Differential methylation of genes in the medial prefrontal cortex of developing and adult rats following exposure to maltreatment or nurturing care during infancy

BNDF methylation in mothers and newborns is associated with maternal exposure to war trauma

BACKGROUND

The BDNF gene codes for brain-derived neurotrophic factor, a growth factor involved in neural development, cell differentiation, and synaptic plasticity. Present in both the brain and periphery, BDNF plays critical roles throughout the body and is essential for placental and fetal development. Rodent studies show that early life stress, including prenatal stress, broadly alters BDNF methylation, with presumed changes in gene expression. No studies have assessed prenatal exposure to maternal traumatic stress and BDNF methylation in humans. This study examined associations of prenatal exposure to maternal stress and BDNF methylation at CpG sites across the BDNF gene.

RESULTS

Among 24 mothers and newborns in the eastern Democratic Republic of Congo, a region with extreme conflict and violence to women, maternal experiences of war trauma and chronic stress were associated with BDNF methylation in umbilical cord blood, placental tissue, and maternal venous blood. Associations of maternal stress and BDNF methylation showed high tissue specificity. The majority of significant associations were observed in putative transcription factor binding regions.

CONCLUSIONS

This is the first study in humans to examine BDNF methylation in relation to prenatal exposure to maternal stress in three tissues simultaneously and the first in any mammalian species to report associations of prenatal stress and BDNF methylation in placental tissue. The findings add to the growing body of evidence highlighting the importance of considering epigenetic effects when examining the impacts of trauma and stress, not only for adults but also for offspring exposed via effects transmitted before birth.

Brain-derived neurotrophic factor and early-life stress: Multifaceted interplay

The brain-derived neurotrophic factor (BDNF) is a key regulator of neural development and plasticity. Longterm changes in the BDNF pathway are associated with childhood adversity and adult depression symptoms. Initially, stress-induced decreases in the BDNF pathway were found in some studies, but subsequent reports indicated the relationship between stress and BDNF to be much more complex, and the concept was significantly revised. In the present mini-review, we focus on the structure and regulation of the Bbnf gene as well as on the stress-BDNF interactions under early-life adverse conditions.

Aberrant development of intrinsic brain activity in a rat model of caregiver maltreatment of offspring

Caregiver maltreatment induces vulnerability to later-life psychopathology. Clinical and preclinical evidence suggest changes in prefrontal and limbic circuitry underlie this susceptibility. We examined this question using a rat model of maternal maltreatment and methods translated from humans, resting-state functional magnetic resonance imaging (R-fMRI). Rat pups were reared by mothers provided with insufficient or abundant bedding for nest building from postnatal (PN) days 8 to 12 and underwent behavioral assessments of affect-related behaviors (forced swim, sucrose preference and social interaction) in adolescence (PN45) and early adulthood (PN60). R-fMRI sessions were conducted under light anesthesia at both ages. Offspring reared with insufficient bedding (that is, maltreated) displayed enduring negative affective behaviors. Amygdala-prefrontal cortex (PFC) functional connectivity increased significantly from adolescence to adulthood in controls, but not in maltreated animals. We computed the fractional amplitude of low-frequency fluctuations (fALFF), an index of intrinsic brain activity, and found that fALFF in medial prefrontal cortex and anterior cingulate cortex (MPFC/ACC) increased significantly with age in controls but remained unchanged in maltreated animals during adolescence and adulthood. We used a seed-based analysis to explore changes in functional connectivity between this region and the whole brain. Compared with controls, maltreated animals demonstrated reduced functional connectivity between MPFC/ACC and left caudate/putamen across both ages. Functional connectivity between MPFC/ACC and right caudate/putamen showed a group by age interaction: decreased in controls but increased in maltreated animals. These data suggest that maltreatment induces vulnerability to psychopathology and is associated with differential developmental trajectories of prefrontal and subcortical circuits underlying affect regulation.

Sex-Specific Effects of Childhood Poverty on Neurocircuitry of Processing of Emotional Cues: A Neuroimaging Study

BACKGROUND

There is accumulating evidence on the negative impacts of childhood poverty on physical and mental health. Previous work has suggested hyperactive neural response to social fear cues, as well as impairment in neural regulatory functions. However, despite differences found between males and females in stress-related and anxiety disorders, possible sex-specific effects of poverty on emotional processing have not been explored.

METHODS

We analyzed data from three previously reported experiments of childhood poverty effects on emotional processing and regulation, for sex-specific effects. Participants were 52 healthy Caucasian males and females, from a longitudinal cohort of poverty development study, who were recruited for examining the long-term effects of childhood poverty and stress. The three functional MRI studies included emotion regulation task, emotional face assessment task, and shifted attention emotion appraisal task. Brain activations that associated with childhood poverty previously were entered into a regression analysis with interaction of gender by childhood income-to-need ratio as the independent variable, and age and current income-to-need ratio as variables of no interest, separately for males and females.

