Prenatal stress-induced programming of genome-wide promoter DNA methylation in 5-HTT-deficient mice

Effects of developmental exposure to arsenic species on behavioral stress responses in larval zebrafish and implications for stress-related disorders

Arsenic (As) is globally detected in drinking water and food products at levels repeatedly surpassing regulatory thresholds. Several neurological and mental health risks linked to arsenic exposure are proposed; however, the nature of these effects and their association with the chemical forms of arsenic are not fully understood. Gaining a clear understanding of the etiologies and characteristics of these effects is crucial, particularly in association with developmental exposures where the nervous system is most vulnerable. In this study, we investigated the effects of early developmental exposure (6- to 120-h postfertilization [hpf]) of larval zebrafish to environmentally relevant concentrations of arsenic species-trivalent/pentavalent, inorganic/organic forms-on developmental, behavioral, and molecular endpoints to determine their effect on stress response and their potential association with stress-related disorders. At 120 hpf, the developing larvae were assessed for a battery of endpoints including survival, developmental malformities, background activity, and behavioral responses to acute visual and acoustic stimuli. Pooled larval samples were analyzed for alterations in the transcript levels of genes associated with developmental neurotoxicity and stress-related disorders. Developmental exposures at target concentrations did not significantly alter survival, overall development, or background activity, and had minor effects on developmental morphology. Sodium arsenate and monomethylarsonic acid exaggerated the behavioral responses of larval zebrafish, whereas sodium arsenite depressed them. Sodium arsenate induced significant effects on molecular biomarkers. This study highlights the effects of developmental exposure to arsenicals on the behavioral stress response, the role chemical formulation plays in exerting toxicological effects, and the possible association with stress-related disorders.

The translational genetics of ADHD and related phenotypes in model organisms

Attention-deficit/hyperactivity disorder (ADHD) is a highly prevalent neurodevelopmental disorder resulting from the interaction between genetic and environmental risk factors. It is well known that ADHD co-occurs frequently with other psychiatric disorders due, in part, to shared genetics factors. Although many studies have contributed to delineate the genetic landscape of psychiatric disorders, their specific molecular underpinnings are still not fully understood. The use of animal models can help us to understand the role of specific genes and environmental stimuli-induced epigenetic modifications in the pathogenesis of ADHD and its comorbidities. The aim of this review is to provide an overview on the functional work performed in rodents, zebrafish and fruit fly and highlight the generated insights into the biology of ADHD, with a special focus on genetics and epigenetics. We also describe the behavioral tests that are available to study ADHD-relevant phenotypes and comorbid traits in these models. Furthermore, we have searched for new models to study ADHD and its comorbidities, which can be useful to test potential pharmacological treatments.

Gene by environment interaction mouse model reveals a functional role for 5-hydroxymethylcytosine in neurodevelopmental disorders

Mouse knockouts of Cntnap2 show altered neurodevelopmental behavior, deficits in striatal GABAergic signaling, and a genome-wide disruption of an environmentally sensitive DNA methylation modification (5-hydroxymethylcytosine [5hmC]) in the orthologs of a significant number of genes implicated in human neurodevelopmental disorders. We tested adult Cntnap2 heterozygous mice (Cntnap2 +/-; lacking behavioral or neuropathological abnormalities) subjected to a prenatal stress and found that prenatally stressed Cntnap2 +/- female mice show repetitive behaviors and altered sociability, similar to the homozygote phenotype. Genomic profiling revealed disruptions in hippocampal and striatal 5hmC levels that are correlated to altered transcript levels of genes linked to these phenotypes (e.g., Reln, Dst, Trio, and Epha5). Chromatin immunoprecipitation coupled with high-throughput sequencing and hippocampal nuclear lysate pull-down data indicated that 5hmC abundance alters the binding of the transcription factor CLOCK near the promoters of these genes (e.g., Palld, Gigyf1, and Fry), providing a mechanistic role for 5hmC in gene regulation. Together, these data support gene-by-environment hypotheses for the origins of mental illness and provide a means to identify the elusive factors contributing to complex human diseases.

Genome-wide DNA methylation alteration in prenatally stressed Brahman heifer calves with the advancement of age

Possible phenotypic impairments associated with maternal stress during gestation in beef cattle may be explained by epigenetic effects. This study examined the impact of prenatal transportation stress on DNA methylation of lymphocytes of Brahman cows over the first 5 years of life. Methylation analysis through reduced representation bisulphite sequencing was conducted on DNA from lymphocytes from 28 paired samples from 6 prenatally stressed (PNS) and 8 control (Control) females obtained initially when they were 28 days of age and 5 years of age. There were 14,386 CpG (C = cytosine; p = phosphate; G = guanine) sites differentially methylated (P < 0.01) in 5-yr-old Control cows compared to their lymphocyte DNA at 28 days of age, this number was slightly decreased in 5-yr-old PNS with 13,378 CpG sites. Only 2,749 age-related differentially methylated CpG sites were seen within PNS females. There were 2,637 CpG sites differentially methylated (P < 0.01) in PNS cows relative to Controls at 5 years of age. There were differentially methylated genes in 5-yr-old cows that contributed similarly to altered gene pathways in both treatment groups. Canonical pathways altered in PNS compared to Control cows at 5 years of age were mostly related to development and growth, nervous system development and function, and immune response. Prenatal stress appeared to alter the epigenome in Brahman cows compared to Control at 5 years of age, which implies a persistent intervention in DNA methylation in lymphocytes, and may confer long-lasting effects on gene expression, and consequently relevant phenotypic changes.

