Parallel evolution of male germline epigenetic poising and somatic development in animals

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.

Single-Cell Transcriptome Sequencing Reveals Molecular Expression Differences and Marker Genes in Testes during the Sexual Maturation of Mongolian Horses

This study aimed to investigate differences in testicular tissue morphology, gene expression, and marker genes between sexually immature (1-year-old) and sexually mature (10-year-old) Mongolian horses. The purposes of our research were to provide insights into the reproductive physiology of male Mongolian horses and to identify potential markers for sexual maturity. The methods we applied included the transcriptomic profiling of testicular cells using single-cell sequencing techniques. Our results revealed significant differences in tissue morphology and gene expression patterns between the two age groups. Specifically, 25 cell clusters and 10 cell types were identified, including spermatogonial and somatic cells. Differential gene expression analysis highlighted distinct patterns related to cellular infrastructure in sexually immature horses and spermatogenesis in sexually mature horses. Marker genes specific to each stage were also identified, including APOA1, AMH, TAC3, INHA, SPARC, and SOX9 for the sexually immature stage, and PRM1, PRM2, LOC100051500, PRSS37, HMGB4, and H1-9 for the sexually mature stage. These findings contribute to a deeper understanding of testicular development and spermatogenesis in Mongolian horses and have potential applications in equine reproductive biology and breeding programs. In conclusion, this study provides valuable insights into the molecular mechanisms underlying sexual maturity in Mongolian horses.

PRC1 directs PRC2-H3K27me3 deposition to shield adult spermatogonial stem cells from differentiation

Spermatogonial stem cells functionality reside in the slow-cycling and heterogeneous undifferentiated spermatogonia cell population. This pool of cells supports lifelong fertility in adult males by balancing self-renewal and differentiation to produce haploid gametes. However, the molecular mechanisms underpinning long-term stemness of undifferentiated spermatogonia during adulthood remain unclear. Here, we discover that an epigenetic regulator, Polycomb repressive complex 1 (PRC1), shields adult undifferentiated spermatogonia from differentiation, maintains slow cycling, and directs commitment to differentiation during steady-state spermatogenesis in adults. We show that PRC2-mediated H3K27me3 is an epigenetic hallmark of adult undifferentiated spermatogonia. Indeed, spermatogonial differentiation is accompanied by a global loss of H3K27me3. Disruption of PRC1 impairs global H3K27me3 deposition, leading to precocious spermatogonial differentiation. Therefore, PRC1 directs PRC2-H3K27me3 deposition to maintain the self-renewing state of undifferentiated spermatogonia. Importantly, in contrast to its role in other tissue stem cells, PRC1 negatively regulates the cell cycle to maintain slow cycling of undifferentiated spermatogonia. Our findings have implications for how epigenetic regulators can be tuned to regulate the stem cell potential, cell cycle and differentiation to ensure lifelong fertility in adult males.

The Association between Long-Term DDT or DDE Exposures and an Altered Sperm Epigenome-a Cross-Sectional Study of Greenlandic Inuit and South African VhaVenda Men

BACKGROUND

The organochlorine dichlorodiphenyltrichloroethane (DDT) is banned worldwide owing to its negative health effects. It is exceptionally used as an insecticide for malaria control. Exposure occurs in regions where DDT is applied, as well as in the Arctic, where its endocrine disrupting metabolite, p , p ' -dichlorodiphenyldichloroethylene (p , p ' -DDE) accumulates in marine mammals and fish. DDT and p , p ' -DDE exposures are linked to birth defects, infertility, cancer, and neurodevelopmental delays. Of particular concern is the potential of DDT use to impact the health of generations to come via the heritable sperm epigenome.

OBJECTIVES

The objective of this study was to assess the sperm epigenome in relation to p , p ' -DDE serum levels between geographically diverse populations.

METHODS

In the Limpopo Province of South Africa, we recruited 247 VhaVenda South African men and selected 50 paired blood serum and semen samples, and 47 Greenlandic Inuit blood and semen paired samples were selected from a total of 193 samples from the biobank of the INUENDO cohort, an EU Fifth Framework Programme Research and Development project. Sample selection was based on obtaining a range of p , p ' -DDE serum levels (mean = 870.734 ± 134.030  ng / mL ). We assessed the sperm epigenome in relation to serum p , p ' -DDE levels using MethylC-Capture-sequencing (MCC-seq) and chromatin immunoprecipitation followed by sequencing (ChIP-seq). We identified genomic regions with altered DNA methylation (DNAme) and differential enrichment of histone H3 lysine 4 trimethylation (H3K4me3) in sperm.

RESULTS

Differences in DNAme and H3K4me3 enrichment were identified at transposable elements and regulatory regions involved in fertility, disease, development, and neurofunction. A subset of regions with sperm DNAme and H3K4me3 that differed between exposure groups was predicted to persist in the preimplantation embryo and to be associated with embryonic gene expression.

DISCUSSION

These findings suggest that DDT and p , p ' -DDE exposure impacts the sperm epigenome in a dose-response-like manner and may negatively impact the health of future generations through epigenetic mechanisms. Confounding factors, such as other environmental exposures, genetic diversity, and selection bias, cannot be ruled out. https://doi.org/10.1289/EHP12013.

De novo transcriptome assembly of mouse male germ cells reveals novel genes, stage-specific bidirectional promoter activity, and noncoding RNA expression

In mammals, the adult testis is the tissue with the highest diversity in gene expression. Much of that diversity is attributed to germ cells, primarily meiotic spermatocytes and postmeiotic haploid spermatids. Exploiting a newly developed cell purification method, we profiled the transcriptomes of such postmitotic germ cells of mice. We used a de novo transcriptome assembly approach and identified thousands of novel expressed transcripts characterized by features distinct from those of known genes. Novel loci tend to be short in length, monoexonic, and lowly expressed. Most novel genes have arisen recently in evolutionary time and possess low coding potential. Nonetheless, we identify several novel protein-coding genes harboring open reading frames that encode proteins containing matches to conserved protein domains. Analysis of mass-spectrometry data from adult mouse testes confirms protein production from several of these novel genes. We also examine overlap between transcripts and repetitive elements. We find that although distinct families of repeats are expressed with differing temporal dynamics during spermatogenesis, we do not observe a general mode of regulation wherein repeats drive expression of nonrepetitive sequences in a cell type-specific manner. Finally, we observe many fairly long antisense transcripts originating from canonical gene promoters, pointing to pervasive bidirectional promoter activity during spermatogenesis that is distinct and more frequent compared with somatic cells.

