Stress-induced perinatal and transgenerational epigenetic programming of brain development and mental health

Research efforts during the past decades have provided intriguing evidence suggesting that stressful experiences during pregnancy exert long-term consequences on the future mental wellbeing of both the mother and her baby. Recent human epidemiological and animal studies indicate that stressful experiences in utero or during early life may increase the risk of neurological and psychiatric disorders, arguably via altered epigenetic regulation. Epigenetic mechanisms, such as miRNA expression, DNA methylation, and histone modifications are prone to changes in response to stressful experiences and hostile environmental factors. Altered epigenetic regulation may potentially influence fetal endocrine programming and brain development across several generations. Only recently, however, more attention has been paid to possible transgenerational effects of stress. In this review we discuss the evidence of transgenerational epigenetic inheritance of stress exposure in human studies and animal models. We highlight the complex interplay between prenatal stress exposure, associated changes in miRNA expression and DNA methylation in placenta and brain and possible links to greater risks of schizophrenia, attention deficit hyperactivity disorder, autism, anxiety- or depression-related disorders later in life. Based on existing evidence, we propose that prenatal stress, through the generation of epigenetic alterations, becomes one of the most powerful influences on mental health in later life. The consideration of ancestral and prenatal stress effects on lifetime health trajectories is critical for improving strategies that support healthy development and successful aging.

piRNAs: from biogenesis to function

Distinguishing self from non-self plays a crucial role in safeguarding the germlines of metazoa from mobile DNA elements. Since their discovery less than a decade ago, Piwi-interacting RNAs (piRNAs) have been shown to repress transposable elements in the germline and, hence, have been at the forefront of research aimed at understanding the mechanisms that maintain germline integrity. More recently, roles for piRNAs in gene regulation have emerged. In this Review, we highlight recent advances made in understanding piRNA function, highlighting the divergent nature of piRNA biogenesis in different organisms, and discussing the mechanisms of piRNA action during transcriptional regulation and in transgenerational epigenetic inheritance.

Germ Granules Coordinate RNA-Based Epigenetic Inheritance Pathways

Germ granules are biomolecular condensates that promote germ cell totipotency in animals. In C. elegans, MEG-3 and MEG-4 function redundantly to assemble germ granules in germline blastomeres. Here, we show that meg-3/4 mutant animals exhibit defects in RNA interference (RNAi) that are transgenerationally disconnected from the meg-3/4 genotype. Similar non-Mendelian inheritance is associated with other mutations disrupting germ granule formation, indicating that loss of germ granules is the likely cause of the observed disconnects between genotype and phenotype. meg-3/4 animals produce aberrant siRNAs that are propagated for ≅10 generations in wild-type descendants of meg-3/4 ancestors. Aberrant siRNAs inappropriately and heritably silence germline-expressed genes including the RNAi gene sid-1, suggesting that transgenerational silencing of sid-1 underlies inherited defects in RNAi. We conclude that one function of germ granules is to organize RNA-based epigenetic inheritance pathways and that germ granule loss has consequences that persist for many generations.

Epigenetic modifications and reprogramming in paternal pronucleus: sperm, preimplantation embryo, and beyond

Pronuclear/zygotic stage is the very first stage of life. In this period, paternal pronucleus undergoes massive chromatin remodeling called "paternal reprogramming" including protamine-histone replacement and subsequent acquisition of epigenetic modifications. Although these consecutive events are required for the initiation of maternal-zygotic transition, the precise role of paternal reprogramming and its effect on subsequent embryonic development has been largely unknown to date. Recently, various new techniques, especially next-generation sequencing (NGS) and RNAi microinjection contribute to unveil the epigenetic transition from both paternal and maternal to early preimplantation embryos, suggesting not only the simple transcriptional regulation by transcription factors but also dynamic structural alteration of chromatin to initiate the wave of zygotic gene transcription. This review summarizes such recent progress for understanding the epigenetic transition in sperm and preimplantation embryos, and further argue about its transgenerational effect.