Changes in Deoxyribonucleic Acid Methylation Contribute to the Pathophysiology of Multiple Sclerosis

Multiple sclerosis (MS) is an autoimmune disease of the central nervous system characterized by loss of coordination, weakness, dysfunctions in bladder capacity, bowel movement, and cognitive impairment. Thus, the disease leads to a significant socioeconomic burden. In the pathophysiology of the disease, both genetic and environmental risk factors are involved. Gene x environment interaction is modulated by epigenetic mechanisms. Epigenetics refers to a sophisticated system that regulates gene expression with no changes in the DNA sequence. The most studied epigenetic mechanism is the DNA methylation. In this review, we summarize the data available from the current literature by grouping sets of differentially methylated genes in distinct biological categories: the immune system including innate and adaptive response, the DNA damage, and the central nervous system.

Epigenome-wide association study of narcolepsy-affected lateral hypothalamic brains, and overlapping DNA methylation profiles between narcolepsy and multiple sclerosis

Narcolepsy with cataplexy is a sleep disorder caused by a deficiency in hypocretin neurons in the lateral hypothalamus (LH). Here we performed an epigenome-wide association study (EWAS) of DNA methylation for narcolepsy and replication analyses using DNA samples extracted from two brain regions: LH (Cases: N = 4; Controls: N = 4) and temporal cortex (Cases: N = 7; Controls: N = 7). Seventy-seven differentially methylated regions (DMRs) were identified in the LH analysis, with the top association of a DMR in the myelin basic protein (MBP) region. Only five DMRs were detected in the temporal cortex analysis. Genes annotated to LH DMRs were significantly associated with pathways related to fatty acid response or metabolism. Two additional analyses applying the EWAS data were performed: (1) investigation of methylation profiles shared between narcolepsy and other disorders and (2) an integrative analysis of DNA methylation data and a genome-wide association study for narcolepsy. The results of the two approaches, which included significant overlap of methylated positions associated with narcolepsy and multiple sclerosis, indicated that the two diseases may partly share their pathogenesis. In conclusion, DNA methylation in LH where loss of orexin-producing neurons occurs may play a role in the pathophysiology of the disease.

Paradoxical effects of exercise on hippocampal plasticity and cognition in mice with a heterozygous null mutation in the serotonin transporter gene

BACKGROUND AND PURPOSE

Exercise is known to improve cognitive function, but the exact synaptic and cellular mechanisms remain unclear. We investigated the potential role of the serotonin (5-HT) transporter (SERT) in mediating these effects.

EXPERIMENTAL APPROACH

Hippocampal long-term potentiation (LTP) and neurogenesis were measured in standard-housed and exercising (wheel running) wild-type (WT) and SERT heterozygous (HET) mice. We also assessed hippocampal-dependent cognition using the Morris water maze (MWM) and a spatial pattern separation touchscreen task.

KEY RESULTS

SERT HET mice had impaired hippocampal LTP regardless of the housing conditions. Exercise increased hippocampal neurogenesis in WT mice. However, this was not observed in SERT HET animals, even though both genotypes used the running wheels to a similar extent. We also found that standard-housed SERT HET mice displayed altered cognitive flexibility than WT littermate controls in the MWM reversal learning task. However, SERT HET mice no longer exhibited this phenotype after exercise. Cognitive changes, specific to SERT HET mice in the exercise condition, were also revealed on the touchscreen spatial pattern separation task, especially when the cognitive pattern separation load was at its highest.

CONCLUSIONS AND IMPLICATIONS

Our study is the first evidence of reduced hippocampal LTP in SERT HET mice. We also show that functional SERT is required for exercise-induced increase in adult neurogenesis. Paradoxically, exercise had a negative impact on hippocampal-dependent cognitive tasks, especially in SERT HET mice. Taken together, our results suggest unique complex interactions between exercise and altered 5-HT homeostasis.

Identification of Cholecystokinin by Genome-Wide Profiling as Potential Mediator of Serotonin-Dependent Behavioral Effects of Maternal Separation in the Amygdala

Converging evidence suggests a role of serotonin (5-hydroxytryptamine, 5-HT) and tryptophan hydroxylase 2 (TPH2), the rate-limiting enzyme of 5-HT synthesis in the brain, in modulating long-term, neurobiological effects of early-life adversity. Here, we aimed at further elucidating the molecular mechanisms underlying this interaction, and its consequences for socio-emotional behaviors, with a focus on anxiety and social interaction. In this study, adult, male Tph2 null mutant (Tph2 -/-) and heterozygous (Tph2 +/-) mice, and their wildtype littermates (Tph2 +/+) were exposed to neonatal, maternal separation (MS) and screened for behavioral changes, followed by genome-wide RNA expression and DNA methylation profiling. In Tph2 -/- mice, brain 5-HT deficiency profoundly affected socio-emotional behaviors, i.e., decreased avoidance of the aversive open arms in the elevated plus-maze (EPM) as well as decreased prosocial and increased rule breaking behavior in the resident-intruder test when compared to their wildtype littermates. Tph2 +/- mice showed an ambiguous profile with context-dependent, behavioral responses. In the EPM they showed similar avoidance of the open arm but decreased prosocial and increased rule breaking behavior in the resident-intruder test when compared to their wildtype littermates. Notably, MS effects on behavior were subtle and depended on the Tph2 genotype, in particular increasing the observed avoidance of EPM open arms in wildtype and Tph2 +/- mice when compared to their Tph2 -/- littermates. On the genomic level, the interaction of Tph2 genotype with MS differentially affected the expression of numerous genes, of which a subset showed an overlap with DNA methylation profiles at corresponding loci. Remarkably, changes in methylation nearby and expression of the gene encoding cholecystokinin, which were inversely correlated to each other, were associated with variations in anxiety-related phenotypes. In conclusion, next to various behavioral alterations, we identified gene expression and DNA methylation profiles to be associated with TPH2 inactivation and its interaction with MS, suggesting a gene-by-environment interaction-dependent, modulatory function of brain 5-HT availability.

