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

Trim66's paternal deficiency causes intrauterine overgrowth

The tripartite motif-containing protein 66 (TRIM66, also known as TIF1-delta) is a PHD-Bromo-containing protein primarily expressed in post-meiotic male germ cells known as spermatids. Biophysical assays showed that the TRIM66 PHD-Bromodomain binds to H3 N-terminus only when lysine 4 is unmethylated. We addressed TRIM66's role in reproduction by loss-of-function genetics in the mouse. Males homozygous for Trim66-null mutations produced functional spermatozoa. Round spermatids lacking TRIM66 up-regulated a network of genes involved in histone acetylation and H3K4 methylation. Profiling of H3K4me3 patterns in the sperm produced by the Trim66-null mutant showed minor alterations below statistical significance. Unexpectedly, Trim66-null males, but not females, sired pups overweight at birth, hence revealing that Trim66 mutations cause a paternal effect phenotype.

Reproductive Ageing: Current insights and a potential role of NAD in the reproductive health of aging fathers and their children

In brief

In light of the increasing age of first-time fathers, this article summarizes the current scientific knowledge base on reproductive aging in the male, including sperm quality and health impacts for the offspring. The emerging role of NAD decline in reproductive aging is highlighted.

Abstract

Over the past decades, the age of first-time fathers has been steadily increasing due to socio-economic pressures. While general mechanisms of aging are subject to intensive research, male reproductive aging has remained an understudied area, and the effects of increased age on the male reproductive system are still only poorly understood, despite new insights into the potential dire consequences of advanced paternal age for the health of their progeny. There is also growing evidence that reproductive aging is linked to overall health in men, but this review mainly focuses on pathophysiological consequences of old age in men, such as low sperm count and diminished sperm genetic integrity, with an emphasis on mechanisms underlying reproductive aging. The steady decline of NAD levels observed in aging men represents one of the emerging concepts in that regard. Because it offers some mechanistic rationale explaining the effects of old age on the male reproductive system, some of the NAD-dependent functions in male reproduction are briefly outlined in this review. The overview also provides many questions that remain open about the basic science of male reproductive aging.

Epigenetic priming in the male germline

Epigenetic priming presets chromatin states that allow the rapid induction of gene expression programs in response to differentiation cues. In the germline, it provides the blueprint for sexually dimorphic unidirectional differentiation. In this review, we focus on epigenetic priming in the mammalian male germline and discuss how cellular memories are regulated and inherited to the next generation. During spermatogenesis, epigenetic priming predetermines cellular memories that ensure the lifelong maintenance of spermatogonial stem cells and their subsequent commitment to meiosis and to the production of haploid sperm. The paternal chromatin state is also essential for the recovery of totipotency after fertilization and contributes to paternal epigenetic inheritance. Thus, epigenetic priming establishes stable but reversible chromatin states during spermatogenesis and enables epigenetic inheritance and reprogramming in the next generation.

Future in the past: paternal reprogramming of offspring phenotype and the epigenetic mechanisms

That certain preconceptual paternal exposures reprogram the developmental phenotypic plasticity in future generation(s) has conceptualized the "paternal programming of offspring health" hypothesis. This transgenerational effect is transmitted primarily through sperm epigenetic mechanisms-DNA methylation, non-coding RNAs (ncRNAs) and associated RNA modifications, and histone modifications-and potentially through non-sperm-specific mechanisms-seminal plasma and circulating factors-that create 'imprinted' memory of ancestral information. The epigenetic landscape in sperm is highly responsive to environmental cues, due to, in part, the soma-to-germline communication mediated by epididymosomes. While human epidemiological studies and experimental animal studies have provided solid evidences in support of transgenerational epigenetic inheritance, how ancestral information is memorized as epigenetic codes for germline transmission is poorly understood. Particular elusive is what the downstream effector pathways that decode those epigenetic codes into persistent phenotypes. In this review, we discuss the paternal reprogramming of offspring phenotype and the possible underlying epigenetic mechanisms. Cracking these epigenetic mechanisms will lead to a better appreciation of "Paternal Origins of Health and Disease" and guide innovation of intervention algorithms to achieve 'healthier' outcomes in future generations. All this will revolutionize our understanding of human disease etiology.