Distinctions between transgenerational and non-transgenerational epimutations

Recent studies have described numerous environmentally-induced disruptions of the epigenome (epimutations) in mammals. While some of these appear to be corrected by normal germline-specific epigenetic reprogramming and are therefore not transmitted transgenerationally, others are not corrected and are transmitted over multiple subsequent generations. The mechanism(s) that distinguish transgenerational and non-transgenerational epimutations have not been delineated. This review examines several potential molecular and developmental distinctions between transgenerational and non-transgenerational epimutations in the context of the likelihood that any of these may or may not contribute to transgenerational inheritance of epimutations.

Multigenerational epigenetic inheritance: Transmitting information across generations

Inherited epigenetic information has been observed to regulate a variety of complex organismal phenotypes across diverse taxa of life. This continually expanding body of literature suggests that epigenetic inheritance plays a significant, and potentially fundamental, role in inheritance. Despite the important role these types of effects play in biology, the molecular mediators of this non-genetic transmission of information are just now beginning to be deciphered. Here we provide an intellectual framework for interpreting these findings and how they can interact with each other. We also define the different types of mechanisms that have been found to mediate epigenetic inheritance and to regulate whether epigenetic information persists for one or many generations. The field of epigenetic inheritance is entering an exciting phase, in which we are beginning to understand the mechanisms by which non-genetic information is transmitted to, and deciphered by, subsequent generations to maintain essential environmental information without permanently altering the genetic code. A more complete understanding of how and when epigenetic inheritance occurs will advance our understanding of numerous different aspects of biology ranging from how organisms cope with changing environments to human pathologies influenced by a parent's environment.

Evidence for germline non-genetic inheritance of human phenotypes and diseases

It is becoming increasingly apparent that certain phenotypes are inherited across generations independent of the information contained in the DNA sequence, by factors in germ cells that remain largely uncharacterized. As evidence for germline non-genetic inheritance of phenotypes and diseases continues to grow in model organisms, there are fewer reports of this phenomenon in humans, due to a variety of complications in evaluating this mechanism of inheritance in humans. This review summarizes the evidence for germline-based non-genetic inheritance in humans, as well as the significant challenges and important caveats that must be considered when evaluating this process in human populations. Most reports of this process evaluate the association of a lifetime exposure in ancestors with changes in DNA methylation or small RNA expression in germ cells, as well as the association between ancestral experiences and the inheritance of a phenotype in descendants, down to great-grandchildren in some cases. Collectively, these studies provide evidence that phenotypes can be inherited in a DNA-independent manner; the extent to which this process contributes to disease development, as well as the cellular and molecular regulation of this process, remain largely undefined.

The insecticide permethrin induces transgenerational behavioral changes linked to transcriptomic and epigenetic alterations in zebrafish (Danio rerio)

The pyrethroid insecticide permethrin is widely used for agricultural and domestic purposes. Previous data indicated that it acts as a developmental neurotoxicant and can induce transgenerational effects in non-target organisms. However, associated underlying mechanisms remain unclear. The aim of this study was to investigate permethrin-related transgenerational effects in the zebrafish model, and to identify possible molecular mechanisms underlying inheritance. Zebrafish (F0) were exposed to permethrin during early-life (2 h post-fertilization up to 28 days). The F1 and F2 offspring generations were obtained by pairing exposed F0 males and females, and were bred unexposed. Locomotor and anxiety behavior were investigated, together with transcriptomic and epigenomic (DNA methylation) changes in brains. Permethrin exposed F0 fish were hypoactive at adulthood, while males from the F1 and F2 generations showed a specific decrease in anxiety-like behavior. In F0, transcriptomic data showed enrichment in pathways related to glutamatergic synapse activity, which may partly underlie the behavioral effects. In F1 and F2 males, dysregulation of similar pathways was observed, including a subset of differentially methylated regions that were inherited from the F0 to the F2 generation and indicated stable dysregulation of glutamatergic signaling. Altogether, the present results provide novel evidence on the transgenerational neurotoxic effects of permethrin, as well as mechanistic insight: a transient exposure induces persistent transcriptional and DNA methylation changes that may translate into transgenerational alteration of glutamatergic signaling and, thus, into behavioral alterations.