Early-life stress impairs developmental programming in Cadherin 13 (CDH13)-deficient mice

OBJECTIVE

Cadherin-13 (CDH13), a member of the calcium-dependent cell adhesion molecule family, has been linked to neurodevelopmental disorders, including autism spectrum (ASD) and attention-deficit/hyperactivity (ADHD) disorders, but also to depression. In the adult brain, CDH13 expression is restricted e.g. to the presynaptic compartment of inhibitory GABAergic synapses in the hippocampus and Cdh13 knockout mice show an increased inhibitory drive onto hippocampal CA1 pyramidal neurons, leading to a shift in excitatory/inhibitory balance. CDH13 is also moderating migration of serotonergic neurons in the dorsal raphe nucleus, establishing projections preferentially to the thalamus and cerebellum during brain development. Furthermore, CDH13 is upregulated by chronic stress as well as in depression, suggesting a role in early-life adaptation to stressful experience. Here, we therefore investigated the interaction between Cdh13 variation and neonatal maternal separation (MS) in mice.

METHODS

Male and female wild-type (Cdh13^+/+), heterozygous (Cdh13^+/-) and homozygous (Cdh13^-/-) knockout mice exposed to MS, or daily handling as control, were subjected to a battery of behavioural tests to assess motor activity, learning and memory as well as anxiety-like behaviour. A transcriptome analysis of the hippocampus was performed in an independent cohort of mice which was exposed to MS or handling, but remained naïve for behavioural testing.

RESULTS

MS lead to increased anxiety-like behaviour in Cdh13^-/- mice compared to the other two MS groups. Cdh13^-/- mice showed a context-dependent effect on stress- and anxiety-related behaviour, impaired extinction learning following contextual fear conditioning and decreased impulsivity, as well as a mild decrease in errors in the Barnes maze and reduced risk-taking in the light-dark transition test after MS. We also show sex differences, with increased locomotor activity in female Cdh13^-/- mice, but unaltered impulsivity and activity in male Cdh13^-/- mice. Transcriptome analysis revealed several pathways associated with cell surface/adhesion molecules to be altered following Cdh13 deficiency, together with an influence on endoplasmic reticulum function.

CONCLUSION

MS resulted in increased stress resilience, increased exploration and an overall anxiolytic behavioural phenotype in male Cdh13^+/+ and Cdh13^+/- mice. Cdh13 deficiency, however, obliterated most of the effects caused by early-life stress, with Cdh13^-/- mice exhibiting delayed habituation, no reduction of anxiety-like behaviour and decreased fear extinction. Our behavioural findings indicate a role of CDH13 in the programming of and adaptation to early-life stress. Finally, our transcriptomic data support the view of CDH13 as a neuroprotective factor as well as a mediator in cell-cell interactions, with an impact on synaptic plasticity.

Editorial: Can dysregulated myelination be linked to ADHD pathogenesis and persistence?

Converging evidence from new epidemiologic, genetic, epigenetic, neuroimaging, and experimental model findings are further refining a long-standing concept, regarding the underlying neurobiology of attention-deficit/hyperactivity disorder (ADHD): that ADHD onset and its persistence into adulthood are the result of dysregulated myelination and associated alterations in neuronal plasticity - linked to disrupted brain maturation and the persistence of cognitive and emotional impairments across the life span. If supported by further work, this concept represents a pathophysiologic mechanism amenable to therapeutic intervention.

Epigenetic Changes Associated with Early Life Experiences: Saliva, A Biospecimen for DNA Methylation Signatures

BACKGROUND

Adverse Childhood Experiences (ACEs), which include traumatic injury, are associated with poor health outcomes in later life, yet the biological mechanisms mediating this association are unknown. Neurocircuitry, immune system and hormone regulation differ from normal in adults reporting ACEs. These systems could be affected by epigenetic changes, including methylation of cytosine (5mC) in genomic DNA, activated by ACEs. Since 5mC levels influence gene expression and can be long-lasting, altered 5mC status at specific sites or throughout the genome is hypothesized to influence mental and physical outcomes after ACE(s). Human and animal studies support this, with animal models allowing experiments for attributing causality. Here we provide a lengthy introduction and background on 5mC and the impact of early life adversity.

OBJECTIVE

Next we address the issue of a mixture of cell types in saliva, the most accessible biospecimen for 5mC analysis. Typical human bio-specimens for 5mC analysis include saliva or buccal swabs, whole blood or types of blood cells, tumors and post-mortem brain. In children saliva is the most accessible biospecimen, but contains a mixture of keratinocytes and white blood cells, as do buccal swabs. Even in saliva from the same individual at different time points, cell composition may differ widely. Similar issues affect analysis in blood, where nucleated cells represent a wide array of white blood cell types. Unless variations in ratios of these cells between each sample are included in the analysis, results can be unreliable.

