H3K79 methylation directly precedes the histone-to-protamine transition in mammalian spermatids and is sensitive to bacterial infections

Insights into the sperm chromatin and implications for male infertility from a protein perspective

Male germ cells undergo an extreme but fascinating process of chromatin remodeling that begins in the testis during the last phase of spermatogenesis and continues through epididymal sperm maturation. Most of the histones are replaced by small proteins named protamines, whose high basicity leads to a tight genomic compaction. This process is epigenetically regulated at many levels, not only by posttranslational modifications, but also by readers, writers, and erasers, in a context of a highly coordinated postmeiotic gene expression program. Protamines are key proteins for acquiring this highly specialized chromatin conformation, needed for sperm functionality. Interestingly, and contrary to what could be inferred from its very specific DNA-packaging function across protamine-containing species, human sperm chromatin contains a wide spectrum of protamine proteoforms, including truncated and posttranslationally modified proteoforms. The generation of protamine knock-out models revealed not only chromatin compaction defects, but also collateral sperm alterations contributing to infertile phenotypes, evidencing the importance of sperm chromatin protamination toward the generation of a new individual. The unique features of sperm chromatin have motivated its study, applying from conventional to the most ground-breaking techniques to disentangle its peculiarities and the cellular mechanisms governing its successful conferment, especially relevant from the protein point of view due to the important epigenetic role of sperm nuclear proteins. Gathering and contextualizing the most striking discoveries will provide a global understanding of the importance and complexity of achieving a proper chromatin compaction and exploring its implications on postfertilization events and beyond. This article is categorized under: Reproductive System Diseases > Genetics/Genomics/Epigenetics Reproductive System Diseases > Molecular and Cellular Physiology.

Epigenetic markers in the embryonal germ cell development and spermatogenesis

Spermatogenesis is the process of generation of male reproductive cells from spermatogonial stem cells in the seminiferous epithelium of the testis. During spermatogenesis, key spermatogenic events such as stem cell self-renewal and commitment to meiosis, meiotic recombination, meiotic sex chromosome inactivation, followed by cellular and chromatin remodeling of elongating spermatids occur, leading to sperm cell production. All the mentioned events are at least partially controlled by the epigenetic modifications of DNA and histones. Additionally, during embryonal development in primordial germ cells, global epigenetic reprogramming of DNA occurs. In this review, we summarized the most important epigenetic modifications in the particular stages of germ cell development, in DNA and histone proteins, starting from primordial germ cells, during embryonal development, and ending with histone-to-protamine transition during spermiogenesis.

Transcriptome profiling of histone writers/erasers enzymes across spermatogenesis, mature sperm and pre-cleavage embryo: Implications in paternal epigenome transitions and inheritance mechanisms

Accumulating evidence points out that sperm carry epigenetic instructions to embryo in the form of retained histones marks and RNA cargo that can transmit metabolic and behavioral traits to offspring. However, the mechanisms behind epigenetic inheritance of paternal environment are still poorly understood. Here, we curated male germ cells RNA-seq data and analyzed the expression profile of all known histone lysine writers and erasers enzymes across spermatogenesis, unraveling the developmental windows at which they are upregulated, and the specific activity related to canonical and non-canonical histone marks deposition and removal. We also characterized the epigenetic enzymes signature in the mature sperm RNA cargo, showing most of them positive translation at pre-cleavage zygote, suggesting that paternally-derived enzymes mRNA cooperate with maternal factors to embryo chromatin assembly. Our study shows several histone modifying enzymes not described yet in spermatogenesis and even more, important mechanistic aspects behind transgenerational epigenetics. Epigenetic enzymes not only can respond to environmental stressors, but could function as vectors of epigenetic information and participate in chromatin organization during maternal-to-zygote transition.