Characterization of sexual maturity-associated N6-methyladenosine in boar testes

BACKGROUND

The health and size of the testes are crucial for boar fertility. Testicular development is tightly regulated by epigenetics. N6-methyladenosine (m6A) modification is a prevalent internal modification on mRNA and plays an important role in development. The mRNA m6A methylation in boar testicular development still needs to be investigated.

RESULTS

Using the MeRIP-seq technique, we identify and profile m6A modification in boar testes between piglets and adults. The results showed 7783 distinct m6A peaks in piglets and 6590 distinct m6A peaks in adults, with 2,471 peaks shared between the two groups. Enrichment of GO and KEGG analysis reveal dynamic m6A methylation in various biological processes and signalling pathways. Meanwhile, we conjointly analyzed differentially methylated and expressed genes in boar testes before and after sexual maturity, and reproductive related genes (TLE4, TSSK3, TSSK6, C11ORF94, PATZ1, PHLPP1 and PAQR7) were identified. Functional enrichment analysis showed that differential genes are associated with important biological functions, including regulation of growth and development, regulation of metabolic processes and protein catabolic processes.

CONCLUSION

The results demonstrate that m6A methylation, differential expression and the related signalling pathways are crucial for boar testicular development. These results suggest a role for m6A modification in boar testicular development and provided a resource for future studies on m6A function in boar testicular development.

A comprehensive analysis of spermatozoal RNA elements in idiopathic infertile males undergoing fertility treatment

Current approaches to diagnosing male infertility inadequately assess the complexity of the male gamete. Beyond the paternal haploid genome, spermatozoa also deliver coding and non-coding RNAs to the oocyte. While sperm-borne RNAs have demonstrated potential involvement in embryo development, the underlying mechanisms remain unclear. In this study, 47 sperm samples from normozoospermic males undergoing fertility treatment using donor oocytes were sequenced and analyzed to evaluate associations between sperm RNA elements (exon-sized sequences) and blastocyst progression. A total of 366 RNA elements (REs) were significantly associated with blastocyst rate (padj < 0.05), some of which were linked to genes related to critical developmental processes, including mitotic spindle formation and both ectoderm and mesoderm specification. Of note, 27 RE-associated RNAs are predicted targets of our previously reported list of developmentally significant miRNAs. Inverse RE-miRNA expression patterns were consistent with miRNA-mediated down-regulation. This study provides a comprehensive set of REs which differ by the patient's ability to produce blastocysts. This knowledge can be leveraged to improve clinical screening of male infertility and ultimately reduce time to pregnancy.

Gene expression programs in mammalian spermatogenesis

Mammalian spermatogenesis, probably the most complex of all cellular developmental processes, is an ideal model both for studying the specific mechanism of gametogenesis and for understanding the basic rules governing all developmental processes, as it entails both cell type-specific and housekeeping molecular processes. Spermatogenesis can be viewed as a mission with many tasks to accomplish, and its success is genetically programmed and ensured by the collaboration of a large number of genes. Here, I present an overview of mammalian spermatogenesis and the mechanisms underlying each step in the process, covering the cellular and molecular activities that occur at each developmental stage and emphasizing their gene regulation in light of recent studies.

Current global status of male reproductive health

BACKGROUND

The widespread interest in male reproductive health (MRH), fueled by emerging evidence, such as the global decline in sperm counts, has intensified concerns about the status of MRH. Consequently, there is a pressing requirement for a strategic, systematic approach to identifying critical questions, collecting pertinent information, and utilizing these data to develop evidence-based strategies. The methods for addressing these questions and the pathways toward their answers will inevitably vary based on the variations in cultural, geopolitical, and health-related contexts. To address these issues, a conjoint ESHRE and Male Reproductive Health Initiative (MRHI) Campus workshop was convened.

OBJECTIVE AND RATIONALE

The three objectives were: first, to assess the current state of MRH around the world; second, to identify some of the key gaps in knowledge; and, third, to examine how MRH stakeholders can collaboratively generate intelligent and effective paths forward.

SEARCH METHODS

Each expert reviewed and summarized the current literature that was subsequently used to provide a comprehensive overview of challenges related to MRH.