METHODS

Several different biochemical assays are available to test for site-specific methylation levels genome-wide, each producing different information, with high-density arrays being the easiest to use, and bisulfite whole genome sequencing the most comprehensive. We compare results from different assays and use high-throughput computational processing to deconvolve cell composition in saliva samples.

RESULTS

Here we present examples demonstrating the critical importance of determining the relative contribution of blood cells versus keratinocytes to the 5mC profile found in saliva. We further describe a strategy to perform a reference-based computational correction for cell composition, and therefore to identify differential methylation patterns due to experience, or for the diagnosis of phenotypes that correlate between traits, such as hormone levels, trauma status and various mental health outcomes.

CONCLUSION

Specific sites that respond to adversity with altered methylation levels in either blood cells, keratinocytes or both can be identified by this rigorous approach, which will then be useful as diagnostic biomarkers and therapeutic targets.

[Social environment, biological embedding and social inequalities in health]

The social gradient in health refers to the fact that the higher individuals rise in the social hierarchy, the better is their health. Understanding the construction of this gradient is a major challenge in social epidemiology. An original approach consists in looking at how the different exposures (chemical, physical, behavioural, psychosocial…) associated with the social environment are ultimately expressed at the biological level influencing health positively or negatively, referring to the concept of biological embedding. Data from animal models and life course epidemiology have shed new light on the biological mechanisms potentially at play. Recent discoveries from the field of epigenetics provide a better understanding of how the social environment, especially the early environment, can influence biological functioning over the long term or even over several generations. The work on the biological embedding of the social environment in connection with epigenetics still needs to be very largely consolidated, but could constitute a change of perspective in human biology, particularly by reconsidering the influence of the environment on biological functioning, which is not without consequences in terms of public health interventions.

Positive and balancing selection on SLC18A1 gene associated with psychiatric disorders and human-unique personality traits

Maintenance of genetic variants susceptible to psychiatric disorders is one of the intriguing evolutionary enigmas. The present study detects three psychiatric disorder‐relevant genes (CLSTN2, FAT1, and SLC18A1) that have been under positive selection during the human evolution. In particular, SLC18A1 (vesicular monoamine transporter 1; VMAT1) gene has a human‐unique variant (rs1390938, Thr136Ile), which is associated with bipolar disorders and/or the anxiety‐related personality traits. 136Ile shows relatively high (20–61%) frequency in non‐African populations, and Tajima's D reports a significant peak around the Thr136Ile site, suggesting that this polymorphism has been positively maintained by balancing selection in non‐African populations. Moreover, Coalescent simulations predict that 136Ile originated around 100,000 years ago, the time being generally associated with the Out‐of‐Africa migration of modern humans. Our study sheds new light on a gene in monoamine pathway as a strong candidate contributing to human‐unique psychological traits.

Prenatal stress and genetic risk: How prenatal stress interacts with genetics to alter risk for psychiatric illness

Risk for neuropsychiatric disorders is complex and includes an individual's internal genetic endowment and their environmental experiences and exposures. Embryonic development captures a particularly complex period, in which genetic and environmental factors can interact to contribute to risk. These environmental factors are incorporated differently into the embryonic brain than postnatal one. Here, we comprehensively review the human and animal model literature for studies that assess the interaction between genetic risks and one particular environmental exposure with strong and complex associations with neuropsychiatric outcomes-prenatal maternal stress. Gene-environment interaction has been demonstrated for stress occurring during childhood, adolescence, and adulthood. Additional work demonstrates that prenatal stress risk may be similarly complex. Animal model studies have begun to address some underlying mechanisms, including particular maternal or fetal genetic susceptibilities that interact with stress exposure and those that do not. More specifically, the genetic underpinnings of serotonin and dopamine signaling and stress physiology mechanisms have been shown to be particularly relevant to social, attentional, and internalizing behavioral changes, while other genetic factors have not, including some growth factor and hormone-related genes. Interactions have reflected both the diathesis-stress and differential susceptibility models. Maternal genetic factors have received less attention than those in offspring, but strongly modulate impacts of prenatal stress. Priorities for future research are investigating maternal response to distinct forms of stress and developing whole-genome methods to examine the contributions of genetic variants of both mothers and offspring, particularly including genes involved in neurodevelopment. This is a burgeoning field of research that will ultimately contribute not only to a broad understanding of psychiatric pathophysiology but also to efforts for personalized medicine.

Impact of varying social experiences during life history on behaviour, gene expression, and vasopressin receptor gene methylation in mice

Both negative and positive social experiences during sensitive life phases profoundly shape brain and behaviour. Current research is therefore increasingly focusing on mechanisms mediating the interaction between varying life experiences and the epigenome. Here, male mice grew up under either adverse or beneficial conditions until adulthood, when they were subdivided into groups exposed to situations that either matched or mismatched previous conditions. It was investigated whether the resulting four life histories were associated with changes in anxiety-like behaviour, gene expression of selected genes involved in anxiety and stress circuits, and arginine vasopressin receptor 1a (Avpr1a) gene methylation. Varying experiences during life significantly modulated (1) anxiety-like behaviour; (2) hippocampal gene expression of Avpr1a, serotonin receptor 1a (Htr1a), monoamine oxidase A (Maoa), myelin basic protein (Mbp), glucocorticoid receptor (Nr3c1), growth hormone (Gh); and (3) hippocampal DNA methylation within the Avpr1a gene. Notably, mice experiencing early beneficial and later adverse conditions showed a most pronounced downregulation of Avpr1a expression, accompanied by low anxiety-like behaviour. This decrease in Avpr1a expression may have been, in part, a consequence of increased methylation in the Avpr1a gene. In summary, this study highlights the impact of interactive social experiences throughout life on the hippocampal epigenotype and associated behaviour.