Abnormal histone replacement following BPA exposure affects spermatogenesis and fertility sequentially

Bisphenol A (BPA) is an endocrine-disrupting chemical widely distributed in the environment. Its exposure has been linked to male infertility in animals and humans due to its ability to induce epigenetic modification. Despite extensive research confirming the impact of BPA on epigenetic regulation, fundamental concerns about how BPA causes epigenetic changes and the underlying mechanism of BPA on the male reproductive system remain unresolved. Therefore, we sought to investigate the effects of BPA on epigenetic regulation and the histone-to-protamine (PRM) transition, which is fundamental process for male fertility in testes and spermatozoa by exposing male mice to BPA for 6 weeks while giving the mice in the control group corn oil by oral gavage. Our results demonstrated that the mRNA levels of the histone family and PRMs were significantly altered by BPA exposure in testes and spermatozoa. Subsequently, core histone proteins, the PRM1/PRM2 ratio, directly linked to male fertility, and transition proteins were significantly reduced. Furthermore, we discovered that BPA significantly caused abnormal histone-to-protamine replacement during spermiogenesis by increased histone variants-related to histone-to-PRM transition. The levels of histone H3 modification in the testes and DNA methylation in spermatozoa were significantly increased. Consequently, sperm concentration/motility/hyperactivation, fertilization, and early embryonic development were adversely affected as a consequence of altered signaling proteins following BPA exposure. To our knowledge, this is the first study to indicate that BPA exposure influences the histone-to-PRM transition via altering epigenetic modification and eventually causing reduced male fertility.

Connecting the DOTs on Cell Identity

DOT1-Like (DOT1L) is the sole methyltransferase of histone H3K79, a modification enriched mainly on the bodies of actively transcribing genes. DOT1L has been extensively studied in leukemia were some of the most frequent onco-fusion proteins contain portions of DOT1L associated factors that mislocalize H3K79 methylation and drive oncogenesis. However, the role of DOT1L in non-transformed, developmental contexts is less clear. Here we assess the known functional roles of DOT1L both in vitro cell culture and in vivo models of mammalian development. DOT1L is evicted during the 2-cell stage when cells are totipotent and massive epigenetic and transcriptional alterations occur. Embryonic stem cell lines that are derived from the blastocyst tolerate the loss of DOT1L, while the reduction of DOT1L protein levels or its catalytic activity greatly enhances somatic cell reprogramming to induced pluripotent stem cells. DOT1L knockout mice are embryonically lethal when organogenesis commences. We catalog the rapidly increasing studies of total and lineage specific knockout model systems that show that DOT1L is broadly required for differentiation. Reduced DOT1L activity is concomitant with increased developmental potential. Contrary to what would be expected of a modification that is associated with active transcription, loss of DOT1L activity results in more upregulated than downregulated genes. DOT1L also participates in various epigenetic networks that are both cell type and developmental stage specific. Taken together, the functions of DOT1L during development are pleiotropic and involve gene regulation at the locus specific and global levels.

Advanced molecular approaches in male infertility diagnosis

In the recent years a special attention has been given to a major health concern namely to male infertility, defined as the inability to conceive after 12 months of regular unprotected sexual intercourse, taken into account the statistics that highlight that sperm counts have dropped by 50-60% in recent decades. According to the WHO, infertility affects approximately 9% of couples globally, and the male factor is believed to be present in roughly 50% of cases, with exclusive responsibility in 30%. The aim of this manuscript is to present an evidence-based approach for diagnosing male infertility that includes finding new solutions for diagnosis and critical outcomes, retrieving up-to-date studies and existing guidelines. The diverse factors that induce male infertility generated in a vast amount of data that needed to be analysed by a clinician before a decision could be made for each individual. Modern medicine faces numerous obstacles as a result of the massive amount of data generated by the molecular biology discipline. To address complex clinical problems, vast data must be collected, analysed, and used, which can be very challenging. The use of artificial intelligence (AI) methods to create a decision support system can help predict the diagnosis and guide treatment for infertile men, based on analysis of different data as environmental and lifestyle, clinical (sperm count, morphology, hormone testing, karyotype, etc.) and "omics" bigdata. Ultimately, the development of AI algorithms will assist clinicians in formulating diagnosis, making treatment decisions, and predicting outcomes for assisted reproduction techniques.

Impact of Endocrine Disruptors upon Non-Genetic Inheritance

Similar to environmental factors, EDCs (endocrine-disrupting chemicals) can influence gene expression without modifying the DNA sequence. It is commonly accepted that the transgenerational inheritance of parentally acquired traits is conveyed by epigenetic alterations also known as “epimutations”. DNA methylation, acetylation, histone modification, RNA-mediated effects and extracellular vesicle effects are the mechanisms that have been described so far to be responsible for these epimutations. They may lead to the transgenerational inheritance of diverse phenotypes in the progeny when they occur in the germ cells of an affected individual. While EDC-induced health effects have dramatically increased over the past decade, limited effects on sperm epigenetics have been described. However, there has been a gain of interest in this issue in recent years. The gametes (sperm and oocyte) represent targets for EDCs and thus a route for environmentally induced changes over several generations. This review aims at providing an overview of the epigenetic mechanisms that might be implicated in this transgenerational inheritance.