RESULTS

Amygdala reactivity to implicitly processed fearful faces was positively correlated with childhood income-to-need in adult females but not males. On the other hand, activation in dorsolateral and ventrolateral prefrontal regions during emotion regulation by reappraisal was positively correlated with childhood income-to-need in males.

CONCLUSION

Childhood poverty may exert sex-specific effects in adulthood as presented by hypersensitive emotional reactivity of the amygdala in females, and impaired emotion regulatory function of the prefrontal cortex in males. Results suggest further focus on sex-specific effects of childhood poverty.

The epigenetic impacts of social stress: how does social adversity become biologically embedded?

Epigenetic mechanisms are implicated in the processes through which social stressors erode health in humans and other animals. Here I review progress in elucidating the biological pathways underlying the social gradient in health, with particular emphasis on how behavioral stresses influence epigenomic variation linked to health. The evidence that epigenetic changes are involved in embedding of social status-linked chronic stress is reviewed in the context of current knowledge about behavior within animal dominance hierarchies and the impacts of social position on behaviors that affect health. The roles of epigenetic mechanisms in responses to trauma and the evidence for their involvement in intergenerational transmission of the biological impacts of traumatic stress are also considered. Taken together, the emerging insights have important implications for development of strategies to improve societal health and well-being.

Depression in cancer: The many biobehavioral pathways driving tumor progression

Major Depressive Disorder (MDD) is common among cancer patients, with prevalence rates up to four-times higher than the general population. Depression confers worse outcomes, including non-adherence to treatment and increased mortality in the oncology setting. Advances in the understanding of neurobiological underpinnings of depression have revealed shared biobehavioral mechanisms may contribute to cancer progression. Moreover, psychosocial stressors in cancer promote: (1) inflammation and oxidative/nitrosative stress; (2) a decreased immunosurveillance; and (3) a dysfunctional activation of the autonomic nervous system and of the hypothalamic-pituitaryadrenal axis. Consequently, the prompt recognition of depression among patients with cancer who may benefit of treatment strategies targeting depressive symptoms, cognitive dysfunction, fatigue and sleep disturbances, is a public health priority. Moreover, behavioral strategies aiming at reducing psychological distress and depressive symptoms, including addressing unhealthy diet and life-style choices, as well as physical inactivity and sleep dysfunction, may represent important strategies not only to treat depression, but also to improve wider cancer-related outcomes. Herein, we provide a comprehensive review of the intertwined biobehavioral pathways linking depression to cancer progression. In addition, the clinical implications of these findings are critically reviewed.

Insight from animal models of environmentally driven epigenetic changes in the developing and adult brain

The efforts of many neuroscientists are directed toward understanding the appreciable plasticity of the brain and behavior. In recent years, epigenetics has become a core of this focus as a prime mechanistic candidate for behavioral modifications. Animal models have been instrumental in advancing our understanding of environmentally driven changes to the epigenome in the developing and adult brain. This review focuses mainly on such discoveries driven by adverse environments along with their associated behavioral outcomes. While much of the evidence discussed focuses on epigenetics within the central nervous system, several peripheral studies in humans who have experienced significant adversity are also highlighted. As we continue to unravel the link between epigenetics and phenotype, discerning the complexity and specificity of epigenetic changes induced by environments is an important step toward understanding optimal development and how to prevent or ameliorate behavioral deficits bred by disruptive environments.

Aging and peripheral lipopolysaccharide can modulate epigenetic regulators and decrease IL-1β promoter DNA methylation in microglia

In aged mice, peripheral stimulation of the innate immune system with lipopolysaccharide (LPS) causes exaggerated neuroinflammation and prolonged sickness behavior due in part to microglial dysfunction. Epigenetic changes to DNA may play a role in microglial dysfunction; therefore, we sought to determine whether aged microglia displayed DNA hypomethylation of the interleukin-1 beta (IL-1β) promoter and altered expression of epigenetic regulators. We further examined whether the demethylating agent 5-azacytidine induced IL-1β expression in BV2 and primary microglia similar to microglia from aged mice. Novel findings indicated that aged mice had decreased methylation of the IL-1β gene promoter in primary microglia basally or following systemic LPS that is associated with increased IL-1β mRNA, intracellular IL-1β production, as well as prolonged sickness behavior. Last, 5-azacytidine increased IL-1β gene expression and decreased DNA methylation of BV2 and primary microglial cells similar to microglia from aged mice. Taken together, these data indicate that DNA methylation promotes heightened microglial activation in the aged brain.