Polycomb protein SCML2 mediates paternal epigenetic inheritance through sperm chromatin

Sperm chromatin retains small amounts of histones, and chromatin states of sperm mirror gene expression programs of the next generation. However, it remains largely unknown how paternal epigenetic information is transmitted through sperm chromatin. Here, we present a novel mouse model of paternal epigenetic inheritance, in which deposition of Polycomb repressive complex 2 (PRC2) mediated-repressive H3K27me3 is attenuated in the paternal germline. By applying modified methods of assisted reproductive technology using testicular sperm, we rescued infertility of mice missing Polycomb protein SCML2, which regulates germline gene expression by establishing H3K27me3 on bivalent promoters with other active marks H3K4me2/3. We profiled epigenomic states (H3K27me3 and H3K4me3) of testicular sperm and epididymal sperm, demonstrating that the epididymal pattern of the sperm epigenome is already established in testicular sperm and that SCML2 is required for this process. In F1 males of X-linked Scml2-knockout mice, which have a wild-type genotype, gene expression is dysregulated in the male germline during spermiogenesis. These dysregulated genes are targets of SCML2-mediated H3K27me3 in F0 sperm. Further, dysregulation of gene expression was observed in the mutant-derived wild-type F1 preimplantation embryos. Together, we present functional evidence that the classic epigenetic regulator Polycomb mediates paternal epigenetic inheritance through sperm chromatin.

Emerging evidence that the mammalian sperm epigenome serves as a template for embryo development

Although more studies are demonstrating that a father's environment can influence child health and disease, the molecular mechanisms underlying non-genetic inheritance remain unclear. It was previously thought that sperm exclusively contributed its genome to the egg. More recently, association studies have shown that various environmental exposures including poor diet, toxicants, and stress, perturbed epigenetic marks in sperm at important reproductive and developmental loci that were associated with offspring phenotypes. The molecular and cellular routes that underlie how epigenetic marks are transmitted at fertilization, to resist epigenetic reprogramming in the embryo, and drive phenotypic changes are only now beginning to be unraveled. Here, we provide an overview of the state of the field of intergenerational paternal epigenetic inheritance in mammals and present new insights into the relationship between embryo development and the three pillars of epigenetic inheritance: chromatin, DNA methylation, and non-coding RNAs. We evaluate compelling evidence of sperm-mediated transmission and retention of paternal epigenetic marks in the embryo. Using landmark examples, we discuss how sperm-inherited regions may escape reprogramming to impact development via mechanisms that implicate transcription factors, chromatin organization, and transposable elements. Finally, we link paternally transmitted epigenetic marks to functional changes in the pre- and post-implantation embryo. Understanding how sperm-inherited epigenetic factors influence embryo development will permit a greater understanding related to the developmental origins of health and disease.

Epigenetic resetting in the human germ line entails histone modification remodeling

Epigenetic resetting in the mammalian germ line entails acute DNA demethylation, which lays the foundation for gametogenesis, totipotency, and embryonic development. We characterize the epigenome of hypomethylated human primordial germ cells (hPGCs) to reveal mechanisms preventing the widespread derepression of genes and transposable elements (TEs). Along with the loss of DNA methylation, we show that hPGCs exhibit a profound reduction of repressive histone modifications resulting in diminished heterochromatic signatures at most genes and TEs and the acquisition of a neutral or paused epigenetic state without transcriptional activation. Efficient maintenance of a heterochromatic state is limited to a subset of genomic loci, such as evolutionarily young TEs and some developmental genes, which require H3K9me3 and H3K27me3, respectively, for efficient transcriptional repression. Accordingly, transcriptional repression in hPGCs presents an exemplary balanced system relying on local maintenance of heterochromatic features and a lack of inductive cues.

Regulation, functions and transmission of bivalent chromatin during mammalian development

Cells differentiate and progress through development guided by a dynamic chromatin landscape that mediates gene expression programmes. During development, mammalian cells display a paradoxical chromatin state: histone modifications associated with gene activation (trimethylated histone H3 Lys4 (H3K4me3)) and with gene repression (trimethylated H3 Lys27 (H3K27me3)) co-occur at promoters of developmental genes. This bivalent chromatin modification state is thought to poise important regulatory genes for expression or repression during cell-lineage specification. In this Review, we discuss recent work that has expanded our understanding of the molecular basis of bivalent chromatin and its contributions to mammalian development. We describe the factors that establish bivalency, especially histone-lysine N-methyltransferase 2B (KMT2B) and Polycomb repressive complex 2 (PRC2), and consider evidence indicating that PRC1 shapes bivalency and may contribute to its transmission between generations. We posit that bivalency is a key feature of germline and embryonic stem cells, as well as other types of stem and progenitor cells. Finally, we discuss the relevance of bivalent chromtin to human development and cancer, and outline avenues of future research.

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.

Paternally inherited H3K27me3 affects chromatin accessibility in mouse embryos produced by round spermatid injection

Round spermatid injection (ROSI) results in a lower birth rate than intracytoplasmic sperm injection, which has hampered its clinical application. Inefficient development of ROSI embryos has been attributed to epigenetic abnormalities. However, the chromatin-based mechanism that underpins the low birth rate in ROSI remains to be determined. Here, we show that a repressive histone mark, H3K27me3, persists from mouse round spermatids into zygotes in ROSI and that round spermatid-derived H3K27me3 is associated with less accessible chromatin and impaired gene expression in ROSI embryos. These loci are initially marked by H3K27me3 but undergo histone modification remodelling in spermiogenesis, resulting in reduced H3K27me3 in normal spermatozoa. Therefore, the absence of epigenetic remodelling, presumably mediated by histone turnover during spermiogenesis, leads to dysregulation of chromatin accessibility and transcription in ROSI embryos. Thus, our results unveil a molecular logic, in which chromatin states in round spermatids impinge on chromatin accessibility and transcription in ROSI embryos, highlighting the importance of epigenetic remodelling during spermiogenesis in successful reproduction.

Dynamic reprogramming of H3K9me3 at hominoid-specific retrotransposons during human preimplantation development

Reprogramming of H3K9me3-dependent heterochromatin is required for early development. How H3K9me3 is involved in early human development remains, however, largely unclear. Here, we resolve the temporal landscape of H3K9me3 during human preimplantation development and its regulation for diverse hominoid-specific retrotransposons. At the 8-cell stage, H3K9me3 reprogramming at hominoid-specific retrotransposons termed SINE-VNTR-Alu (SVA) facilitates interaction between certain promoters and SVA-derived enhancers, promoting the zygotic genome activation. In trophectoderm, de novo H3K9me3 domains prevent pluripotent transcription factors from binding to hominoid-specific retrotransposons-derived regulatory elements for inner cell mass (ICM)-specific genes. H3K9me3 re-establishment at SVA elements in the ICM is associated with higher transcription of DNA repair genes, when compared with naive human pluripotent stem cells. Our data demonstrate that species-specific reorganization of H3K9me3-dependent heterochromatin at hominoid-specific retrotransposons plays important roles during early human development, shedding light on how the epigenetic regulation for early development has evolved in mammals.