OUTCOMES

This narrative report is an overview of the data, opinions, and arguments presented during the workshop. A number of outcomes are presented and can be summarized by the following overarching themes: MRH is a serious global issue and there is a plethora of gaps in our understanding; there is a need for widespread international collaborative networks to undertake multidisciplinary research into fundamental issues, such as lifestyle/environmental exposure studies, and high-quality clinical trials; and there is an urgent requirement for effective strategies to educate young people and the general public to safeguard and improve MRH across diverse population demographics and resources.

LIMITATIONS REASONS FOR CAUTION

This was a workshop where worldwide leading experts from a wide range of disciplines presented and discussed the evidence regarding challenges related to MRH. While each expert summarized the current literature and placed it in context, the data in a number of areas are limited and/or sparse. Equally, important areas for consideration may have been missed. Moreover, there are clear gaps in our knowledge base, which makes some conclusions necessarily speculative and warranting of further study.

WIDER IMPLICATIONS

Poor MRH is a global issue that suffers from low awareness among the public, patients, and heathcare professionals. Addressing this will require a coordinated multidisciplinary approach. Addressing the significant number of knowledge gaps will require policy makers prioritizing MRH and its funding.

STUDY FUNDING/COMPETING INTERESTS

The authors would like to extend their gratitude to ESHRE for providing financial support for the Budapest Campus Workshop, as well as to Microptic S.L. (Barcelona) for kindly sponsoring the workshop. P.B. is the Director of the not-for-profit organization Global Action on Men's Health and receives fees and expenses for his work, (which includes the preparation of this manuscript). Conflicts of interest: C.J.D.J., C.L.R.B., R.A.A., P.B., M.P.C., M.L.E., N.G., N.J., C.K., AAP, M.K.O., S.R.-H., M.H.V.-L.: ESHRE Campus Workshop 2022 (Travel support-personal). C.J.D.J.: Cambridge University Press (book royalties-personal). ESHRE Annual Meeting 2022 and Yale University Panel Meeting 2023 (Travel support-personal). C.L.R.B.: Ferring and IBSA (Lecture), RBMO editor (Honorarium to support travel, etc.), ExSeed and ExScentia (University of Dundee), Bill & Melinda Gates Foundation (for research on contraception). M.P.C.: Previously received funding from pharmaceutical companies for health economic research. The funding was not in relation to this work and had no bearing on the contents of this work. No funding from other sources has been provided in relation to this work (funding was provided to his company Global Market Access Solutions). M.L.E.: Advisor to Ro, Doveras, Next, Hannah, Sandstone. C.K.: European Academy of Andrology (Past president UNPAID), S.K.: CEO of His Turn, a male fertility Diagnostic and Therapeutic company (No payments or profits to date). R.I.M.: www.healthymale.org.au (Australian Government funded not for profit in men's health sector (Employed as Medical Director 0.2 FET), Monash IVF Pty Ltd (Equity holder)). N.J.: Merck (consulting fees), Gedeon Richter (honoraria). S.R.-H.: ESHRE (Travel reimbursements). C.N.: LLC (Nursing educator); COMMIT (Core Outcomes Measures for Infertility Trials) Advisor, meeting attendee, and co-author; COMMA (Core Outcomes in Menopause) Meeting attendee, and co-author; International Federation of Gynecology and Obstetrics (FIGO) Delegate Letters and Sciences; ReproNovo, Advisory board; American Board of Urology Examiner; American Urological Association Journal subsection editor, committee member, guidelines co-author Ferring Scientific trial NexHand Chief Technology Officer, stock ownership Posterity Health Board member, stock ownership. A.P.: Economic and Social Research Council (A collaborator on research grant number ES/W001381/1). Member of an advisory committee for Merck Serono (November 2022), Member of an advisory board for Exceed Health, Speaker fees for educational events organized by Mealis Group; Chairman of the Cryos External Scientific Advisory Committee: All fees associated with this are paid to his former employer The University of Sheffield. Trustee of the Progress Educational Trust (Unpaid). M.K.O.: National Health and Medical Research Council and Australian Research Council (Funding for research of the topic of male fertility), Bill and Melinda Gates Foundation (Funding aimed at the development of male gamete-based contraception), Medical Research Future Fund (Funding aimed at defining the long-term consequences of male infertility). M.H.V.-L.: Department of Sexual and Reproductive Health and Research (SRH)/Human Reproduction Programme (HRP) Research Project Panel RP2/WHO Review Member; MRHI (Core Group Member), COMMIT (member), EGOI (Member); Human Reproduction (Associate Editor), Fertility and Sterility (Editor), AndroLATAM (Founder and Coordinator).