Interplay between maternal Slc6a4 mutation and prenatal stress: a possible mechanism for autistic behavior development

The low activity allele of the maternal polymorphism, 5HTTLPR, in the serotonin transporter, SLC6A4, coupled with prenatal stress is reported to increase the risk for children to develop autism spectrum disorder (ASD). Similarly, maternal Slc6a4 knock-out and prenatal stress in rodents results in offspring demonstrating ASD-like characteristics. The present study uses an integrative genomics approach to explore mechanistic changes in early brain development in mouse embryos exposed to this maternal gene-environment phenomenon. Restraint stress was applied to pregnant Slc6a4 ^+/+ and Slc6a4 ^+/- mice and post-stress embryonic brains were assessed for whole genome level profiling of methylome, transcriptome and miRNA using Next Generation Sequencing. Embryos of stressed Slc6a4 ^+/+ dams exhibited significantly altered methylation profiles and differential expression of 157 miRNAs and 1009 genes affecting neuron development and cellular adhesion pathways, which may function as a coping mechanism to prenatal stress. In striking contrast, the response of embryos of stressed Slc6a4 ^+/- dams was found to be attenuated, shown by significantly reduced numbers of differentially expressed genes (458) and miRNA (0) and genome hypermethylation. This attenuated response may pose increased risks on typical brain development resulting in development of ASD-like characteristics in offspring of mothers with deficits in serotonin related pathways during stressful pregnancies.

Epigenetic programming by stress and glucocorticoids along the human lifespan

Psychosocial stress triggers a set of behavioral, neural, hormonal, and molecular responses that can be a driving force for survival when adaptive and time-limited, but may also contribute to a host of disease states if dysregulated or chronic. The beneficial or detrimental effects of stress are largely mediated by the hypothalamic-pituitary axis, a highly conserved neurohormonal cascade that culminates in systemic secretion of glucocorticoids. Glucocorticoids activate the glucocorticoid receptor, a ubiquitous nuclear receptor that not only causes widespread changes in transcriptional programs, but also induces lasting epigenetic modifications in many target tissues. While the epigenome remains sensitive to stressors throughout life, we propose two key principles that may govern the epigenetics of stress and glucocorticoids along the lifespan: first, the presence of distinct life periods, during which the epigenome shows heightened plasticity to stress exposure, such as in early development and at advanced age; and, second, the potential of stress-induced epigenetic changes to accumulate throughout life both in select chromatin regions and at the genome-wide level. These principles have important clinical and translational implications, and they show striking parallels with the existence of sensitive developmental periods and the cumulative impact of stressful experiences on the development of stress-related phenotypes. We hope that this conceptual mechanistic framework will stimulate fruitful research that aims at unraveling the molecular pathways through which our life stories sculpt genomic function to contribute to complex behavioral and somatic phenotypes.

The Serotonin Transporter and Early Life Stress: Translational Perspectives

The interaction between the serotonin transporter (SERT) linked polymorphic region (5-HTTLPR) and adverse early life stressing (ELS) events is associated with enhanced stress susceptibility and risk to develop mental disorders like major depression, anxiety, and aggressiveness. In particular, human short allele carriers are at increased risk. This 5-HTTLPR polymorphism is absent in the rodent SERT gene, but heterozygous SERT knockout rodents (SERT^+/−) show several similarities to the human S-allele carrier, therefore creating an animal model of the human situation. Many rodent studies investigated ELS interactions in SERT knockout rodents combined with ELS. However, underlying neuromolecular mechanisms of the (mal)adaptive responses to adversity displayed by SERT rodents remain to be elucidated. Here, we provide a comprehensive review including studies describing mechanisms underlying SERT variation × ELS interactions in rodents. Alterations at the level of translation and transcription but also epigenetic alterations considerably contribute to underlying mechanisms of SERT variation × ELS interactions. In particular, SERT^+/− rodents exposed to adverse early rearing environment may be of high translational and predictive value to the more stress sensitive human short-allele carrier, considering the similarity in neurochemical alterations. Therefore, SERT^+/− rodents are highly relevant in research that aims to unravel the complex psychopathology of mental disorders. So far, most studies fail to show solid evidence for increased vulnerability to develop affective-like behavior after ELS in SERT^+/− rodents. Several reasons may underlie these failures, e.g., (1) stressors used might not be optimal or severe enough to induce maladaptations, (2) effects in females are not sufficiently studied, and (3) few studies include both behavioral manifestations and molecular correlates of ELS-induced effects in SERT^+/− rodents. Of course, one should not exclude the (although unlikely) possibility of SERT^+/− rodents not being sensitive to ELS. In conclusion, future studies addressing ELS-induced effects in the SERT^+/− rodents should extensively study both long-term behavioral and (epi)genetic aspects in both sexes. Finally, further research is warranted using more severe stressors in animal models. From there on, we should be able to draw solid conclusions whether the SERT^+/− exposed to ELS is a suitable translational animal model for studying 5-HTTLPR polymorphism and stress interactions.