Globalization, climate change, and transgenerational epigenetic inheritance: will our descendants be at risk?

Transgenerational epigenetic inheritance has gained increased attention due to the possibility that exposure to environmental contaminants induce diseases that propagate across generations through epigenomic alterations in gametes. In laboratory animals, exposure to environmental toxicants such as fungicides, pesticides, or plastic compounds has been shown to produce abnormal reproductive or metabolic phenotypes that are transgenerationally transmitted. Human exposures to environmental toxicants have increased due to industrialization and globalization, as well as the incidence of diseases shown to be transgenerationally transmitted in animal models. This new knowledge poses an urgent call to study transgenerational consequences of current human exposures to environmental toxicants.

The social construction of the social epigenome and the larger biological context

Epigenetics researchers in developmental, cell, and molecular biology greatly diverge in their understanding and definitions of epigenetics. In contrast, social epigeneticists, e.g., sociologists, scholars of STS, and behavioural scientists, share a focus and definition of epigenetics that is environmentally caused and trans-generationally inherited. This article demonstrates that this emphasis on the environment and on so-called Lamarckian inheritance, in addition to other factors, reflects an interdisciplinary power struggle with genetics, in which epigenetics appears to grant the social sciences a higher epistemic status. Social scientists' understanding of epigenetics, thus, appears in part to be socially constructed, i.e., the result of extra-scientific factors, such as social processes and the self-interest of the discipline. This article argues that social epigeneticists make far-reaching claims by selecting elements from research labelled epigenetics in biology while ignoring widely confirmed scientific facts in genetics and cell biology, such as the dependence of epigenetic marks on DNA sequence-specific events, or the lack of evidence for the lasting influence of the environment on epigenetic marks or the epigenome. Moreover, they treat as a given crucial questions that are far from resolved, such as what role, if any, DNA methylation plays in the complex biochemical system of regulating gene activity. The article also points out incorrect perceptions and media hypes among biological epigeneticists and calls attention to an apparent bias among scientific journals that prefer papers that promote transgenerational epigenetic inheritance over articles that critique it. The article concludes that while research labelled epigenetics contributes significantly to our knowledge about chromatin and the genome, it does not, as is often claimed, rehabilitate Lamarck or overthrow the fundamental biological principles of gene regulation, which are based on specific regulatory sequences of the genome.

An evolutionary perspective of lifespan and epigenetic inheritance

In the last decade epigenetics has come to the fore as a discipline which is central to biogerontology. Age associated epigenetic changes are routinely linked with pathologies, including cardiovascular disease, cancer, and Alzheimer's disease; moreover, epigenetic clocks are capable of correlating biological age with chronological age in many species including humans. Recent intriguing empirical observations also suggest that inherited epigenetic effects could influence lifespan/longevity in a variety of organisms. If this is the case, an imperative exists to reconcile lifespan/longevity associated inherited epigenetic processes with the evolution of ageing. This review will critically evaluate inherited epigenetic effects from an evolutionary perspective. The overarching aim is to integrate the evidence which suggests epigenetic inheritance modulates lifespan/longevity with the main evolutionary theories of ageing.

Transgenerational inheritance and its modulation by environmental cues

The epigenome plays an important role in shaping phenotypes. However, whether the environment can alter an organism's phenotype across several generations through epigenetic remodeling in the germline is still a highly debated topic. In this chapter, we briefly review the mechanisms of epigenetic inheritance and their connection with germline development before highlighting specific developmental windows of susceptibility to environmental cues. We further discuss the evidence of transgenerational inheritance to a range of different environmental cues, both epidemiological in humans and experimental in rodent models. Doing so, we pinpoint the current challenges in demonstrating transgenerational inheritance to environmental cues and offer insight in how recent technological advances may help deciphering the epigenetic mechanisms at play. Together, we draw a detailed picture of how our environment can influence our epigenomes, ultimately reshaping our phenotypes, in an extended theory of inheritance.