Sperm DNA methylation defects in a new mouse model of the 5,10-methylenetetrahydrofolate reductase 677C>T variant and correction with moderate dose folic acid supplementation

5,10-Methylenetetrahydrofolate reductase (MTHFR) is an enzyme that plays a key role in providing methyl groups for DNA methylation, including during spermatogenesis. A common genetic variant in humans (MTHFR 677C>T) results in reduced enzyme activity and has been linked to various disorders, including male infertility. A new animal model has been created by reproducing the human equivalent of the polymorphism in mice using CRISPR/Cas9. Biochemical parameters in the Mthfr 677TT mice recapitulate alterations found in MTHFR 677TT men. Our aims were to characterize the sperm DNA methylome of the Mthfr 677CC and TT mice on a control diet (2 mg folic acid/kg diet) and assess the effects of folic acid supplementation (10 mg/kg diet) on the sperm DNA methylome. Body and reproductive organ weights, testicular sperm counts, and histology were examined. DNA methylation in sperm was assessed using bisulfite pyrosequencing and whole-genome bisulfite sequencing (WGBS). Reproductive parameters and locus-specific imprinted gene methylation were unaffected by genotype or diet. Using WGBS, sperm from 677TT mice had 360 differentially methylated tiles as compared to 677CC mice, predominantly hypomethylation (60% of tiles). Folic acid supplementation mostly caused hypermethylation in sperm of males of both genotypes and was found to partially correct the DNA methylation alterations in sperm associated with the TT genotype. The new mouse model will be useful in understanding the role of MTHFR deficiency in male fertility and in designing folate supplementation regimens for the clinic.

Adverse effects of bisphenol A and its analogues on male fertility: An epigenetic perspective

In recent years, there has been growing concern about the adverse effects of endocrine disrupting chemicals (EDCs) on male fertility. Epigenetic modification is critical for male germline development, and has been suggested as a potential mechanism for impaired fertility induced by EDCs. Bisphenol A (BPA) has been recognized as a typical EDC. BPA and its analogues, which are still widely used in various consumer products, have garnered increasing attention due to their reproductive toxicity and the potential to induce epigenetic alteration. This literature review provides an overview of studies investigating the adverse effects of bisphenol exposures on epigenetic modifications and male fertility. Existing studies provide evidence that exposure to bisphenols can lead to adverse effects on male fertility, including declined semen quality, altered reproductive hormone levels, and adverse reproductive outcomes. Epigenetic patterns, including DNA methylation, histone modification, and non-coding RNA expression, can be altered by bisphenol exposures. Transgenerational effects, which influence the fertility and epigenetic patterns of unexposed generations, have also been identified. However, the magnitude and direction of certain outcomes varied across different studies. Investigations into the dynamics of histopathological and epigenetic alterations associated with bisphenol exposures during developmental stages can enhance the understanding of the epigenetic effects of bisphenols, the implication of epigenetic alteration on male fertility, and the health of successive generation.

Could the sperm epigenome become a diagnostic tool for evaluation of the infertile man?

Although male infertility is currently diagnosed when abnormal sperm parameters are found, the poor predictive ability of sperm parameters on natural fecundity and medically assisted reproduction outcome poses the need for improved diagnostic techniques for male infertility. The accumulating evidence about the role played by the sperm epigenome in modulation of the early phases of embryonic development has led researchers to focus on the epigenetic mechanisms within the sperm epigenome to find new molecular markers of male infertility. Indeed, sperm epigenome abnormalities could explain some cases of unexplained male infertility in men showing normal sperm parameters and were found to be associated with poor embryo development in IVF cycles. The present mini-review summarizes the current knowledge about this interesting topic, starting from a description of the epigenetic mechanisms of gene expression regulation (i.e. DNA methylation, histone modifications, and non-coding RNAs' activity). We also discuss possible mechanisms by which environmental factors might cause epigenetic changes in the human germline and affect embryonic development, as well as subsequent generations' phenotypes. Studies demonstrating sperm epigenome abnormalities in men with male infertility are reviewed, with particular emphasis on those with the more severe form of spermatogenic dysfunction. Observations demonstrate that the diagnostic and prognostic efficacy of sperm epigenome evaluation will help facilitate the management of men with male factor infertility.