A gene deriving from the ancestral sex chromosomes was lost from the X and retained on the Y chromosome in eutherian mammals

Background

The mammalian X and Y chromosomes originated from a pair of ordinary autosomes. Over the past ~180 million years, the X and Y have become highly differentiated and now only recombine with each other within a short pseudoautosomal region. While the X chromosome broadly preserved its gene content, the Y chromosome lost ~92% of the genes it once shared with the X chromosome. PRSSLY is a Y-linked gene identified in only a few mammalian species that was thought to be acquired, not ancestral. However, PRSSLY’s presence in widely divergent species—bull and mouse—led us to further investigate its evolutionary history.

Results

We discovered that PRSSLY is broadly conserved across eutherians and has ancient origins. PRSSLY homologs are found in syntenic regions on the X chromosome in marsupials and on autosomes in more distant animals, including lizards, indicating that PRSSLY was present on the ancestral autosomes but was lost from the X and retained on the Y in eutherian mammals. We found that across eutheria, PRSSLY’s expression is testis-specific, and, in mouse, it is most robustly expressed in post-meiotic germ cells. The closest paralog to PRSSLY is the autosomal gene PRSS55, which is expressed exclusively in testes, involved in sperm differentiation and migration, and essential for male fertility in mice. Outside of eutheria, in species where PRSSLY orthologs are not Y-linked, we find expression in a broader range of somatic tissues, suggesting that PRSSLY has adopted a germ-cell-specific function in eutherians. Finally, we generated Prssly mutant mice and found that they are fully fertile but produce offspring with a modest female-biased sex ratio compared to controls.

Conclusions

PRSSLY appears to be the first example of a gene that derives from the mammalian ancestral sex chromosomes that was lost from the X and retained on the Y. Although the function of PRSSLY remains to be determined, it may influence the sex ratio by promoting the survival or propagation of Y-bearing sperm.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01338-8.

Sperm Histone H3 Lysine 4 tri-methylation serves as a metabolic sensor of paternal obesity and is associated with the inheritance of metabolic dysfunction

Objective

Parental environmental exposures can strongly influence descendant risks for adult disease. How paternal obesity changes the sperm chromatin leading to the acquisition of metabolic disease in offspring remains controversial and ill-defined. The objective of this study was to assess (1) whether obesity induced by a high-fat diet alters sperm histone methylation; (2) whether paternal obesity can induce metabolic disturbances across generations; (3) whether there could be cumulative damage to the sperm epigenome leading to enhanced metabolic dysfunction in descendants; and (4) whether obesity-sensitive regions associate with embryonic epigenetic and transcriptomic profiles. Using a genetic mouse model of epigenetic inheritance, we investigated the role of histone H3 lysine 4 methylation (H3K4me3) in the paternal transmission of metabolic dysfunction. This transgenic mouse overexpresses the histone demethylase enzyme KDM1A in the developing germline and has an altered sperm epigenome at the level of histone H3K4 methylation. We hypothesized that challenging transgenic sires with a high-fat diet would further erode the sperm epigenome and lead to enhanced metabolic disturbances in the next generations.

Methods

To assess whether paternal obesity can have inter- or transgenerational impacts, and if so to identify potential mechanisms of this non-genetic inheritance, we used wild-type C57BL/6NCrl and transgenic males with a pre-existing altered sperm epigenome. To induce obesity, sires were fed either a control or high-fat diet (10% or 60% kcal fat, respectively) for 10–12 weeks, then bred to wild-type C57BL/6NCrl females fed a regular diet. F₁ and F₂ descendants were characterized for metabolic phenotypes by examining the effects of paternal obesity by sex, on body weight, fat mass distribution, the liver transcriptome, intraperitoneal glucose, and insulin tolerance tests. To determine whether obesity altered the F₀ sperm chromatin, native chromatin immunoprecipitation-sequencing targeting H3K4me3 was performed. To gain insight into mechanisms of paternal transmission, we compared our sperm H3K4me3 profiles with embryonic and placental chromatin states, histone modification, and gene expression profiles.

Results

Obesity-induced alterations in H3K4me3 occurred in genes implicated in metabolic, inflammatory, and developmental processes. These processes were associated with offspring metabolic dysfunction and corresponded to genes enriched for H3K4me3 in embryos and overlapped embryonic and placenta gene expression profiles. Transgenerational susceptibility to metabolic disease was only observed when obese F₀ had a pre-existing modified sperm epigenome. This coincided with increased H3K4me3 alterations in sperm and more severe phenotypes affecting their offspring.

Conclusions

Our data suggest sperm H3K4me3 might serve as a metabolic sensor that connects paternal diet with offspring phenotypes via the placenta. This non-DNA-based knowledge of inheritance has the potential to improve our understanding of how environment shapes heritability and may lead to novel routes for the prevention of disease. This study highlights the need to further study the connection between the sperm epigenome, placental development, and children's health.

Summary sentence

Paternal obesity impacts sperm H3K4me3 and is associated with placenta, embryonic and metabolic outcomes in descendants.

•

Sperm H3K4me3 serves as a metabolic sensor of HFD-induced obesity.

•

Obesity-altered sperm H3K4me3 corresponds to embryonic transcription and chromatin profiles.

•

HFD- and KDM1A-induced cumulative sperm epimutations enhanced F₁ metabolic dysfunction.

•

Sperm epimutations may influence placenta function inducing F₁ metabolic phenotypes.