Considering the use of the terms strain and adaptation in prion research

Evolutionary biologists and disease biologists use the terms strain and adaptation in Chronic Wasting Disease (CWD) research in different ways. In evolutionary biology, a strain is a nascent genetic lineage that can be described by a genealogy, and a phylogenetic nomenclature constructed to reflect that genealogy. Prion strains are described as showing distinct host range, clinical presentation, disease progression, and neuropathological and PrP biochemical profiles, and lack information that would permit phylogenetic reconstruction of their history. Prion strains are alternative protein conformations, sometimes derived from the same genotype. I suggest referring to prion strains as ecotypes, because the variant phenotypic conformations ("strains") are a function of the interaction between PRNP amino acid genotype and the host environment. In the case of CWD, a prion ecotype in white-tailed deer would be described by its genotype and the host in which it occurs, such as the H95 + ecotype. However, an evolutionary nomenclature is difficult because not all individuals with the same PRNP genotype show signs of CWD, therefore creating a nomenclature reflecting and one-to-one relationship between PRNP genealogy and CWD presence is difficult. Furthermore, very little information exists on the phylogenetic distribution of CWD ecotypes in wild deer populations. Adaptation has a clear meaning in evolutionary biology, the differential survival and reproduction of individual genotypes. If a new prion ecotype arises in a particular host and kills more hosts or kills at an earlier age, it is the antithesis of the evolutionary definition of adaptation. However, prion strains might be transmitted across generations epigenetically, but whether this represents adaptation depends on the fitness consequences of the strain. Protein phenotypes of PRNP that cause transmissible spongiform encephalopathies (TSEs), and CWD, are maladaptive and would not be propagated genetically or epigenetically via a process consistent with an evolutionary view of adaptation. I suggest terming the process of prion strain origination "phenotypic transformation", and only adaptation if evidence shows they are not maladaptive and persist over evolutionary time periods (e.g., thousands of generations) and across distinct species boundaries (via inheritance). Thus, prion biologists use strain and adaptation, historically evolutionary terms, in quite different ways.

How epigenetic inheritance fails to explain the Black-White health gap

Slavery, legal segregation, and ongoing discrimination have exacted an unfathomable toll on the black population in the United States, particularly with respect to the impact on health outcomes. In recent years, various researchers and activists have suggested that racial disparities in the modern era can be attributed directly to the trauma of slavery, postulating that these unspeakable traumas led to epigenetic changes in slaves-changes that have since been passed down to subsequent generations. Investigating those claims in this paper, we comprise a review of previous literature that considers the potential for transgenerational epigenetic transmission of trauma in humans. However, we find that there is little evidence to indicate the presence of transgenerational epigenetic transmission of trauma in humans. We find no prior evidence that supports (or is relevant to) the notion that the black-white health gap stems from the inherited trauma of slavery. We conclude that, given the ongoing traumas black Americans are exposed to in modern America, it is much more likely that present-day racial health disparities are due to more direct and current mechanisms than transgenerational transmission of slavery-era trauma.

Transgenerational inheritance of mitochondrial hormetic oxidative stress mediated by histone H3K4me3 and H3K27me3 modifications

Mitochondrial hormetic oxidative stress (mtHOS) is crucial in physiology and disease; however, its effects on epigenetic inheritance and organism fitness across generations remains elusive. Utilizing the C. elegans as a model, we elucidate that parental exposure to mtHOS not only elicits a lifespan extension in the exposed individuals but also confers this longevity advantage to the progeny through the transgenerational epigenetic inheritance (TEI) mechanism. This transgenerational transmission of lifespan prolongation depends on the activation of the UPRmt and the synergistic action of the transcription factors DAF-16/FOXO and SKN-1/Nrf2. Additionally, the H3K4me3 and H3K27me3 serve as epigenetic mediators, selectively marking and regulating the expression of genes associated with oxidative stress response and longevity determination. Our findings illuminate the mechanisms underlying the implementation and transmission of mtHOS, revealing a sophisticated interplay among oxidative stress response genes and chromatin remodeling that collectively enhances the progeny's adaptive resilience to future challenges.