The Art of Packaging the Sperm Genome: Molecular and Structural Basis of the Histone-To-Protamine Exchange

Male fertility throughout life hinges on the successful production of motile sperm, a developmental process that involves three coordinated transitions: mitosis, meiosis, and spermiogenesis. Germ cells undergo both mitosis and meiosis to generate haploid round spermatids, in which histones bound to the male genome are replaced with small nuclear proteins known as protamines. During this transformation, the chromatin undergoes extensive remodeling to become highly compacted in the sperm head. Despite its central role in spermiogenesis and fertility, we lack a comprehensive understanding of the molecular mechanisms underlying the remodeling process, including which remodelers/chaperones are involved, and whether intermediate chromatin proteins function as discrete steps, or unite simultaneously to drive successful exchange. Furthermore, it remains largely unknown whether more nuanced interactions instructed by protamine post-translational modifications affect chromatin dynamics or gene expression in the early embryo. Here, we bring together past and more recent work to explore these topics and suggest future studies that will elevate our understanding of the molecular basis of the histone-to-protamine exchange and the underlying etiology of idiopathic male infertility.

Battle of the Sex Chromosomes: Competition between X and Y Chromosome-Encoded Proteins for Partner Interaction and Chromatin Occupancy Drives Multicopy Gene Expression and Evolution in Muroid Rodents

Transmission distorters (TDs) are genetic elements that favor their own transmission to the detriments of others. Slx/Slxl1 (Sycp3-like-X-linked and Slx-like1) and Sly (Sycp3-like-Y-linked) are TDs, which have been coamplified on the X and Y chromosomes of Mus species. They are involved in an intragenomic conflict in which each favors its own transmission, resulting in sex ratio distortion of the progeny when Slx/Slxl1 versus Sly copy number is unbalanced. They are specifically expressed in male postmeiotic gametes (spermatids) and have opposite effects on gene expression: Sly knockdown leads to the upregulation of hundreds of spermatid-expressed genes, whereas Slx/Slxl1-deficiency downregulates them. When both Slx/Slxl1 and Sly are knocked down, sex ratio distortion and gene deregulation are corrected. Slx/Slxl1 and Sly are, therefore, in competition but the molecular mechanism remains unknown. By comparing their chromatin-binding profiles and protein partners, we show that SLX/SLXL1 and SLY proteins compete for interaction with H3K4me3-reader SSTY1 (Spermiogenesis-specific-transcript-on-the-Y1) at the promoter of thousands of genes to drive their expression, and that the opposite effect they have on gene expression is mediated by different abilities to recruit SMRT/N-Cor transcriptional complex. Their target genes are predominantly spermatid-specific multicopy genes encoded by the sex chromosomes and the autosomal Speer/Takusan. Many of them have coamplified with not only Slx/Slxl1/Sly but also Ssty during muroid rodent evolution. Overall, we identify Ssty as a key element of the X versus Y intragenomic conflict, which may have influenced gene content and hybrid sterility beyond Mus lineage since Ssty amplification on the Y predated that of Slx/Slxl1/Sly.

Genome-wide occupancy reveals the localization of H1T2 (H1fnt) to repeat regions and a subset of transcriptionally active chromatin domains in rat spermatids

Background

H1T2/H1FNT is a germ cell-specific linker histone variant expressed during spermiogenesis specifically in round and elongating spermatids. Infertile phenotype of homozygous H1T2 mutant male mice revealed the essential function of H1T2 for the DNA condensation and histone-to-protamine replacement in spermiogenesis. However, the mechanism by which H1T2 imparts the inherent polarity within spermatid nucleus including the additional protein partners and the genomic domains occupied by this linker histone are unknown.