Histone Methylation Regulates Gene Expression in the Round Spermatids to Set the RNA Payloads of Sperm

Gene expression during spermatogenesis undergoes significant changes due to a demanding sequence of mitosis, meiosis, and differentiation. We investigated the contribution of H3 histone modifications to gene regulation in the round spermatids. Round spermatids were purified from rat testes using centrifugal elutriation and Percoll density-gradient centrifugation. After enzymatic chromatin shearing, immuno-precipitation using antibodies against histone marks H3k4me3 and H3K9me3 was undertaken. The immunoprecipitated DNA fragments were subjected to massive parallel sequencing. Gene expression in round spermatids and sperm was analyzed by transcriptome sequencing using next-generation sequencing methods. ChIP-seq analysis showed significant peak enrichment in H3K4me3 marks in active chromatin regions and H3K9me3 peak enrichment in repressive regions. We found 53 genes which showed overlapping peak enrichment in both H3K4me3 and H3K9me3 marks. Some of the top H3K4me3-enriched genes were involved in sperm tail formation (Odf1, Odf3, Odf4, Oaz3, Ccdc42, Ccdc63, and Ccdc181), chromatin condensation (Dync1h1, Dynll1, and Kdm3a), and sperm functions such as acrosome reaction (Acrbp and Fabp9), energy generation (Gapdhs), and signaling for motility (Tssk1b, Tssk2, and Tssk4). Transcriptome sequencing in round spermatids found 64% transcripts of the H3K4me3-enriched genes at high levels and of about 25% of H3K9me3-enriched genes at very low levels. Transcriptome sequencing in sperm found that more than 99% of the ChIP-seq corresponding transcripts were also present in sperm. H3K4me3 enrichment in the round spermatids correlates significantly with gene expression and H3K9me3 correlates with gene silencing that contribute to sperm differentiation and setting the RNA payloads of sperm.

Unraveling three-dimensional chromatin structural dynamics during spermatogonial differentiation

Spermatogonial stem cells (SSCs) are able to undergo both self-renewal and differentiation. Unlike self-renewal, which replenishes the SSC and progenitor pool, differentiation is an irreversible process committing cells to meiosis. Although the preparations for meiotic events in differentiating spermatogonia (Di-SG) are likely to be accompanied by alterations in chromatin structure, the three-dimensional chromatin architectural differences between SSCs and Di-SG, and the higher-order chromatin dynamics during spermatogonial differentiation, have not been systematically investigated. Here, we performed in situ high-throughput chromosome conformation capture, RNA-seq, and chromatin immunoprecipitation-sequencing analyses on porcine undifferentiated spermatogonia (which consist of SSCs and progenitors) and Di-SG. We identified that Di-SG exhibited less compact chromatin structural organization, weakened compartmentalization, and diminished topologically associating domains in comparison with undifferentiated spermatogonia, suggesting that diminished higher-order chromatin architecture in meiotic cells, as shown by recent reports, might be preprogrammed in Di-SG. Our data also revealed that A/B compartments, representing open or closed chromatin regions respectively, and topologically associating domains were related to dynamic gene expression during spermatogonial differentiation. Furthermore, we unraveled the contribution of promoter-enhancer interactions to premeiotic transcriptional regulation, which has not been accomplished in previous studies due to limited cell input and resolution. Together, our study uncovered the three-dimensional chromatin structure of SSCs/progenitors and Di-SG, as well as the interplay between higher-order chromatin architecture and dynamic gene expression during spermatogonial differentiation. These findings provide novel insights into the mechanisms for SSC self-renewal and differentiation and have implications for diagnosis and treatment of male sub-/infertility.

Non-coding RNAs and chromatin: key epigenetic factors from spermatogenesis to transgenerational inheritance

Cellular fate and gene expression patterns are modulated by different epigenetic factors including non-coding RNAs (ncRNAs) and chromatin organization. Both factors are dynamic throughout male germ cell differentiation on the seminiferous tubule, despite the transcriptional inactivation in the last stages of spermatogenesis. Sperm maturation during the caput-to-cauda transit on the epididymis involves changes in chromatin organization and the soma-to-germ line transference of ncRNAs that are essential to obtain a functional sperm for fertilization and embryo development. Here, the male environment (diseases, drugs, mental stress) is crucial to modulate these epigenetic factors throughout sperm maturation, affecting the corresponding offspring. Paternal transgenerational inheritance has been directly related to sperm epigenetic changes, most of them associated with variations in the ncRNA content and chromatin marks. Our aim is to give an overview about how epigenetics, focused on ncRNAs and chromatin, is pivotal to understand spermatogenesis and sperm maturation, and how the male environment impacts the sperm epigenome modulating the offspring gene expression pattern.

Validation of baboon pluripotent cells as a model for translational stem cell research

Translation of stem cell therapies to the clinic will be most successful following optimization of efficacy and safety in appropriate preclinical model systems. Among available models, nonhuman primates (NHPs) provide the most accurate recapitulation of human anatomy, physiology, genetics and epigenetics. Here, we show that baboon pluripotent cells (PSCs) recapitulate key molecular features of human PSCs with greater accuracy than that found in PSCs from non-primate species such as mice. Specifically, baboon and human PSCs exhibit greater conservation of gene expression patterns, higher sequence and structural homology among pluripotency factors, more equivalent genome-wide patterns of histone and DNA methylation modifications, and similar maintenance of bivalent programming of developmental genes than that found between human and non-primate PSCs.

Evolutionary epigenomic analyses in mammalian early embryos reveal species-specific innovations and conserved principles of imprinting

While mouse remains the most popular model, the conservation of parental-to-embryonic epigenetic transition across mammals is poorly defined. Through analysis of oocytes and early embryos in human, bovine, porcine, rat, and mouse, we revealed remarkable species-specific innovations as no single animal model fully recapitulates the human epigenetic transition. In rodent oocytes, transcription-dependent DNA methylation allows methylation of maternal imprints but not intergenic paternal imprints. Unexpectedly, prevalent DNA hypermethylation, paralleled by H3K36me2/3, also occurs in nontranscribed regions in porcine and bovine oocytes, except for megabase-long “CpG continents (CGCs)” where imprinting control regions preferentially reside. Broad H3K4me3 and H3K27me3 domains exist in nonhuman oocytes, yet only rodent H3K27me3 survives beyond genome activation. Coincidently, regulatory elements preferentially evade H3K27me3 in rodent oocytes, and failure to do so causes aberrant embryonic gene repression. Hence, the diverse mammalian innovations of parental-to-embryonic transition center on a delicate “to-methylate-or-not” balance in establishing imprints while protecting other regulatory regions.

INO80 promotes H2A.Z occupancy to regulate cell fate transition in pluripotent stem cells

The INO80 chromatin remodeler is involved in many chromatin-dependent cellular functions. However, its role in pluripotency and cell fate transition is not fully defined. We examined the impact of Ino80 deletion in the naïve and primed pluripotent stem cells. We found that Ino80 deletion had minimal effect on self-renewal and gene expression in the naïve state, but led to cellular differentiation and de-repression of developmental genes in the transition toward and maintenance of the primed state. In the naïve state, INO80 pre-marked gene promoters that would adopt bivalent histone modifications by H3K4me3 and H3K27me3 upon transition into the primed state. In the primed state, in contrast to its known role in H2A.Z exchange, INO80 promoted H2A.Z occupancy at these bivalent promoters and facilitated H3K27me3 installation and maintenance as well as downstream gene repression. Together, our results identified an unexpected function of INO80 in H2A.Z deposition and gene regulation. We showed that INO80-dependent H2A.Z occupancy is a critical licensing step for the bivalent domains, and thereby uncovered an epigenetic mechanism by which chromatin remodeling, histone variant deposition and histone modification coordinately control cell fate.

