase; the widespread existing gap between transcription and translation in meiotic cells; the expression patterns and potential roles of meiotic long non-coding RNAs; and the visualization of meiotic sex chromosome inactivation from the RNAseq perspective.

Contributions of Flow Cytometry to the Molecular Study of Spermatogenesis in Mammals

Mammalian testes are very heterogeneous organs, with a high number of different cell types. Testicular heterogeneity, together with the lack of reliable in vitro culture systems of spermatogenic cells, have been an obstacle for the characterization of the molecular bases of the unique events that take place along the different spermatogenic stages. In this context, flow cytometry has become an invaluable tool for the analysis of testicular heterogeneity, and for the purification of stage-specific spermatogenic cell populations, both for basic research and for clinical applications. In this review, we highlight the importance of flow cytometry for the advances on the knowledge of the molecular groundwork of spermatogenesis in mammals. Moreover, we provide examples of different approaches to the study of spermatogenesis that have benefited from flow cytometry, including the characterization of mutant phenotypes, transcriptomics, epigenetic and genome-wide chromatin studies, and the attempts to establish cell culture systems for research and/or clinical aims such as infertility treatment.

Isolation of Murine Spermatogenic Cells using a Violet-Excited Cell-Permeable DNA Binding Dye

Isolation of meiotic spermatocytes is essential to investigate molecular mechanisms underlying meiosis and spermatogenesis. Although there are established cell isolation protocols using Hoechst 33342 staining in combination with fluorescence-activated cell sorting, it requires cell sorters equipped with an ultraviolet laser. Here we describe a cell isolation protocol using the DyeCycle Violet (DCV) stain, a low cytotoxicity DNA binding dye structurally similar to Hoechst 33342. DCV can be excited by both ultraviolet and violet lasers, which improves the flexibility of equipment choice, including a cell sorter not equipped with an ultraviolet laser. Using this protocol, one can isolate three live-cell subpopulations in meiotic prophase I, including leptotene/zygotene, pachytene, and diplotene spermatocytes, as well as post-meiotic round spermatids. We also describe a protocol to prepare single-cell suspension from mouse testes. Overall, the procedure requires a short time to complete (4-5 hours depending on the number of needed cells), which facilitates many downstream applications.

Protective effect of bone marrow mesenchymal stem cell-derived exosomes against the reproductive toxicity of cyclophosphamide is associated with the p38MAPK/ERK and AKT signaling pathways

Spermatogenic dysfunction caused by cyclophosphamide (CP) chemotherapy has seriously influenced the life quality of patients. Unfortunately, treatments for CP-induced testicular spermatogenic dysfunction are limited, and the molecular mechanisms are not fully understood. For the first time, here, we explored the effects of bone marrow mesenchymal stem cell-derived exosomes (BMSC-exos) on CP-induced testicular spermatogenic dysfunction in vitro and in vivo. BMSC-exos could be taken up by spermatogonia (GC1-spg cells). CP-injured GC1-spg cells and BMSC-exos were cocultured at various doses, and then, cell proliferation was measured using 3-[4,5-dimethylthiazol-2-yl]-2,5 diphenyl tetrazolium bromide (MTT) assay. In addition, photophosphorylation of extracellular-regulated kinase (ERK), p38 mitogen-activated protein kinase (p38MAPK), and protein kinase B (AKT) proteins was evaluated by western blotting as well as apoptosis in GC1-spg cells measured using flow cytometry. Treatment with BMSC-exos enhanced cell proliferation and reduced apoptosis of CP-injured GCI-spg cells. Phosphorylated levels of ERK, AKT, and p38MAPK proteins were reduced in CP-injured spermatogonia when co-treated with BMSC-exos, indicating that BMSC-exos acted against the reproductive toxicity of CP via the p38MAPK/ERK and AKT signaling pathways. In experiments in vivo, CP-treated rats received BMSC-exos by injection into the tail vein, and testis morphology was compared between treated and control groups. Histology showed that transfusion of BMSC-exos inhibited the pathological changes in CP-injured testes. Thus, BMSC-exos could counteract the reproductive toxicity of CP via the p38MAPK/ERK and AKT signaling pathways. The findings provide a potential treatment for CP-induced male spermatogenic dysfunction using BMSC-exos.