Results

Sequence analysis revealed the presence of Walker motif, SR domains and putative coiled-coil domains in the C-terminal domain of rat H1T2 protein. Genome-wide occupancy analysis using highly specific antibody against the CTD of H1T2 demonstrated the binding of H1T2 to the LINE L1 repeat elements and to a significant percentage of the genic regions (promoter-TSS, exons and introns) of the rat spermatid genome. Immunoprecipitation followed by mass spectrometry analysis revealed the open chromatin architecture of H1T2 occupied chromatin encompassing the H4 acetylation and other histone PTMs characteristic of transcriptionally active chromatin. In addition, the present study has identified the interacting protein partners of H1T2-associated chromatin mainly as nucleo-skeleton components, RNA-binding proteins and chaperones.

Conclusions

Linker histone H1T2 possesses unique domain architecture which can account for the specific functions associated with chromatin remodeling events facilitating the initiation of histone to transition proteins/protamine transition in the polar apical spermatid genome. Our results directly establish the unique function of H1T2 in nuclear shaping associated with spermiogenesis by mediating the interaction between chromatin and nucleo-skeleton, positioning the epigenetically specialized chromatin domains involved in transcription coupled histone replacement initiation towards the apical pole of round/elongating spermatids.

Maternal DOT1L is dispensable for mouse development

A battery of chromatin modifying enzymes play essential roles in remodeling the epigenome in the zygote and cleavage stage embryos, when the maternal genome is the sole contributor. Here we identify an exemption. DOT1L methylates lysine 79 in the globular domain of histone H3 (H3K79). Dot1l is an essential gene, as homozygous null mutant mouse embryos exhibit multiple developmental abnormalities and die before 11.5 days of gestation. To test if maternally deposited DOT1L is required for embryo development, we carried out a conditional Dot1l knockout in growing oocytes using the Zona pellucida 3-Cre (Zp3-Cre) transgenic mice. We found that the resulting maternal mutant Dot1l^mat−/+ offspring displayed normal development and fertility, suggesting that the expression of the paternally inherited copy of Dot1l in the embryo is sufficient to support development. In addition, Dot1l maternal deletion did not affect the parental allele-specific expression of imprinted genes, indicating that DOT1L is not needed for imprint establishment in the oocyte or imprint protection in the zygote. In summary, uniquely and as opposed to other histone methyltransferases and histone marks, maternal DOT1L deposition and H3K79 methylation in the zygote and in the preimplantation stage embryo is dispensable for mouse development.

Bacterial infection of the male reproductive system causing infertility

Bacterial infections play a disruptive and hidden role in male reproductive failure. Different kinds of bacteria are often able to interfere with reproductive function in both sexes and lead to infertility. In this study, to further evaluate the role of bacterial infections in male reproduction we provided an extensive overview of so far researches investigating the effects of bacterial infections on male fertility. We searched Medline, PubMed, Scopus and Google scholar databases to identify the potentially relevant studies on bacterial infections and their implications in male infertility. All the bacteria included in this article have negative effects on the male reproductive function; however, there is ample evidence to blame bacteria such as Escherichia coli, Chlamydia trachomatis, Ureaplasma, Mycoplasma and Staphylococcus aureus for reduced fertility and deterioration of sperm parameters. More studies are needed to clarify the molecular mechanisms by which different bacteria exert their detrimental effects on male reproductive system. Getting more insight into probable mechanisms, would significantly facilitate the production of new, advanced, and effective remedies in the future. In view of all evidence, we strongly suggest increasing awareness among people and considering screening programs for patients seeking fertility both to avoid transmission and to improve fertility outcomes among them.

Comprehensive histochemical profiles of histone modification in male germline cells during meiosis and spermiogenesis: Comparison of young and aged testes in mice