Analysis of Local Chromatin States Reveals Gene Transcription Potential during Mouse Neural Progenitor Cell Differentiation

Chromatin dynamics play a critical role in cell fate determination and maintenance by regulating the expression of genes essential for development and differentiation. In mouse embryonic stem cells (mESCs), maintenance of pluripotency coincides with a poised chromatin state containing active and repressive histone modifications. However, the structural features of poised chromatin are largely uncharacterized. By adopting mild time-course MNase-seq with computational analysis, the low-compact chromatin in mESCs is featured in two groups: one in more open regions, corresponding to an active state, and the other enriched with bivalent histone modifications, considered the poised state. A parameter called the chromatin opening potential index (COPI) is also devised to quantify the transcription potential based on the dynamic changes of MNase-seq signals at promoter regions. Use of COPI provides effective prediction of gene activation potential and, more importantly, reveals a few developmental factors essential for mouse neural progenitor cell (NPC) differentiation.

Germ cell determination and the developmental origin of germ cell tumors

In each generation, the germline is tasked with producing somatic lineages that form the body, and segregating a population of cells for gametogenesis. During animal development, when do cells of the germline irreversibly commit to producing gametes? Integrating findings from diverse species, we conclude that the final commitment of the germline to gametogenesis - the process of germ cell determination - occurs after primordial germ cells (PGCs) colonize the gonads. Combining this understanding with medical findings, we present a model whereby germ cell tumors arise from cells that failed to undertake germ cell determination, regardless of their having colonized the gonads. We propose that the diversity of cell types present in these tumors reflects the broad developmental potential of migratory PGCs.

Histone H3 lysine 4 trimethylation in sperm is transmitted to the embryo and associated with diet-induced phenotypes in the offspring

A father's lifestyle impacts offspring health; yet, the underlying molecular mechanisms remain elusive. We hypothesized that a diet that changes methyl donor availability will alter the sperm and embryo epigenomes to impact embryonic gene expression and development. Here, we demonstrate that a folate-deficient (FD) diet alters histone H3 lysine 4 trimethylation (H3K4me3) in sperm at developmental genes and putative enhancers. A subset of H3K4me3 alterations in sperm are retained in the pre-implantation embryo and associated with deregulated embryonic gene expression. Using a genetic mouse model in which sires have pre-existing altered H3K4me2/3 in sperm, we show that a FD diet exacerbates alterations in sperm H3K4me3 and embryonic gene expression, leading to an increase in developmental defect severity. These findings imply that paternal H3K4me3 is transmitted to the embryo and influences gene expression and development. It further suggests that epigenetic errors can accumulate in sperm to worsen offspring developmental outcomes.

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.

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

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

Meiotic Cells Counteract Programmed Retrotransposon Activation via RNA-Binding Translational Repressor Assemblies

Retrotransposon proliferation poses a threat to germline integrity. While retrotransposons must be activated in developing germ cells in order to survive and propagate, how they are selectively activated in the context of meiosis is unclear. We demonstrate that the transcriptional activation of Ty3/Gypsy retrotransposons and host defense are controlled by master meiotic regulators. We show that budding yeast Ty3/Gypsy co-opts binding sites of the essential meiotic transcription factor Ndt80 upstream of the integration site, thereby tightly linking its transcriptional activation to meiotic progression. We also elucidate how yeast cells thwart Ty3/Gypsy proliferation by blocking translation of the retrotransposon mRNA using amyloid-like assemblies of the RNA-binding protein Rim4. In mammals, several inactive Ty3/Gypsy elements are undergoing domestication. We show that mammals utilize equivalent master meiotic regulators (Stra8, Mybl1, Dazl) to regulate Ty3/Gypsy-derived genes in developing gametes. Our findings inform how genes that are evolving from retrotransposons can build upon existing regulatory networks during domestication.

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.

Sperm histone H3 lysine 4 trimethylation is altered in a genetic mouse model of transgenerational epigenetic inheritance

Advancing the molecular knowledge surrounding fertility and inheritance has become critical given the halving of sperm counts in the last 40 years, and the rise in complex disease which cannot be explained by genetics alone. The connection between both these trends may lie in alterations to the sperm epigenome and occur through environmental exposures. Changes to the sperm epigenome are also associated with health risks across generations such as metabolic disorders and cancer. Thus, it is imperative to identify the epigenetic modifications that escape reprogramming during spermatogenesis and embryogenesis. Here, we aimed to identify the chromatin signature(s) involved in transgenerational phenotypes in our genetic mouse model of epigenetic inheritance that overexpresses the histone demethylase KDM1A in their germ cells. We used sperm-specific chromatin immunoprecipitation followed by in depth sequencing (ChIP-seq), and computational analysis to identify whether differential enrichment of histone H3 lysine 4 trimethylation (H3K4me3), and histone H3 lysine 27 trimethylation (H3K27me3) serve as mechanisms for transgenerational epigenetic inheritance through the paternal germline. Our analysis on the sperm of KDM1A transgenic males revealed specific changes in H3K4me3 enrichment that predominantly occurred independently from bivalent H3K4me3/H3K27me3 regions. Many regions with altered H3K4me3 enrichment in sperm were identified on the paternal allele of the pre-implantation embryo. These findings suggest that sperm H3K4me3 functions in the transmission of non-genetic phenotypes transgenerationally.

A dual enhancer-silencer element, DES-K16, in mouse spermatocyte-derived GC-2spd(ts) cells

The multifunctionality of genome is suggested at some loci in different species but not well understood. Here we identified a DES-K16 region in an intron of the Kctd16 gene as the chromatin highly marked with epigenetic modifications of both enhancers (H3K4me1 and H3K27ac) and silencers (H3K27me3) in mouse spermatocytes. In vitro reporter gene assay demonstrated that DES-K16 exhibited significant enhancer activity in spermatocyte-derived GC-2spd(ts) and hepatic tumor-derived Hepa1-6 cells, and a deletion of this sequence in GC-2spd(ts) cells resulted in a decrease and increase of Yipf5 and Kctd16 expression, respectively. This was consistent with increased and decreased expression of Yipf5 and Kctd16, respectively, in primary spermatocytes during testis development. While known dual enhancer-silencers exert each activity in different tissues, our data suggest that DES-K16 functions as both enhancer and silencer in a single cell type, GC-2spd(ts) cells. This is the first report on a dual enhancer-silencer element which activates and suppresses gene expression in a single cell type.