Associations between Sperm Epigenetic Age and Semen Parameters: An Evaluation of Clinical and Non-Clinical Cohorts

The well-documented relationship between chronological age and the sperm methylome has allowed for the construction of epigenetic clocks that estimate the biological age of sperm based on DNA methylation, which we previously termed sperm epigenetic age (SEA). Our lab demonstrated that SEA is positively associated with the time taken to achieve pregnancy; however, its relationship with semen parameters is unknown. A total of 379 men from the Longitudinal Investigation of Fertility and Environment (LIFE) study, a non-clinical cohort, and 192 men seeking fertility treatment from the Sperm Environmental Epigenetics and Development Study (SEEDS) were included in the study. Semen analyses were conducted for both cohorts, and SEA was previously generated using a machine learning algorithm and DNA methylation array data. Association analyses were conducted via multivariable linear regression models adjusting for BMI and smoking status. We found that SEA was not associated with standard semen characteristics in SEEDS and LIFE cohorts. However, SEA was significantly associated with higher sperm head length and perimeter, the presence of pyriform and tapered sperm, and lower sperm elongation factor in the LIFE study (p < 0.05). Based on our results, SEA is mostly associated with defects in sperm head morphological factors that are less commonly evaluated during male infertility assessments. SEA shows promise to be an independent biomarker of sperm quality to assess male fecundity.

Early Life Stress and Major Depressive Disorder-An Update on Molecular Mechanisms and Synaptic Impairments

Early life stress (ELS), characterized as abuse, neglect, and abandonment, can cause several adverse consequences in the lives of affected individuals. ELS experiences can affect an individual's development in variable ways, persisting in the long term and promoting lasting impacts, considering that early exposure to stressors can be biologically incorporated, as prolonged stimulation of stress response systems affects the development of the brain structure and other body systems, increasing the risk of diseases associated with stress and cognitive impairment. This type of stress increases the risk of developing major depressive disorder (MDD) in a severe form that does not respond adequately to traditional antidepressant treatments. Several alterations are studied as mechanisms that relate ELS with MDD, such as epigenetic alterations, neurotransmitters, and neuronal signaling. This review discusses research that brings evidence about the ELS mechanisms involved in synaptic impairments and MDD. The processes involved in epigenetic changes and the HPA axis are highlighted, as well as changes in neurotransmitters and neuronal signaling mechanisms.

Frequency, morbidity and equity - the case for increased research on male fertility

Currently, most men with infertility cannot be given an aetiology, which reflects a lack of knowledge around gamete production and how it is affected by genetics and the environment. A failure to recognize the burden of male infertility and its potential as a biomarker for systemic illness exists. The absence of such knowledge results in patients generally being treated as a uniform group, for whom the strategy is to bypass the causality using medically assisted reproduction (MAR) techniques. In doing so, opportunities to prevent co-morbidity are missed and the burden of MAR is shifted to the woman. To advance understanding of men's reproductive health, longitudinal and multi-national centres for data and sample collection are essential. Such programmes must enable an integrated view of the consequences of genetics, epigenetics and environmental factors on fertility and offspring health. Definition and possible amelioration of the consequences of MAR for conceived children are needed. Inherent in this statement is the necessity to promote fertility restoration and/or use the least invasive MAR strategy available. To achieve this aim, protocols must be rigorously tested and the move towards personalized medicine encouraged. Equally, education of the public, governments and clinicians on the frequency and consequences of infertility is needed. Health options, including male contraceptives, must be expanded, and the opportunities encompassed in such investment understood. The pressing questions related to male reproductive health, spanning the spectrum of andrology are identified in the Expert Recommendation.