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

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

The role of tyrosine phosphatase Shp2 in spermatogonial differentiation and spermatocyte meiosis

The transition from spermatogonia to spermatocytes and the initiation of meiosis are key steps in spermatogenesis and are precisely regulated by a plethora of proteins. However, the underlying molecular mechanism remains largely unknown. Here, we report that Src homology domain tyrosine phosphatase 2 (Shp2; encoded by the protein tyrosine phosphatase, nonreceptor type 11 [Ptpn11] gene) is abundant in spermatogonia but markedly decreases in meiotic spermatocytes. Conditional knockout of Shp2 in spermatogonia in mice using stimulated by retinoic acid gene 8 (Stra8)-cre enhanced spermatogonial differentiation and disturbed the meiotic process. Depletion of Shp2 in spermatogonia caused many meiotic spermatocytes to die; moreover, the surviving spermatocytes reached the leptotene stage early at postnatal day 9 (PN9) and the pachytene stage at PN11–13. In preleptotene spermatocytes, Shp2 deletion disrupted the expression of meiotic genes, such as disrupted meiotic cDNA 1 (Dmc1), DNA repair recombinase rad51 (Rad51), and structural maintenance of chromosome 3 (Smc3), and these deficiencies interrupted spermatocyte meiosis. In GC-1 cells cultured in vitro, Shp2 knockdown suppressed the retinoic acid (RA)-induced phosphorylation of extracellular-regulated protein kinase (Erk) and protein kinase B (Akt/PKB) and the expression of target genes such as synaptonemal complex protein 3 (Sycp3) and Dmc1. Together, these data suggest that Shp2 plays a crucial role in spermatogenesis by governing the transition from spermatogonia to spermatocytes and by mediating meiotic progression through regulating gene transcription, thus providing a potential treatment target for male infertility.

Sequence analysis in Bos taurus reveals pervasiveness of X-Y arms races in mammalian lineages

Studies of Y Chromosome evolution have focused primarily on gene decay, a consequence of suppression of crossing-over with the X Chromosome. Here, we provide evidence that suppression of X-Y crossing-over unleashed a second dynamic: selfish X-Y arms races that reshaped the sex chromosomes in mammals as different as cattle, mice, and men. Using super-resolution sequencing, we explore the Y Chromosome of Bos taurus (bull) and find it to be dominated by massive, lineage-specific amplification of testis-expressed gene families, making it the most gene-dense Y Chromosome sequenced to date. As in mice, an X-linked homolog of a bull Y-amplified gene has become testis-specific and amplified. This evolutionary convergence implies that lineage-specific X-Y coevolution through gene amplification, and the selfish forces underlying this phenomenon, were dominatingly powerful among diverse mammalian lineages. Together with Y gene decay, X-Y arms races molded mammalian sex chromosomes and influenced the course of mammalian evolution.

What has single-cell RNA-seq taught us about mammalian spermatogenesis?

Mammalian spermatogenesis is a complex developmental program that transforms mitotic testicular germ cells (spermatogonia) into mature male gametes (sperm) for production of offspring. For decades, it has been known that this several-weeks-long process involves a series of highly ordered and morphologically recognizable cellular changes as spermatogonia proliferate, spermatocytes undertake meiosis, and spermatids develop condensed nuclei, acrosomes, and flagella. Yet, much of the underlying molecular logic driving these processes has remained opaque because conventional characterization strategies often aggregated groups of cells to meet technical requirements or due to limited capability for cell selection. Recently, a cornucopia of single-cell transcriptome studies has begun to lift the veil on the full compendium of gene expression phenotypes and changes underlying spermatogenic development. These datasets have revealed the previously obscured molecular heterogeneity among and between varied spermatogenic cell types and are reinvigorating investigation of testicular biology. This review describes the extent of available single-cell RNA-seq profiles of spermatogenic and testicular somatic cells, how those data were produced and evaluated, their present value for advancing knowledge of spermatogenesis, and their potential future utility at both the benchtop and bedside.