Unique Epigenetic Programming Distinguishes Regenerative Spermatogonial Stem Cells in the Developing Mouse Testis

Spermatogonial stem cells (SSCs) both self-renew and give rise to progenitors that initiate spermatogenic differentiation in the mammalian testis. Questions remain regarding the extent to which the SSC and progenitor states are functionally distinct. Here we provide the first multiparametric integrative analysis of mammalian germ cell epigenomes comparable with that done for >100 somatic cell types by the ENCODE Project. Differentially expressed genes distinguishing SSC- and progenitor-enriched spermatogonia showed distinct histone modification patterns, particularly for H3K27ac and H3K27me3. Motif analysis predicted transcription factors that may regulate spermatogonial subtype-specific fate, and immunohistochemistry and gene-specific chromatin immunoprecipitation analyses confirmed subtype-specific differences in target gene binding of a subset of these factors. Taken together, these results show that SSCs and progenitors display distinct epigenetic profiling consistent with these spermatogonial subtypes being differentially programmed to either self-renew and maintain regenerative capacity as SSCs or lose regenerative capacity and initiate lineage commitment as progenitors.

Endogenous retroviruses drive species-specific germline transcriptomes in mammals

Gene regulation in the germline ensures the production of high-quality gametes, long-term maintenance of the species, and speciation. Male germline transcriptomes undergo dynamic changes after the mitosis-to-meiosis transition and have been subject to evolutionary divergence among mammals. However, the mechanisms underlying germline regulatory divergence remain undetermined. Here, we show that endogenous retroviruses (ERVs) influence species-specific germline transcriptomes. After the mitosis-to-meiosis transition in male mice, specific ERVs function as active enhancers to drive germline genes, including a mouse-specific gene set, and bear binding motifs for critical regulators of spermatogenesis such as A-MYB. This raises the possibility that a genome-wide transposition of ERVs rewired germline gene expression in a species-specific manner. Of note, independently evolved ERVs are associated with the expression of human-specific germline genes, demonstrating the prevalence of ERV-driven mechanisms in mammals. Together, we propose that ERVs fine-tune species-specific transcriptomes in the mammalian germline.

Torpor expression is associated with differential spermatogenesis in hibernating eastern chipmunks

Hibernators suppress physiological processes when expressing torpor, yet little is known about the effects of torpor on male reproductive physiology. Studies of hibernating mammals suggest that deep torpor negatively impacts spermatogenesis and that transitions between torpor and euthermic arousals increase cellular oxidative stress, with potentially damaging effects on sperm. Here, we hypothesize that variation in torpor expression affects the reproductive readiness of hibernators by impacting their sperm production. To test this, we examined the relationship between torpor expression and spermatogenesis in captive eastern chipmunks (Tamias striatus). We determined torpor depth with temperature data loggers and assessed its relationship with spermatogenesis by examining spermatogenic progression, cell division, sperm counts, sperm maturity, and DNA damage. We show that deep hibernators (high levels of torpor) largely halted spermatogenesis in late hibernation in comparison with shallow hibernators (low levels of torpor), where ongoing spermatogenesis was observed. Despite these differences in spermatogenic state during hibernation, spermatogenic progression, sperm numbers, and maturity did not differ in spring, potentially reflecting similar degrees of reproductive readiness. Interestingly, shallow hibernators exhibited higher rates of DNA damage in spermatogenic cells during hibernation, with this trend reversing in spring. Our results thus indicate that once heterothermy is terminated, deep hibernators resume spermatogenesis but are characterized by higher rates of DNA damage in spermatogenic cells at the seasonal stage when spring mating commences. Therefore, our study confirmed posthibernation recovery of sperm production but also a potential impact of deep torpor expression during winter on DNA damage in spring.

Two Separation-of-Function Isoforms of Human TPP1 Dictate Telomerase Regulation in Somatic and Germ Cells

Telomerase replicates chromosome ends in germ and somatic stem cells to facilitate their continued proliferation. Telomerase action depends on the telomeric protein TPP1, which recruits telomerase to telomeres and facilitates processive DNA synthesis. Here, we identify separation-of-function long (TPP1-L) and short (TPP1-S) isoforms of TPP1 that appear to be generated from separate transcripts and differ only in 86 amino acids at their N terminus. Although both isoforms retain the ability to recruit telomerase, only TPP1-S facilitates efficient telomere synthesis. We find that TPP1-S is the predominant isoform in somatic cells, and strikingly, TPP1-L is the major isoform in differentiated male germ cells. We observed that TERT expression persists in these germ cells, suggesting that TPP1-L could restrain telomerase in this context. We show how differential expression of TPP1 isoforms determines telomerase function and demonstrate how alternative transcription start sites allow one gene to perform distinct functions in different biological contexts.

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.

Conservation, acquisition, and functional impact of sex-biased gene expression in mammals

Sex differences abound in human health and disease, as they do in other mammals used as models. The extent to which sex differences are conserved at the molecular level across species and tissues is unknown. We surveyed sex differences in gene expression in human, macaque, mouse, rat, and dog, across 12 tissues. In each tissue, we identified hundreds of genes with conserved sex-biased expression-findings that, combined with genomic analyses of human height, explain ~12% of the difference in height between females and males. We surmise that conserved sex biases in expression of genes otherwise operating equivalently in females and males contribute to sex differences in traits. However, most sex-biased expression arose during the mammalian radiation, which suggests that careful attention to interspecies divergence is needed when modeling human sex differences.

DNA methylation in bull spermatozoa: evolutionary impacts, interindividual variability, and contribution to the embryo

The DNA methylome of spermatozoa results from a unique epigenetic reprogramming crucial for chromatin compaction and the protection of the paternal genetic heritage. Although bull semen is widely used for artificial insemination (AI), little is known about the sperm epigenome in cattle. The purpose of this review is to synthetize recent work on the bull sperm methylome in light of the knowledge accumulated in humans and model species. We will address sperm-specific DNA methylation features and their potential evolutionary impacts, with particular emphasis on hypomethylated regions and repetitive elements. We will review recent examples of interindividual variability and intra-individual plasticity of the bull sperm methylome as related to fertility and age, respectively. Finally, we will address paternal methylome reprogramming after fertilization, as well as the mechanisms potentially involved in epigenetic inheritance, and provide some examples of disturbances that alter the dynamics of reprogramming in cattle. Because the selection of AI bulls is closely based on their genotypes, we will also discuss the complex interplay between sequence polymorphism and DNA methylation, which represents both a difficulty in addressing the role of DNA methylation in shaping phenotypes and an opportunity to better understand genome plasticity.