The Epigenetic Link between Prenatal Adverse Environments and Neurodevelopmental Disorders

Prenatal adverse environments, such as maternal stress, toxicological exposures, and viral infections, can disrupt normal brain development and contribute to neurodevelopmental disorders, including schizophrenia, depression, and autism. Increasing evidence shows that these short- and long-term effects of prenatal exposures on brain structure and function are mediated by epigenetic mechanisms. Animal studies demonstrate that prenatal exposure to stress, toxins, viral mimetics, and drugs induces lasting epigenetic changes in the brain, including genes encoding glucocorticoid receptor (Nr3c1) and brain-derived neurotrophic factor (Bdnf). These epigenetic changes have been linked to changes in brain gene expression, stress reactivity, and behavior, and often times, these effects are shown to be dependent on the gestational window of exposure, sex, and exposure level. Although evidence from human studies is more limited, gestational exposure to environmental risks in humans is associated with epigenetic changes in peripheral tissues, and future studies are required to understand whether we can use peripheral biomarkers to predict neurobehavioral outcomes. An extensive research effort combining well-designed human and animal studies, with comprehensive epigenomic analyses of peripheral and brain tissues over time, will be necessary to improve our understanding of the epigenetic basis of neurodevelopmental disorders.

Circadian behavior of adult mice exposed to stress and fluoxetine during development

INTRODUCTION

Women of child-bearing age are the population at greatest risk for depression. The stress experienced during pregnancy and the associated antidepressant treatments can both affect fetal development. Fluoxetine (FLX) is among the most common antidepressants used by pregnant women. We have previously demonstrated that perinatal exposure to FLX can alter expression of circadian rhythms in adulthood. Here, we examine the combined effects of maternal stress during pregnancy and perinatal exposure to the antidepressant FLX on the circadian behavior of mice as adults.

METHODS

Mouse dams were exposed to chronic unpredictable stress (embryonic (E) day 7 to E18), FLX (E15 to postnatal day 12), a combination of both stress and FLX, or were left untreated. At 2 months of age, male offspring were placed in recording chambers and circadian organization of wheel running rhythms and phase shifts to photic and non-photic stimuli were assessed.

RESULTS

Mice exposed to prenatal stress (PS) had smaller light-induced phase delays. Mice exposed to perinatal FLX required more days to re-entrainment to an 8-h phase advance of their light-dark cycle. Mice subjected to either perinatal FLX or to PS had larger light-induced phase advances and smaller phase advances to 8-OH-DPAT. FLX treatment partially reversed the effect of PS on phase shifts to late-night light exposure and to 8-OH-DPAT.

CONCLUSIONS

Our results suggest that, in mice, perinatal exposure to either FLX, or PS, or their combination, leads to discernible, persistent changes in their circadian systems as adults.

Epigenetic dysregulation of brainstem nuclei in the pathogenesis of Alzheimer's disease: looking in the correct place at the right time?

Even though the etiology of Alzheimer's disease (AD) remains unknown, it is suggested that an interplay among genetic, epigenetic and environmental factors is involved. An increasing body of evidence pinpoints that dysregulation in the epigenetic machinery plays a role in AD. Recent developments in genomic technologies have allowed for high throughput interrogation of the epigenome, and epigenome-wide association studies have already identified unique epigenetic signatures for AD in the cortex. Considerable evidence suggests that early dysregulation in the brainstem, more specifically in the raphe nuclei and the locus coeruleus, accounts for the most incipient, non-cognitive symptomatology, indicating a potential causal relationship with the pathogenesis of AD. Here we review the advancements in epigenomic technologies and their application to the AD research field, particularly with relevance to the brainstem. In this respect, we propose the assessment of epigenetic signatures in the brainstem as the cornerstone of interrogating causality in AD. Understanding how epigenetic dysregulation in the brainstem contributes to AD susceptibility could be of pivotal importance for understanding the etiology of the disease and for the development of novel diagnostic and therapeutic strategies.

Early life adversity: Lasting consequences for emotional learning

The early postnatal period is a highly sensitive time period for the developing brain, both in humans and rodents. During this time window, exposure to adverse experiences can lastingly impact cognitive and emotional development. In this review, we briefly discuss human and rodent studies investigating how exposure to adverse early life conditions - mainly related to quality of parental care - affects brain activity, brain structure, cognition and emotional responses later in life. We discuss the evidence that early life adversity hampers later hippocampal and prefrontal cortex functions, while increasing amygdala activity, and the sensitivity to stressors and emotional behavior later in life. Exposure to early life stress may thus on the one hand promote behavioral adaptation to potentially threatening conditions later in life -at the cost of contextual memory formation in less threatening situations- but may on the other hand also increase the sensitivity to develop stress-related and anxiety disorders in vulnerable individuals.

Life stress, glucocorticoid signaling, and the aging epigenome: Implications for aging-related diseases

Life stress has been associated with accelerated cellular aging and increased risk for developing aging-related diseases; however, the underlying molecular mechanisms remain elusive. A highly relevant process that may underlie this association is epigenetic regulation. In this review, we build upon existing evidence to propose a model whereby exposure to life stress, in part via its effects on the hypothalamic-pituitary axis and the glucocorticoid signaling system, may alter the epigenetic landscape across the lifespan and, consequently, influence genomic regulation and function in ways that are conducive to the development of aging-related diseases. This model is supported by recent studies showing that life stressors and stress-related phenotypes can accelerate epigenetic aging, a measure that is based on DNA methylation prediction of chronological age and has been associated with several aging-related disease phenotypes. We discuss the implications of this model for the prevention and treatment of aging-related diseases, as well as the challenges and limitations of this line of research.