Super-enhancer switching drives a burst in gene expression at the mitosis-to-meiosis transition

Due to bursts in the expression of thousands of germline-specific genes, the testis has the most diverse and complex transcriptome of all organs. By analyzing the male germline of mice, we demonstrate that the genome-wide reorganization of super-enhancers (SEs) drives bursts in germline gene expression after the mitosis-to-meiosis transition. SE reorganization is regulated by two molecular events: the establishment of meiosis-specific SEs via A-MYB (MYBL1), a key transcription factor for germline genes, and the resolution of SEs in mitotically proliferating cells via SCML2, a germline-specific Polycomb protein required for spermatogenesis-specific gene expression. Prior to entry into meiosis, meiotic SEs are preprogrammed in mitotic spermatogonia to ensure the unidirectional differentiation of spermatogenesis. We identify key regulatory factors for both mitotic and meiotic enhancers, revealing a molecular logic for the concurrent activation of mitotic enhancers and suppression of meiotic enhancers in the somatic and/or mitotic proliferation phases.

H3K4me1 Distribution Predicts Transcription State and Poising at Promoters

Monomethylation on lysine 4 of histone H3 (H3K4me1) is commonly associated with distal enhancers, but H3K4me1 is also present at promoter regions proximal to transcription start sites. To assess a possible role for H3K4me1 in dictating gene regulatory states at promoters, we examined H3K4me1 peak density around promoters in human and mouse germ cells using an analytic strategy that allowed us to assess relationships between different epigenetic marks on a promoter-by-promoter basis. We found that H3K4me1 exhibits either a bimodal pattern at active promoters, where it flanks H3K4me3, or a unimodal pattern at poised promoters, where it coincides with both H3K4me3 and H3K27me3. This pattern is correlated with gene expression level, but is more strongly linked to a poised chromatin state, defined by the simultaneous presence of H3K4me3 and H3K27me3, than to transcriptional activity. The pattern is especially prominent in germ cells, but is also present in other cell types, including embryonic stem cells and differentiated somatic cells. We propose that H3K4me1 is a key feature of the poised epigenetic state, and suggest possible roles for this mark in epigenetic memory.

A male germ cell assay and supporting somatic cells: its application for the detection of phase specificity of genotoxins in vitro

Male germ stem cells are responsible for transmission of genetic information to the next generation. Some chemicals exert a negative impact on male germ cells, either directly, or indirectly affecting them through their action on somatic cells. Ultimately, these effects might inhibit fertility, and may exhibit negative consequences on future offspring. Genotoxic anticancer agents may interact with DNA in germ cells potentially leading to a heritable germline mutation. Experimental information in support of this theory has not always been reproducible and suitable in vivo studies remain limited. Thus, alternative male germ cell tests, which are now able to detect phase specificity of such agents, might be used by regulatory agencies to help evaluate the potential risk of mutation. However, there is an urgent need for such approaches for identification of male reproductive genotoxins since this area has until recently been dependent on in vivo studies. Many factors drive alternative approaches, including the (1) commitment to the principles of the 3R's (Replacement, Reduction, and Refinement), (2) time-consuming nature and high cost of animal experiments, and (3) new opportunities presented by new molecular analytical assays. There is as yet currently no apparent appropriate model of full mammalian spermatogenesis in vitro, under the REACH initiative, where new tests introduced to assess genotoxicity and mutagenicity need to avoid unnecessary testing on animals. Accordingly, a battery of tests used in conjunction with the high throughput STAPUT gravity sedimentation was recently developed for purification of male germ cells to investigate genotoxicity for phase specificity in germ cells. This system might be valuable for the examination of phases previously only available in mammals with large-scale studies of germ cell genotoxicity in vivo. The aim of this review was to focus on this alternative approach and its applications as well as on chemicals of known in vivo phase specificities used during this test system development.