The Role of Chromatin Accessibility in cis-Regulatory Evolution

Transcription factor (TF) binding is determined by sequence as well as chromatin accessibility. Although the role of accessibility in shaping TF-binding landscapes is well recorded, its role in evolutionary divergence of TF binding, which in turn can alter cis-regulatory activities, is not well understood. In this work, we studied the evolution of genome-wide binding landscapes of five major TFs in the core network of mesoderm specification, between Drosophila melanogaster and Drosophila virilis, and examined its relationship to accessibility and sequence-level changes. We generated chromatin accessibility data from three important stages of embryogenesis in both Drosophila melanogaster and Drosophila virilis and recorded conservation and divergence patterns. We then used multivariable models to correlate accessibility and sequence changes to TF-binding divergence. We found that accessibility changes can in some cases, for example, for the master regulator Twist and for earlier developmental stages, more accurately predict binding change than is possible using TF-binding motif changes between orthologous enhancers. Accessibility changes also explain a significant portion of the codivergence of TF pairs. We noted that accessibility and motif changes offer complementary views of the evolution of TF binding and developed a combined model that captures the evolutionary data much more accurately than either view alone. Finally, we trained machine learning models to predict enhancer activity from TF binding and used these functional models to argue that motif and accessibility-based predictors of TF-binding change can substitute for experimentally measured binding change, for the purpose of predicting evolutionary changes in enhancer activity.

Putative contributions of the sex chromosome proteins SOX3 and SRY to neurodevelopmental disorders

The male-biased prevalence of certain neurodevelopmental disorders and the sex-biased outcomes associated with stress exposure during gestation have been previously described. Here, we hypothesized that genes distinctively targeted by only one or both homologous proteins highly conserved across therian mammals, SOX3 and SRY, could induce sexual adaptive changes that result in a differential risk for neurodevelopmental disorders. ChIP-seq/chip data showed that SOX3/SRY gene targets were expressed in different brain cell types in mice. We used orthologous human genes in rodent genomes to extend the number of SOX3/SRY set (1,721). These genes were later found to be enriched in five modules of coexpressed genes during the early and mid-gestation periods (FDR < 0.05), independent of sexual hormones. Genes with differential expression (24, p < 0.0001) and methylation (40, p < 0.047) between sexes were overrepresented in this set. Exclusive SOX3 or SRY target genes were more associated with the late gestational and postnatal periods. Using autism as a model sex-biased disorder, the SOX3/SRY set was enriched in autism gene databases (FDR ≤ 0.05), and there were more de novo variations from the male autism spectrum disorder (ASD) samples under the SRY peaks compared to the random peaks (p < 0.024). The comparison of coexpressed networks of SOX3/SRY target genes between male autism and control samples revealed low preservation in gene modules related to stress response (99 genes) and neurogenesis (78 genes). This study provides evidence that while SOX3 is a regulatory mechanism for both sexes, the male-exclusive SRY also plays a role in gene regulation, suggesting a potential mechanism for sex bias in ASD.

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.

EpiAlignment: alignment with both DNA sequence and epigenomic data

Comparative epigenomics, which subjects both epigenome and genome to interspecies comparison, has become a powerful approach to reveal regulatory features of the genome. Thus elucidated regulatory features surpass the information derived from comparison of genomic sequences alone. Here, we present EpiAlignment, a web-based tool to align genomic regions with both DNA sequence and epigenomic data. EpiAlignment takes DNA sequence and epigenomic profiles derived by ChIP-seq from two species as input data, and outputs the best semi-global alignments. These alignments are based on EpiAlignment scores, computed by a dynamic programming algorithm that accounts for both sequence alignment and epigenome similarity. For timely response, the EpiAlignment web server automatically initiates up to 140 computing threads depending on the size of user input data. For users’ convenience, we have pre-compiled the comparable human and mouse epigenome datasets in matched cell types and tissues from the Roadmap Epigenomics and ENCODE consortia. Users can either upload their own data or select pre-compiled datasets as inputs for EpiAlignment analyses. Results are presented in graphical and tabular formats where the entries can be interactively expanded to visualize additional features of these aligned regions. EpiAlignment is available at https://epialign.ucsd.edu/.

On the Regulatory Evolution of New Genes Throughout Their Life History

Every gene has a birthplace and an age, that is, a cis-regulatory environment and an evolution lifespan since its origination, yet how the two shape the evolution trajectories of genes remains unclear. Here, we address this basic question by comparing phylogenetically dated new genes in the context of both their ages and origination mechanisms. In both Drosophila and vertebrates, we confirm a clear "out of the testis" transition from the specifically expressed young genes to the broadly expressed old housekeeping genes, observed only in testis but not in other tissues. Many new genes have gained important functions during embryogenesis, manifested as either specific activation at maternal-zygotic transition, or different spatiotemporal expressions from their parental genes. These expression patterns are largely driven by an age-dependent evolution of cis-regulatory environment. We discover that retrogenes are more frequently born in a pre-existing repressive regulatory domain, and are more diverged in their enhancer repertoire than the DNA-based gene duplications. During evolution, new gene duplications gradually gain active histone modifications and undergo more enhancer turnovers when becoming older, but exhibit complex trends of gaining or losing repressive histone modifications in Drosophila or vertebrates, respectively. Interestingly, vertebrate new genes exhibit an "into the testis" epigenetic transition that older genes become more likely to be co-occupied by both active and repressive ("bivalent") histone modifications specifically in testis. Our results uncover the regulatory mechanisms underpinning the stepwise acquisition of novel and complex functions by new genes, and illuminate the general evolution trajectory of genes throughout their life history.

Reprogramming of Meiotic Chromatin Architecture during Spermatogenesis

Chromatin organization undergoes drastic reconfiguration during gametogenesis. However, the molecular reprogramming of three-dimensional chromatin structure in this process remains poorly understood for mammals, including primates. Here, we examined three-dimensional chromatin architecture during spermatogenesis in rhesus monkey using low-input Hi-C. Interestingly, we found that topologically associating domains (TADs) undergo dissolution and reestablishment in spermatogenesis. Strikingly, pachytene spermatocytes, where synapsis occurs, are strongly depleted for TADs despite their active transcription state but uniquely show highly refined local compartments that alternate between transcribing and non-transcribing regions (refined-A/B). Importantly, such chromatin organization is conserved in mouse, where it remains largely intact upon transcription inhibition. Instead, it is attenuated in mutant spermatocytes, where the synaptonemal complex failed to be established. Intriguingly, this is accompanied by the restoration of TADs, suggesting that the synaptonemal complex may restrict TADs and promote local compartments. Thus, these data revealed extensive reprogramming of higher-order meiotic chromatin architecture during mammalian gametogenesis.