Human epidemiological studies have shown that paternal aging as one of the risk factors for neurodevelopmental disorders, such as autism, in offspring. A recent study has suggested that factors other than de novo mutations due to aging can influence the biology of offspring. Here, we focused on epigenetic alterations in sperm that can influence developmental programs in offspring. In this study, we qualitatively and semiquantitatively evaluated histone modification patterns in male germline cells throughout spermatogenesis based on immunostaining of testes taken from young (3 months old) and aged (12 months old) mice. Although localization patterns were not obviously changed between young and aged testes, some histone modification showed differences in their intensity. Among histone modifications that repress gene expression, histone H3 lysine 9 trimethylation (H3K9me3) was decreased in the male germline cells of the aged testis, while H3K27me2/3 was increased. The intensity of H3K27 acetylation (ac), an active mark, was lower/higher depending on the stages in the aged testis. Interestingly, H3K27ac was detected on the putative sex chromosomes of round spermatids, while other chromosomes were occupied by a repressive mark, H3K27me3. Among other histone modifications that activate gene expression, H3K4me2 was drastically decreased in the male germline cells of the aged testis. In contrast, H3K79me3 was increased in M-phase spermatocytes, where it accumulates on the sex chromosomes. Therefore, aging induced alterations in the amount of histone modifications and in the differences of patterns for each modification. Moreover, histone modifications on the sex chromosomes and on other chromosomes seems to be differentially regulated by aging. These findings will help elucidate the epigenetic mechanisms underlying the influence of paternal aging on offspring development.

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

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

Expression profile of Toll-like receptor 4 in rat testis and epididymis throughout postnatal development

Toll-like receptors (TLRs) belonging to pattern recognition receptors are involved in maintaining testicular and epididymal immune homeostasis. The purpose of the current study was to investigate TLR4 expression in rat testis and epididymis throughout postnatal development. Weak staining was detected in peritubular myoid cells and immature Sertoli cells while no staining was observed in gonocytes during prepubertal period. However, TLR4 expression began to appear in spermatocytes in pubertal period and gradually increased in spermatids. An intense staining was observed in steps 5-19 spermatids in post pubertal and mature periods. Similarly, TLR4 expression in the testes steadily increased from pubertal period to mature period. Puberty also caused a significant increase in TLR4 expression in epididymis. TLR4 expression in cauda epididymis was lower as compared to those of other epididymal segments. The majority of epididymal epithelial cells exhibited apical TLR4 expression, whereas basal cells showed intense intracytoplasmic immunoreaction. We detected an intense staining in epididymal smooth muscle cells. The expression levels of TLR4 showed dynamic changes in both spermatogenic cells, and entire testicular and epididymal tissues during postnatal development. These results suggest that TLR4 expression contributes not only to inflammation but also to the development of spermatogenic cells.

Essential Role of Histone Replacement and Modifications in Male Fertility

Spermiogenesis is a complex cellular differentiation process that the germ cells undergo a distinct morphological change, and the protamines replace the core histones to facilitate chromatin compaction in the sperm head. Recent studies show the essential roles of epigenetic events during the histone-to-protamine transition. Defects in either the replacement or the modification of histones might cause male infertility with azoospermia, oligospermia or teratozoospermia. Here, we summarize recent advances in our knowledge of how epigenetic regulators, such as histone variants, histone modification and their related chromatin remodelers, facilitate the histone-to-protamine transition during spermiogenesis. Understanding the molecular mechanism underlying the modification and replacement of histones during spermiogenesis will enable the identification of epigenetic biomarkers of male infertility, and shed light on potential therapies for these patients in the future.

Genetic Factors Affecting Sperm Chromatin Structure

Spermatozoa genome has unique features that make it a fascinating field of investigation: first, because, with oocyte genome, it can be transmitted generation after generation; second, because of genetic shuffling during meiosis, each spermatozoon is virtually unique in terms of genetic content, with consequences for species evolution; and finally, because its chromatin organization is very different from that of somatic cells or oocytes, as it is not based on nucleosomes but on nucleoprotamines which confer a higher order of packaging. Histone-to-protamine transition involves many actors, such as regulators of spermatid gene expression, components of the nuclear envelop, histone-modifying enzymes and readers, chaperones, histone variants, transition proteins, protamines, and certainly many more to be discovered.In this book chapter, we will present what is currently known about sperm chromatin structure and how it is established during spermiogenesis, with the aim to list the genetic factors that regulate its organization.

[Endocrine disrupters: Towards an unsatisfying regulation]

Endocrine disruptors commonly make the headlines of newscasts and magazines, which is not without generating questions, even anxieties, with the general public. In a recent survey, 90% of French people considered it desirable to set up regulations concerning endocrine disruptors. However, under pressure from the lobbies of the chemical industry, and also due to scientific conflicts, the European Union is slow to legislate and has even been condemned before the European Court for failing to fulfill its obligations. This article does not intend to be exhaustive on the issue of endocrine disrupters, but rather to give the reader a certain number of keys enabling him to understand why national or European regulators are slow to establish specific regulation.