Lack of AKAP3 disrupts integrity of the subcellular structure and proteome of mouse sperm and causes male sterility

The development and maintenance of the correct morphology of sperm is important for their functions. Cellular morphogenesis of sperm occurs during the post-meiotic developmental stage; however, little is known about what coordinates this process. In the present study, we investigated the role of A-kinase anchoring protein 3 (AKAP3) during mouse spermiogenesis, using both mouse genetics and proteomics. It was found that AKAP3 is essential for the formation of the specific subcellular structure of the sperm flagellum, motility of sperm and male fertility. Additionally, lack of AKAP3 caused global changes of the sperm proteome and mislocalization of sperm proteins, including accumulation of RNA metabolism and translation factors and displacement of PKA subunits in mature sperm, which may underlie misregulated PKA activity and immotility in sperm. Interestingly, sperm lacking a complete fibrous sheath from both Akap3 and Akap4 null mice accumulated F-actin filaments and morphological defects during post-testicular maturation in the epididymis. These results suggest that the subcellular structures of sperm could be formed via independent pathways, and elucidate the roles of AKAP3 during the coordinated synthesis and organization of the sperm proteome and sperm morphology.

Bovid microRNAs involved in the process of spermatogonia differentiation into spermatocytes

The male infertility of cattleyak resulted from spermatogenic arrest has greatly restricted the effective utilization of the heterosis from crossbreeding of cattle and yak. Based on our previous studies, the significant divergences of the transcriptomic and proteomic sequencing between yak and cattleyak prompt us to investigate the critical roles of microRNAs in post-transcriptional regulation of gene expression during spermatogenesis. TUNEL-POD analysis presented sharply decreased spermatogenic cell types and the increased apoptotic spermatogonia in cattleyak. The STA-PUT velocity sedimentation was employed to obtain spermatogonia and spermatocytes from cattle, yak and cattleyak and these spermatogenic cells were verified by the morphological and phenotypic identification. MicroRNA microarray showed that 27 differentially expressed miRNAs were simultaneously identified both in cattleyak vs cattle and in cattleyak vs yak comparisons. Further analysis revealed that the down-regulation of bta-let-7 families, bta-miR-125 and bta-miR-23a might impair the RA-induced differentiation of spermatogonia. Target gene analysis for differentially expressed miRNAs revealed that miRNAs targeted major players involved in vesicle-mediated transport, regulation of protein kinase activity and Pathways in cancer. In addition, spermatogonia transfection analysis revealed that the down-regulation of bta-miR-449a in the cattleyak might block the transition of male germ cells from the mitotic cycle to the meiotic program. The present study provided valuable information for future elucidating the regulatory roles of miRNAs involved in spermatogenic arrest of cattleyak.

Inactivation of glycogen synthase kinase-3α is required for mitochondria-mediated apoptotic germ cell phagocytosis in Sertoli cells

The rapid and efficient clearance of apoptotic germ cells (GCs) by Sertoli cells (SCs) is important for spermatogenesis. High mitochondrial activity in phagocytes is critical for continued clearance of apoptotic cells. However, the underlying molecular mechanism is poorly understood. Glycogen synthase kinase-3α (GSK3α) is a protein kinase that participates in the regulation of mitochondrial activity. Immunohistochemistry evidenced the predominant presence of the Ser21 phosphorylation GSK3α (inactivation) signal in SCs. Heat shock-induced apoptosis of GCs and dephosphorylation of GSK3α in SCs is a perfect model to investigate the role of GSK3α in phagocytic action. The number of apoptotic GCs was significantly lower in GSK3α inhibitor pre-treated mice with HS compared to normal control. In vitro phagocytosis assays shown that the phagocytic activity in GSK3α activated SCs was downregulated, while GSK3α inhibitor supplementation restored this process. Moreover, GSK3α activation participates in the alteration of the mitochondrial ultrastructure and activity. In particular, GSK3α activation inhibits mitochondrial fission via phosphorylation of dynamin related protein 1 at Ser637. Changes of mitochondrial activity resulted in the accumulation of lipid droplets and the alteration of metabolism pattern in SCs. In summary, our results demonstrate that inactivation of GSK3α is required for mitochondria-mediated apoptotic GCs phagocytosis in SCs.