Influence of 5-HTT variation, childhood trauma and self-efficacy on anxiety traits: a gene-environment-coping interaction study

Environmental vulnerability factors such as adverse childhood experiences in interaction with genetic risk variants, e.g., the serotonin transporter gene linked polymorphic region (5-HTTLPR), are assumed to play a role in the development of anxiety and affective disorders. However, positive influences such as general self-efficacy (GSE) may exert a compensatory effect on genetic disposition, environmental adversity, and anxiety traits. We, thus, assessed childhood trauma (Childhood Trauma Questionnaire, CTQ) and GSE in 678 adults genotyped for 5-HTTLPR/rs25531 and their interaction on agoraphobic cognitions (Agoraphobic Cognitions Questionnaire, ACQ), social anxiety (Liebowitz Social Anxiety Scale, LSAS), and trait anxiety (State-Trait Anxiety Inventory, STAI-T). The relationship between anxiety traits and childhood trauma was moderated by self-efficacy in 5-HTTLPR/rs25531 LALA genotype carriers: LALA probands maltreated as children showed high anxiety scores when self-efficacy was low, but low anxiety scores in the presence of high self-efficacy despite childhood maltreatment. Our results extend previous findings regarding anxiety-related traits showing an interactive relationship between 5-HTT genotype and adverse childhood experiences by suggesting coping-related measures to function as an additional dimension buffering the effects of a gene-environment risk constellation. Given that anxiety disorders manifest already early in childhood, this insight could contribute to the improvement of psychotherapeutic interventions by including measures strengthening self-efficacy and inform early targeted preventive interventions in at-risk populations, particularly within the crucial time window of childhood and adolescence.

Failure to upregulate Agrp and Orexin in response to activity based anorexia in weight loss vulnerable rats characterized by passive stress coping and prenatal stress experience

We hypothesize that anorexia nervosa (AN) poses a physiological stress. Therefore, the way an individual copes with stress may affect AN vulnerability. Since prenatal stress (PNS) exposure alters stress responsivity in offspring this may increase their risk of developing AN. We tested this hypothesis using the activity based anorexia (ABA) rat model in control and PNS rats that were characterized by either proactive or passive stress-coping behavior. We found that PNS passively coping rats ate less and lost more weight during the ABA paradigm. Exposure to ABA resulted in higher baseline corticosterone and lower insulin levels in all groups. However, leptin levels were only decreased in rats with a proactive stress-coping style. Similarly, ghrelin levels were increased only in proactively coping ABA rats. Neuropeptide Y (Npy) expression was increased and proopiomelanocortin (Pomc) expression was decreased in all rats exposed to ABA. In contrast, agouti-related peptide (Agrp) and orexin (Hctr) expression were increased in all but the PNS passively coping ABA rats. Furthermore, DNA methylation of the orexin gene was increased after ABA in proactive coping rats and not in passive coping rats. Overall our study suggests that passive PNS rats have innate impairments in leptin and ghrelin in responses to starvation combined with prenatal stress associated impairments in Agrp and orexin expression in response to starvation. These impairments may underlie decreased food intake and associated heightened body weight loss during ABA in the passively coping PNS rats.

Effects of maternal stress and perinatal fluoxetine exposure on behavioral outcomes of adult male offspring

Women of child-bearing age are the population group at highest risk for depression. In pregnant women, fluoxetine (Flx) is the most widely prescribed selective serotonin reuptake inhibitor (SSRI) used for the treatment of depression. While maternal stress, depression, and Flx exposure have been shown to effect neurodevelopment of the offspring, separately, combined effects of maternal stress and Flx exposure have not been extensively examined. The present study investigated the effects of prenatal maternal stress and perinatal exposure to the SSRI Flx on the behavior of male mice as adults.

METHODS

C57BL/6 dams exposed to chronic unpredictable stress from embryonic (E) day 4 to E18 and non-stressed dams were administered Flx (25 mg/kg/d) in the drinking water from E15 to postnatal day 12. A separate control group consisted of animals that were not exposed to stress or Flx. At 12 days of age, brain levels of serotonin were assessed in the male offspring. At two months of age, the male offspring of mothers exposed to prenatal stress (PS), perinatal Flx, PS and Flx, or neither PS or Flx, went through a comprehensive behavioral test battery. At the end of testing brain-derived neurotropic factor (BDNF) levels were assessed in the frontal cortex of the offspring.

RESULTS

Maternal behavior was not altered by either stress or Flx treatment. Treatment of the mother with Flx led to detectible Flx and NorFlx levels and lead to a decrease in serotonin levels in pup brains. In the adult male offspring, while perinatal exposure to Flx increased aggressive behavior, prenatal maternal stress decreased aggressive behavior. Interestingly, the combined effects of stress and Flx normalized aggressive behavior. Furthermore, perinatal Flx treatment led to a decrease in anxiety-like behavior in male offspring. PS led to hyperactivity and a decrease in BDNF levels in the frontal cortex regardless of Flx exposure. Neither maternal stress or Flx altered offspring performance in tests of cognitive abilities, memory, sensorimotor information processing, or risk assessment behaviors. These results demonstrate that maternal exposure to stress and Flx have a number of sustained effects on the male offspring.

METHYLATION OF SEROTONIN RECEPTOR 3A IN ADHD, BORDERLINE PERSONALITY, AND BIPOLAR DISORDERS: LINK WITH SEVERITY OF THE DISORDERS AND CHILDHOOD MALTREATMENT

BACKGROUND

Serotonin 3A receptor (5-HT3A R) is associated at the genetic and epigenetic levels with a variety of psychiatric disorders and interacts with early-life stress such as childhood maltreatment. We studied the impact of childhood maltreatment on the methylation status of the 5-HT3A R and its association with clinical severity outcomes in relation with a functional genetic polymorphism.