Nejire/dCBP-mediated histone H3 acetylation during spermatogenesis is essential for male fertility in Drosophila melanogaster

Spermatogenesis in many species including Drosophila melanogaster is accompanied by major reorganisation of chromatin in post-meiotic stages, involving a nearly genome-wide displacement of histones by protamines, Mst77F and Protamine-like 99C. A proposed prerequisite for the histone-to-protamine transition is massive histone H4 hyper-acetylation prior to the switch. Here, we investigated the pattern of histone H3 lysine acetylation and general lysine crotonylation in D. melanogaster spermiogenesis to elucidate a possible role of these marks in chromatin reorganisation. Lysine crotonylation was strongest prior to remodelling and the deposition of this mark depended on the acetylation status of the spermatid chromatin. In contrast to H4 acetylation, individual H3 acetylation marks displayed surprisingly distinct patterns during the histone-to-protamine transition. We observed that Nejire, a histone acetyl transferase, is expressed during the time of histone-to-protamine transition. Nejire knock down led to strongly reduced fertility, which correlated with misshaped spermatid nuclei and a lack of mature sperm. protA and prtl99C transcript levels were reduced after knocking down Nejire. ProtB-eGFP, Mst77F-eGFP and Prtl99C-eGFP were synthesized at the late canoe stage, while histones were often not detectable. However, in some cysts histones persist in parallel to protamines. Therefore, we hypothesize that complete histone removal requires multiple histone modifications besides H3K18ac and H3K27ac. In summary, H3K18 and H3K27 acetylation during Drosophila spermatogenesis is dependent on Nejire or a yet uncharacterized acetyl transferase. We show that Nejire is required for male fertility since Nejire contributes to efficient transcription of protA and prtl99C, but not Mst77F, in spermatocytes, and to maturation of sperm.

A Decade of Exploring the Mammalian Sperm Epigenome: Paternal Epigenetic and Transgenerational Inheritance

The past decade has seen a tremendous increase in interest and progress in the field of sperm epigenetics. Studies have shown that chromatin regulation during male germline development is multiple and complex, and that the spermatozoon possesses a unique epigenome. Its DNA methylation profile, DNA-associated proteins, nucleo-protamine distribution pattern and non-coding RNA set up a unique epigenetic landscape which is delivered, along with its haploid genome, to the oocyte upon fertilization, and therefore can contribute to embryogenesis and to the offspring health. An emerging body of compelling data demonstrates that environmental exposures and paternal lifestyle can change the sperm epigenome and, consequently, may affect both the embryonic developmental program and the health of future generations. This short review will attempt to provide an overview of what is currently known about sperm epigenome and the existence of transgenerational epigenetic inheritance of paternally acquired traits that may contribute to the offspring phenotype.

SLY regulates genes involved in chromatin remodeling and interacts with TBL1XR1 during sperm differentiation

Sperm differentiation requires unique transcriptional regulation and chromatin remodeling after meiosis to ensure proper compaction and protection of the paternal genome. Abnormal sperm chromatin remodeling can induce sperm DNA damage, embryo lethality and male infertility, yet, little is known about the factors which regulate this process. Deficiency in Sly, a mouse Y chromosome-encoded gene expressed only in postmeiotic male germ cells, has been shown to result in the deregulation of hundreds of sex chromosome-encoded genes associated with multiple sperm differentiation defects and subsequent male infertility. The underlying mechanism remained, to date, unknown. Here, we show that SLY binds to the promoter of sex chromosome-encoded and autosomal genes highly expressed postmeiotically and involved in chromatin regulation. Specifically, we demonstrate that Sly knockdown directly induces the deregulation of sex chromosome-encoded H2A variants and of the H3K79 methyltransferase DOT1L. The modifications prompted by loss of Sly alter the postmeiotic chromatin structure and ultimately result in abnormal sperm chromatin remodeling with negative consequences on the sperm genome integrity. Altogether our results show that SLY is a regulator of sperm chromatin remodeling. Finally we identified that SMRT/N-CoR repressor complex is involved in gene regulation during sperm differentiation since members of this complex, in particular TBL1XR1, interact with SLY in postmeiotic male germ cells.