Histone hyperacetylation and DNA methylation interplay during murine spermatogenesis

Male germ cell development is a critical period during which epigenetic patterns are established and maintained. The progression from diploid spermatogonia to haploid spermatozoa involves the incorporation of testis-specific histone variants, mitotic and meiotic divisions, haploid gene expression, histone–protamine transitions and massive epigenetic reprogramming. Understanding the protein players and the epigenetic mark network involved in the setting of the epigenetic programme in spermatogenesis is an exciting new clue in the field of reproductive biology with translational outcomes. As information in the existing literature regarding cross-talk between DNA methylation and histone hyperacetylation in the advanced stages of murine spermatogenesis is still scarce and controversial we have investigated the effect of a DNA-methyltransferase inhibitor, 5-aza-2′-deoxycytidine, at the cytological and molecular level (by transmission electron microscopy, immunocytochemistry and immunoprecipitation methods). Our results revealed an important role for regulation of DNA methylation in controlling histone hyperacetylation and chromatin remodelling during spermatogenesis.

Dynamic reorganization of open chromatin underlies diverse transcriptomes during spermatogenesis

During spermatogenesis, germ cells undergo massive cellular reconstruction and dynamic chromatin remodeling to facilitate highly diverse transcriptomes, which are required for the production of functional sperm. However, it remains unknown how germline chromatin is organized to promote the dynamic, complex transcriptomes of spermatogenesis. Here, using ATAC-seq, we establish the varied landscape of open chromatin during spermatogenesis. We identify the reorganization of accessible chromatin in intergenic and intronic regions during the mitosis-to-meiosis transition. During the transition, mitotic-type open chromatin is closed while the de novo formation of meiotic-type open chromatin takes place. Contrastingly, differentiation processes such as spermatogonial differentiation and the meiosis-to-postmeiosis transition involve chromatin closure without the de novo formation of accessible chromatin. In spermiogenesis, the germline-specific Polycomb protein SCML2 promotes the closure of open chromatin at autosomes for gene suppression. Paradoxically, we identify the massive de novo formation of accessible chromatin when the sex chromosomes undergo meiotic sex chromosome inactivation, and this is also mediated by SCML2. These results reveal meiotic sex chromosome inactivation as an active process for chromatin organization. Together, our results unravel the genome-wide, dynamic reorganization of open chromatin and reveal mechanisms that underlie diverse transcriptomes during spermatogenesis.

BPDE and B[a]P induce mitochondrial compromise by ROS-mediated suppression of the SIRT1/TERT/PGC-1α pathway in spermatogenic cells both in vitro and in vivo

There is increasing evidence that indicates benzo[a]pyrene (B[a]P) and its active metabolite benzo[a]pyrene-7, 8-dihydrodiol-9, 10-epoxide (BPDE) are endocrine disruptors that can cause reproductive toxicity. Nevertheless, the underlying mechanisms are still obscure. The present study investigates the impacts of B[a]P and BPDE on mitochondria, a sensitive target affected by multiple chemicals, in spermatogenic cells. It showed that BPDE treatment induced mitochondrial dysfunction and the inhibition of mitochondrial biogenesis in mouse spermatocyte-derived cells (GC-2). These effects were efficiently mitigated by pretreatment with ZLN005, an activator of PGC-1α, in GC-2 cells. TERT knockdown and re-expression cell models were established to demonstrate that TERT regulated the BPDE-induced mitochondrial damage via PGC-1α signaling in GC-2 cells. Moreover, upregulating or knockdown SIRT1 expression attenuated or aggravated BPDE-induced mitochondrial compromise by activating or inhibiting, respectively, the TERT and PGC-1α molecules in GC-2 cells. Finally, we observed that BPDE markedly elevated oxidative stress in GC-2 cells. Resveratrol and N-acetylcysteine, as reactive oxygen species (ROS) scavengers, attenuated BPDE-mediated mitochondrial damage by increasing SIRT1 activity and expression in GC-2 cells. The in vitro results were corroborated by in vivo experiments in rats treated with B[a]P for 4 weeks. B[a]P administration caused mitochondrial damage and mitochondria-dependent apoptosis in spermatogenic cells, as well as the decreased expression of SIRT1, TERT, and PGC-1α. In summary, the results of the present study demonstrate that B[a]P and BPDE induce mitochondrial damage through ROS production that suppresses SIRT1/TERT/PGC-1a signaling and mediate B[a]P- and BPDE-mediated reproductive toxicity.