The Dynamics of Histone Modifications during Mammalian Zygotic Genome Activation

Mammalian fertilization initiates the reprogramming of oocytes and sperm, forming a totipotent zygote. During this intricate process, the zygotic genome undergoes a maternal-to-zygotic transition (MZT) and subsequent zygotic genome activation (ZGA), marking the initiation of transcriptional control and gene expression post-fertilization. Histone modifications are pivotal in shaping cellular identity and gene expression in many mammals. Recent advances in chromatin analysis have enabled detailed explorations of histone modifications during ZGA. This review delves into conserved and unique regulatory strategies, providing essential insights into the dynamic changes in histone modifications and their variants during ZGA in mammals. The objective is to explore recent advancements in leading mechanisms related to histone modifications governing this embryonic development phase in depth. These considerations will be useful for informing future therapeutic approaches that target epigenetic regulation in diverse biological contexts. It will also contribute to the extensive areas of evolutionary and developmental biology and possibly lay the foundation for future research and discussion on this seminal topic.

Advances in human reproductive biomarkers

Reproductive biomarkers are important regulators in women, especially during pregnancy and childbirth. Because of their essential role in women's health, the discovery and quantification of reproductive biomarkers is of great clinical importance. Nowadays, there are many detection strategies to detect these biomarkers, including VEGF, human chorionic gonadotropin (hCG), etc. Consider the limitations and problems of conventional diagnostic methods, new methods are being developed, one of the most important being methods based on nanotechnology. This review includes a review of methods for diagnosing reproductive biomarkers, ranging from mainstream to nanotechnology-based methods. The bulk of this article is an in-depth introduction to the latest advances in biosensor and nanosensor research for the detection and quantitative identification of reproductive biomarkers.

When Dad's Stress Gets under Kid's Skin-Impacts of Stress on Germline Cargo and Embryonic Development

Multiple lines of evidence suggest that paternal psychological stress contributes to an increased prevalence of neuropsychiatric and metabolic diseases in the progeny. While altered paternal care certainly plays a role in such transmitted disease risk, molecular factors in the germline might additionally be at play in humans. This is supported by findings on changes to the molecular make up of germ cells and suggests an epigenetic component in transmission. Several rodent studies demonstrate the correlation between paternal stress induced changes in epigenetic modifications and offspring phenotypic alterations, yet some intriguing cases also start to show mechanistic links in between sperm and the early embryo. In this review, we summarise efforts to understand the mechanism of intergenerational transmission from sperm to the early embryo. In particular, we highlight how stress alters epigenetic modifications in sperm and discuss the potential for these modifications to propagate modified molecular trajectories in the early embryo to give rise to aberrant phenotypes in adult offspring.

Epigenetic aging of mammalian gametes

The process of aging refers to physiological changes that occur to an organism as time progresses and involves changes to DNA, proteins, metabolism, cells, and organs. Like the rest of the cells in the body, gametes age, and it is well established that there is a decline in reproductive capabilities in females and males with aging. One of the major pathways known to be involved in aging is epigenetic changes. The epigenome is the multitude of chemical modifications performed on DNA and chromatin that affect the ability of chromatin to be transcribed. In this review, we explore the effects of aging on female and male gametes with a focus on the epigenetic changes that occur in gametes throughout aging. Quality decline in oocytes occurs at a relatively early age. Epigenetic changes constitute an important part of oocyte aging. DNA methylation is reduced with age, along with reduced expression of DNA methyltransferases (DNMTs). Histone deacetylases (HDAC) expression is also reduced, and a loss of heterochromatin marks occurs with age. As a consequence of heterochromatin loss, retrotransposon expression is elevated, and aged oocytes suffer from DNA damage. In sperm, aging affects sperm number, motility and fecundity, and epigenetic changes may constitute a part of this process. 5 methyl-cytosine (5mC) methylation is elevated in sperm from aged men, but methylation on Long interspersed nuclear elements (LINE) elements is reduced. Di and trimethylation of histone 3 lysine 9 (H3K9me2/3) is reduced in sperm from aged men and trimethylation of histone 3 lysine 27 (H3K27me3) is elevated. The protamine makeup of sperm from aged men is also changed, with reduced protamine expression and a misbalanced ratio between protamine proteins protamine P1 and protamine P2. The study of epigenetic reproductive aging is recently gaining interest. The current status of the field suggests that many aspects of gamete epigenetic aging are still open for investigation. The clinical applications of these investigations have far-reaching consequences for fertility and sociological human behavior.