Epigenetic Remodeling in Male Germline Development

In mammals, germ cells guarantee the inheritance of genetic and epigenetic information across generations and are the origin of a new organism. During embryo development, the blastocyst is formed in the early stage, is comprised of an inner cell mass which is pluripotent, and could give rise to the embryonic stem cells (ESCs). The inner cell mass undergoes demethylation processes and will reestablish a methylated state that is similar to that of somatic cells later in epiblast stage. Primordial germ cells (PGCs) will be formed very soon and accompanied by the process of genome-wide demethylation. With the input of male sex determination genes, spermatogonial stem cells (SSCs) are generated and undergo the process of spermatogenesis. Spermatogenesis is a delicately regulated process in which various regulations are launched to guarantee normal mitosis and meiosis in SSCs. During all these processes, especially during spermatid development, DNA methylation profile and histone modifications are of crucial importance. In this review, we will discuss the epigenetic modifications from zygote formation to mature sperm generation and their significance to these development processes.

Epigenetics: A key paradigm in reproductive health

It is well established that there is a heritable element of susceptibility to chronic human ailments, yet there is compelling evidence that some components of such heritability are transmitted through non-genetic factors. Due to the complexity of reproductive processes, identifying the inheritance patterns of these factors is not easy. But little doubt exists that besides the genomic backbone, a range of epigenetic cues affect our genetic programme. The inter-generational transmission of epigenetic marks is believed to operate via four principal means that dramatically differ in their information content: DNA methylation, histone modifications, microRNAs and nucleosome positioning. These epigenetic signatures influence the cellular machinery through positive and negative feedback mechanisms either alone or interactively. Understanding how these mechanisms work to activate or deactivate parts of our genetic programme not only on a day-to-day basis but also over generations is an important area of reproductive health research.

Epigenetic regulation of the histone-to-protamine transition during spermiogenesis

In mammals, male germ cells differentiate from haploid round spermatids to flagella-containing motile sperm in a process called spermiogenesis. This process is distinct from somatic cell differentiation in that the majority of the core histones are replaced sequentially, first by transition proteins and then by protamines, facilitating chromatin hyper-compaction. This histone-to-protamine transition process represents an excellent model for the investigation of how epigenetic regulators interact with each other to remodel chromatin architecture. Although early work in the field highlighted the critical roles of testis-specific transcription factors in controlling the haploid-specific developmental program, recent studies underscore the essential functions of epigenetic players involved in the dramatic genome remodeling that takes place during wholesale histone replacement. In this review, we discuss recent advances in our understanding of how epigenetic players, such as histone variants and histone writers/readers/erasers, rewire the haploid spermatid genome to facilitate histone substitution by protamines in mammals.

Epigenetic regulation of germ cells-remember or forget?

Unlike somatic cells, germ cells retain the potential to reproduce an entire new organism upon fertilization. In order to accomplish the process of fertilization, germ cells undergo an extreme cellular differentiation process known as gametogenesis in order to produce morphologically and functionally distinct oocyte and sperm. In addition to changes in genetic content changes from diploid to haploid, epigenetic mechanisms that modify chromatin state without altering primary DNA sequences have profound influence on germ cell differentiation and moreover, the transgenerational effect. In this review, we will go over the most recent discoveries on epigenetic regulations in germline differentiation and transgenerational inheritance across different metazoan species.

Multimerization of Drosophila sperm protein Mst77F causes a unique condensed chromatin structure

Despite insights on the cellular level, the molecular details of chromatin reorganization in sperm development, which involves replacement of histone proteins by specialized factors to allow ultra most condensation of the genome, are not well understood. Protamines are dispensable for DNA condensation during Drosophila post-meiotic spermatogenesis. Therefore, we analyzed the interaction of Mst77F, another very basic testis-specific protein with chromatin and DNA as well as studied the molecular consequences of such binding. We show that Mst77F on its own causes severe chromatin and DNA aggregation. An intrinsically unstructured domain in the C-terminus of Mst77F binds DNA via electrostatic interaction. This binding results in structural reorganization of the domain, which induces interaction with an N-terminal region of the protein. Via putative cooperative effects Mst77F is induced to multimerize in this state causing DNA aggregation. In agreement, overexpression of Mst77F results in chromatin aggregation in fly sperm. Based on these findings we postulate that Mst77F is crucial for sperm development by giving rise to a unique condensed chromatin structure.