Methylome Dynamics of Bovine Gametes and in vivo Early Embryos

DNA methylation undergoes drastic fluctuation during early mammalian embryogenesis. The dynamics of global DNA methylation in bovine embryos, however, have mostly been studied by immunostaining. We adopted the whole genome bisulfite sequencing (WGBS) method to characterize stage-specific genome-wide DNA methylation in bovine sperm, immature oocytes, oocytes matured in vivo and in vitro, as well as in vivo developed single embryos at the 2-, 4-, 8-, and 16-cell stages. We found that the major wave of genome-wide DNA demethylation was complete by the 8-cell stage when de novo methylation became prominent. Sperm and oocytes were differentially methylated in numerous regions (DMRs), which were primarily intergenic, suggesting that these non-coding regions may play important roles in gamete specification. DMRs were also identified between in vivo and in vitro matured oocytes, suggesting environmental effects on epigenetic modifications. In addition, virtually no (less than 1.5%) DNA methylation was found in mitochondrial DNA. Finally, by using RNA-seq data generated from embryos at the same developmental stages, we revealed a weak inverse correlation between gene expression and promoter methylation. This comprehensive analysis provides insight into the critical features of the bovine embryo methylome, and serves as an important reference for embryos produced in vitro, such as by in vitro fertilization and cloning. Lastly, these data can also provide a model for the epigenetic dynamics in human early embryos.

Bivalency in Drosophila embryos is associated with strong inducibility of Polycomb target genes

Polycomb group (PcG) and Trithorax group (TrxG) proteins orchestrate development of a multicellular organism by faithfully maintaining cell fate decisions made early in embryogenesis. An important chromatin mark connected to PcG/TrxG regulation is bivalent domains, the simultaneous presence of H3K27me3 and H3K4me3 on a given locus, originally identified in mammalian embryonic stem cells but considered to be absent in invertebrates. Here, we provide evidence for the existence of bivalency in fly embryos. Using a recently described PcG reporter fly line, we observed a strong reporter inducibility in the embryo and its sharp decrease in larval and adult stages. Analysis of the chromatin landscape of the reporter revealed a strong signal for the repressive PcG mark, H3K27me3, in all three developmental stages and, surprisingly, a strong signal for a transcriptionally activating H3K4me3 mark in the embryo. Using re-chromatin immunoprecipitation experiments, bivalent domains were also uncovered at endogenous PcG targets like the Hox genes.

Pach-ing It in: The Peculiar Organization of Mammalian Pachytene Chromosomes

In this issue of Molecular Cell, Wang et al. (2019) use Hi-C to visualize at high resolution the complex reprogramming of chromatin architecture during spermatogenesis in rhesus monkeys and mice. They find that pachytene spermatocytes have a unique chromosome organization that may result from the presence of the synaptonemal complex and transcription-associated proteins.

Germ cell reprogramming

Germ cells undergo epigenome reprogramming for proper development of the next generation. The achievement of in vitro germ cell derivation from human and mouse pluripotent stem cells and further differentiation in a plane culture and in aggregation with gonadal somatic cells offers unprecedented opportunities for investigation of the germ cell development. Moreover, advances in low-input/single-cell genomics have enabled detailed investigation of epigenome dynamics during germ cell development. These technologies have advanced our knowledge of epigenome reprogramming during the specification and development of primordial germ cells, their sex differentiation, and gametogenesis. Key findings include details of chromatin remodeling and transcriptional regulation, progressive and comprehensive DNA demethylation, and tight links between DNA demethylation and histone marks during the development of primordial germ cells, acquisition of unique totipotent epigenome during oogenesis (e.g., broad H3K4me3 domains and low-level three-dimensional genomic organization), and unexpected organization of the sperm genome. Moreover, these studies suggest the importance of epigenome analyses for in-depth evaluations of in vitro gametogenesis.

Intergenerational epigenetic inheritance of cancer susceptibility in mammals

Susceptibility to cancer is heritable, but much of this heritability remains unexplained. Some 'missing' heritability may be mediated by epigenetic changes in the parental germ line that do not involve transmission of genetic variants from parent to offspring. We report that deletion of the chromatin regulator Kdm6a (Utx) in the paternal germ line results in elevated tumor incidence in genetically wild type mice. This effect increases following passage through two successive generations of Kdm6a male germline deletion, but is lost following passage through a wild type germ line. The H3K27me3 mark is redistributed in sperm of Kdm6a mutants, and we define approximately 200 H3K27me3-marked regions that exhibit increased DNA methylation, both in sperm of Kdm6a mutants and in somatic tissue of progeny. Hypermethylated regions in enhancers may alter regulation of genes involved in cancer initiation or progression. Epigenetic changes in male gametes may therefore impact cancer susceptibility in adult offspring.

Reprogramming histone modification patterns to coordinate gene expression in early zebrafish embryos

BACKGROUND

Multicellular organisms require precise gene regulation during ontogeny, and epigenetic modifications, such as DNA methylation and histone modification, facilitate this precise regulation. The conservative reprogramming patterns of DNA methylation in vertebrates have been well described. However, knowledge of how histone modifications are passed on from gametes to early embryos is limited, and whether histone modification reprogramming is conserved is not clear.

RESULTS

We profiled H3K4me3/H3K27me3 modifications in gametes and early embryos in zebrafish and found that the patterns in gene promoter regions have been largely set to either co-occupied or active states in gametes and then passed on to early embryos. Co-occupied states are partially maintained, while active states are largely restored to nearly match the sperm's pattern prior to zygotic genome activation (ZGA). However, repressive H3K27me3 modifications in promoter regions are largely discarded in early embryos. Prior to ZGA, patterns of genes that initialize ZGA are converted to nonrepressive states to coordinate gene expression. Moreover, promoter peaks that mark stage-specific genes are hypermethylated, and histone modifications in these regions are erased independently of DNA methylation reprogramming. Furthermore, comparative analysis revealed that the functions of co-occupied and active genes passed on from gametes are conserved in vertebrates. Gene age preferences by co-occupied and active histone modifications are also confirmed in vertebrates.

CONCLUSIONS

Our data provide fundamental resources for understanding H3K4me3/H3K27me3 modifications in early zebrafish embryos. The data also reveal that the reprogramming progress of histone modifications is conserved in vertebrates and coordinates with gene expression during ZGA.