Teratogenesis and the epigenetic programming of congenital defects: Why paternal exposures matter

Until recently, clinicians and researchers did not realize paternal exposures could impact child developmental outcomes. Indeed, although there is growing recognition that sperm carry a large amount of non-genomic information and that paternal stressors influence the health of the next generation, toxicologists are only now beginning to explore the role paternal exposures have in dysgenesis and the incidence of congenital malformations. In this commentary, I will briefly summarize the few studies describing congenital malformations resulting from preconception paternal stressors, argue for the theoretical expansion of teratogenic perspectives into the male preconception period, and discuss some of the challenges in this newly emerging branch of toxicology. I argue that we must consider gametes the same as any other malleable precursor cell type and recognize that environmentally-induced epigenetic changes acquired during the formation of the sperm and oocyte hold equal teratogenic potential as exposures during early development. Here, I propose the term epiteratogen to reference agents acting outside of pregnancy that, through epigenetic mechanisms, induce congenital malformations. Understanding the interactions between the environment, the essential epigenetic processes intrinsic to spermatogenesis, and their cumulative influences on embryo patterning is essential to addressing a significant blind spot in the field of developmental toxicology.

Small Non-Coding RNAs in Male Reproduction

Male reproductive functions are strictly regulated in order to maintain sperm production and fertility. All processes are controlled by precise regulation of gene expression, which creates specific gene expression programs for different developmental stages and cell types, and forms the functional basis for the reproductive system. Small non-coding RNAs (sncRNAs) are involved in gene regulation by targeting mRNAs for translational repression and degradation through complementary base pairing to recognize their targets. This review article summarizes the current knowledge on the function of different classes of sncRNAs, in particular microRNAs (miRNAs) and PIWI-interacting RNAs (piRNAs), during male germ cell differentiation, with the focus on sncRNAs expressed in the germline. Although transcriptionally inactive, mature spermatozoa contain a complex population of sncRNAs, and we also discuss the recently identified role of sperm sncRNAs in the intergenerational transmission of epigenetic information on father's environmental and lifestyle exposures to offspring. Finally, we summarize the current information on the utility of sncRNAs as potential biomarkers of infertility that may aid in the diagnosis and prediction of outcomes of medically assisted reproduction.

Paternal effects on fetal programming

Paternal programming is the concept that the environmental signals from the sire's experiences leading up to mating can alter semen and ultimately affect the phenotype of resulting offspring. Potential mechanisms carrying the paternal effects to offspring can be associated with epigenetic signatures (DNA methylation, histone modification and non-coding RNAs), oxidative stress, cytokines, and the seminal microbiome. Several opportunities exist for sperm/semen to be influenced during development; these opportunities are within the testicle, the epididymis, or accessory sex glands. Epigenetic signatures of sperm can be impacted during the pre-natal and pre-pubertal periods, during sexual maturity and with advancing sire age. Sperm are susceptible to alterations as dictated by their developmental stage at the time of the perturbation, and sperm and seminal plasma likely have both dependent and independent effects on offspring. Research using rodent models has revealed that many factors including over/under nutrition, dietary fat, protein, and ingredient composition (e.g., macro- or micronutrients), stress, exercise, and exposure to drugs, alcohol, and endocrine disruptors all elicit paternal programming responses that are evident in offspring phenotype. Research using livestock species has also revealed that sire age, fertility level, plane of nutrition, and heat stress can induce alterations in the epigenetic, oxidative stress, cytokine, and microbiome profiles of sperm and/or seminal plasma. In addition, recent findings in pigs, sheep, and cattle have indicated programming effects in blastocysts post-fertilization with some continuing into post-natal life of the offspring. Our research group is focused on understanding the effects of common management scenarios of plane of nutrition and growth rates in bulls and rams on mechanisms resulting in paternal programming and subsequent offspring outcomes. Understanding the implication of paternal programming is imperative as short-term feeding and management decisions have the potential to impact productivity and profitability of our herds for generations to come.