METHODS

Clinical severity indexes of 346 bipolar, borderline personality, and adult attention deficit hyperactivity disorders patients were tested for association with the DNA methylation status of eight 5-HT3A R gene CpGs. Relationship between the functional variant rs1062613 (C > T) and methylation status on severity of the disorders were also assessed.

RESULTS

Childhood maltreatment was associated with higher severity of the disease (higher number of mood episodes, history of suicide attempts, hospitalization, and younger age at onset) across disorders and within each individual disorder. This effect was mediated by two 5-HT3A R CpGs. Compared to T allele carriers, CC carriers had higher methylation status at one CpG located 1 bp upstream of this variant.

CONCLUSIONS

This study shows that epigenetic modification of the 5-HT3A R is involved in the mechanism underlying the relationship between maltreatment in childhood and the severity of several psychiatric disorders in adulthood.

Stress In Utero: Prenatal Programming of Brain Plasticity and Cognition

Animal studies confirm earlier anecdotal observations in humans to indicate that early life experience has a profound impact on adult behavior, years after the original experience has vanished. These studies also highlight the role of early life adversaries in the shaping of a disordered brain. Evidence is accumulating to indicate that the epigenome, through which the environment regulates gene expression, is responsible for long-lasting effects of stress during pregnancy on brain and behavior. A possible differential effect of the environment on the epigenome may underlie the observation that only a small fraction of a population with similar genetic background deteriorates into mental disorders. Considerable progress has been made in the untangling of the epigenetic mechanisms that regulate emotional brain development. The present review focuses on the lasting effects of prenatal stress on brain plasticity and cognitive functions in human and rodent models. Although human studies stress the significance of early life experience in functional maturation, they lack the rigor inherent in controlled animal experiments. Furthermore, the analysis of molecular and cellular mechanisms affected by prenatal stress is possible only in experimental animals. The present review attempts to link human and animal studies while proposing molecular mechanisms that interfere with functional brain development.

Genetics of anxiety disorders: Genetic epidemiological and molecular studies in humans

This review provides a broad overview of the state of research in the genetics of anxiety disorders (AD). Genetic epidemiological studies report a moderate level of familial aggregation (odds ratio: 4-6) and heritability estimates are about 30-50%. Twin studies suggest that the genetic architecture of AD is not isomorphic with their classifications, sharing risk factors with each other. So far, linkage and association studies of AD have produced inconclusive results. Genome-wide association studies of AD can provide an unbiased survey of common genetic variations across the entire genome. Given the shared causes of AD that transcend our current diagnostic classifications, clustering anxiety phenotypes into broader groups may be a powerful approach to identifying susceptibility locus for AD. Using such a shared genetic risk factor, meta-analyses of genome-wide association studies of AD conducted by large consortia are needed. Environmental factors also make a substantial contribution to the cause of AD. Although candidate gene studies of gene by environmental (G × E) interaction have appeared recently, no genome-wide search for G × E interactions have been performed. Epigenetic modification of DNA appears to have important effects on gene expression mediating environmental influences on disease risk. Given that G × E can be linked to an epigenetic modification, a combination analysis of genome-wide G × E interaction and methylation could be an alternative method to find risk variants for AD. This genetic research will enable us to utilize more effective strategies for the prevention and treatment of AD in the near future.

Annual research review: The (epi)genetics of neurodevelopmental disorders in the era of whole-genome sequencing--unveiling the dark matter

BACKGROUND AND SCOPE

Neurodevelopmental disorders (NDDs) are defined by a wide variety of behavioural phenotypes, psychopathology and clinically informed categorical classifications. Diagnostic entities include intellectual disability (ID), the autism spectrum (ASD) and attention-deficit/hyperactivity disorder (ADHD). The aetiopathogenesis of these conditions and disorders involves an interaction between both genetic and environmental risk factors on the developmental trajectory. Despite their remarkable genetic heterogeneity and complexity of pathophysiological mechanisms, NDDs display an overlap in their phenotypic features, a considerable degree of comorbidity as well as sharing of genetic and environmental risk factors. This review aims to provide an overview of the genetics and epigenetic of NDDs.

FINDINGS

Recent evidence suggests a critical role of defined and tightly regulated neurodevelopmental programs running out of control in NDDs, most notably neuronal proliferation and migration, synapse formation and remodelling, as well as neural network configuration resulting in compromised systems connectivity and function. Moreover, the machinery of epigenetic programming, interacting with genetic liability, impacts many of those processes and pathways, thus modifying vulnerability of, and resilience to, NDDs. Consequently, the categorically defined entities of ID, ADHD and ASD are increasingly viewed as disorders on a multidimensional continuum of molecular and cellular deficiencies in neurodevelopment. As such, this range of NDDs displays a broad phenotypic diversity, which may be explained by a combination and interplay of underlying loss- and potential gain-of-function traits.

CONCLUSION

In this overview, we discuss a backbone continuum concept of NDDs by summarizing pertinent findings in genetics and epigenetics. We also provide an appraisal of the genetic overlap versus differences, with a focus on genome-wide screening approaches for (epi)genetic variation. Finally, we conclude with insights from evolutionary psychobiology suggesting positive selection for discrete NDD-associated traits.