Cadmium exposure induces multigenerational inheritance of germ cell apoptosis and fertility suppression in Caenorhabditis elegans

Cadmium is a significant environmental pollutant that poses a substantial health hazard to humans due to its propensity to accumulate in the body and resist excretion. We have a comprehensive understanding of the damage caused by Cd exposure and the mechanisms of tolerance, however, the intricate mechanisms underlying multigenerational effects resulting from Cd exposure remain poorly understood. In this study, Caenorhabditis elegans were used as a model organism to investigate Cd-induced multigenerational effects and its association with epigenetic modifications. The results showed that Cd exposure leads to an increase in germ cell apoptosis and a decrease in fertility, which can be passed down to subsequent generations. Further analysis revealed that transcription factors DAF-16/FOXO and SKN-1/Nrf2 play essential roles in responding to Cd exposure and in the transgenerational induction of germ cell apoptosis. Additionally, histone H3K4 trimethylation (H3K4me3) marks stress-responsive genes and enhances their transcription, ultimately triggering multigenerational germ cell apoptosis. This study provides compelling evidence that the detrimental effects of Cd on the reproductive system can be inherited across generations. These findings enhance our understanding of the multigenerational effects of environmental pollutants and may inform strategies for the prevention and control of such pollutants.

Transgenerational epigenetic inheritance increases trait variation but is not adaptive

Understanding processes that can produce adaptive phenotypic shifts in response to rapid environmental change is critical to reducing biodiversity loss. The ubiquity of environmentally induced epigenetic marks has led to speculation that epigenetic inheritance could potentially enhance population persistence in response to environmental change. Yet, the magnitude and fitness consequences of epigenetic marks carried beyond maternal inheritance are largely unknown. Here, we tested how transgenerational epigenetic inheritance (TEI) shapes the phenotypic response of Daphnia clones to the environmental stressor Microcystis. We split individuals from each of eight genotypes into exposure and control treatments (F0 generation) and tracked the fitness of their descendants to the F3 generation. We found transgenerational epigenetic exposure to Microcystis led to reduced rates of survival and individual growth and no consistent effect on offspring production. Increase in trait variance in the F3 relative to F0 generations suggests potential for heritable bet hedging driven by TEI, which could impact population dynamics. Our findings are counter to the working hypothesis that TEI is a generally adaptive mechanism likely to prevent extinction for populations inhabiting rapidly changing environments.

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.

Calling the question: what is mammalian transgenerational epigenetic inheritance?

While transgenerational epigenetic inheritance has been extensively documented in plants, nematodes, and fruit flies, its existence in mammals remains controversial. Several factors have contributed to this debate, including the lack of a clear distinction between intergenerational and transgenerational epigenetic inheritance (TEI), the inconsistency of some studies, the potential confounding effects of in-utero vs. epigenetic factors, and, most importantly, the biological challenge of epigenetic reprogramming. Two waves of epigenetic reprogramming occur: in the primordial germ cells and the developing embryo after fertilization, characterized by global erasure of DNA methylation and remodelling of histone modifications. Consequently, TEI can only occur if specific genetic regions evade this reprogramming and persist through embryonic development. These challenges have revived the long-standing debate about the possibility of inheriting acquired traits, which has been strongly contested since the Lamarckian and Darwinian eras. As a result, coupled with the absence of universally accepted criteria for transgenerational epigenetic studies, a vast body of literature has emerged claiming evidence of TEI. Therefore, the goal of this study is to advocate for establishing fundamental criteria that must be met for a study to qualify as evidence of TEI. We identified five criteria based on the consensus of studies that critically evaluated TEI. To assess whether published original research papers adhere to these criteria, we examined 80 studies that either claimed or were cited as supporting TEI. The findings of this analysis underscore the widespread confusion in this field and highlight the urgent need for a unified scientific consensus on TEI requirements.