Epigenetic RELN Dysfunction in Schizophrenia and Related Neuropsychiatric Disorders

REELIN (RELN) is a large (420 kDa) glycoprotein that in adulthood is mostly synthesized in GABAergic neurons of corticolimbic structures. Upon secretion in the extracellular matrix (ECM), RELN binds to VLDL, APOE2, and α3β2 Integrin receptors located on dendritic shafts and spines of postsynaptic pyramidal neurons. Reduced levels of RELN expression in the adult brain induce cognitive impairment and dendritic spine density deficits. RELN supplementation recovers these deficits suggesting a trophic action for RELN in synaptic plasticity. We and others have shown that altered RELN expression in schizophrenia (SZ) and bipolar (BP) disorder patients is difficult to reconcile with classical Mendelian genetic disorders and it is instead plausible to associate these disorders with altered epigenetic homeostasis. Support for the contribution of altered epigenetic mechanisms in the down-regulation of RELN expression in corticolimbic structures of psychotic patients includes the concomitant increase of DNA-methyltransferases and the increased levels of the methyl donor S-adenosylmethionine (SAM). It is hypothesized that these conditions lead to RELN promoter hypermethylation and a reduction in RELN protein amounts in psychotic patients. The decreased synthesis and release of RELN from GABAergic corticolimbic neurons could serve as a model to elucidate the epigenetic pathophysiological mechanisms acting at pyramidal neuron dendrites that regulate synaptic plasticity and cognition in psychotic and non-psychotic subjects.

Environmental influences on the female epigenome and behavior

Environmental factors have long-lasting effects on brain development and behavior. One way experiences are propagated is via epigenetic modifications to the genome. Environmentally-driven epigenetic modifications show incredible brain region- and sex-specificity, and many brain regions affected are ones involved in maternal behavior. In rodent models, females are typically the primary caregiver and thus, any environmental factors that modulate the epigenotype of the mother could have consequences for her current and future offspring. Here we review evidence of the susceptibility of the female epigenome to environmental factors, with a focus on brain regions involved in maternal behavior. Accordingly, implications for interventions that target the mother's epigenome and parenting behavior are discussed.

Recurrent Moderate Hypoglycemia Suppresses Brain-Derived Neurotrophic Factor Expression in the Prefrontal Cortex and Impairs Sensorimotor Gating in the Posthypoglycemic Period in Young Rats

Recurrent hypoglycemia is common in infants and children. In developing rat models, recurrent moderate hypoglycemia leads to neuronal injury in the medial prefrontal cortex. To understand the effects beyond neuronal injury, 3-week-old male rats were subjected to 5 episodes of moderate hypoglycemia (blood glucose concentration, approx. 30 mg/dl for 90 min) once daily from postnatal day 24 to 28. Neuronal injury was determined using Fluoro-Jade B histochemistry on postnatal day 29. The effects on brain-derived neurotrophic factor (BDNF) and its cognate receptor, tyrosine kinase receptor B (TrkB) expression, which is critical for prefrontal cortex development, were determined on postnatal day 29 and at adulthood. The effects on prefrontal cortex-mediated function were determined by assessing the prepulse inhibition of the acoustic startle reflex on postnatal day 29 and 2 weeks later, and by testing for fear-potentiated startle at adulthood. Recurrent hypoglycemia led to neuronal injury confined primarily to the medial prefrontal cortex. BDNF/TrkB expression in the prefrontal cortex was suppressed on postnatal day 29 and was accompanied by lower prepulse inhibition, suggesting impaired sensorimotor gating. Following the cessation of recurrent hypoglycemia, the prepulse inhibition had recovered at 2 weeks. BDNF/TrkB expression in the prefrontal cortex had normalized and fear-potentiated startle was intact at adulthood. Recurrent moderate hypoglycemia during development has significant adverse effects on the prefrontal cortex in the posthypoglycemic period.

Administration of Non-Absorbable Antibiotics to Pregnant Mice to Perturb the Maternal Gut Microbiota Is Associated with Alterations in Offspring Behavior

There is increasing evidence that the gut microbiota plays a major role in host health and disease. In this study, we examined whether perturbation of the maternal gut microbiota during pregnancy, induced by administration of non-absorbable antibiotics to pregnant dams, influences the behavior of offspring. Terminal restriction fragment length polymorphism analyses of fecal bacterial composition showed that the relative abundance of the bacterial order Lactobacillales was lower in offspring born from antibiotic-treated dams (20.7±3.4%) than in control offspring (42.1±6.2%) at P24, while the relative abundance of the bacterial family Clostridium subcluster XIVa was higher in offspring born from antibiotic-treated dams (34.2±5.0%) than in control offspring (16.4±3.3%). Offspring born from antibiotic-treated dams exhibited low locomotor activity in both familiar and novel environments, and preferred to explore in the peripheral area of an unfamiliar field at postnatal week 4. At postnatal weeks 7–8, no difference was observed in the level of locomotor activity between control offspring and offspring from antibiotic-treated dams, while the tendency for the offspring from antibiotic-treated dams to be less engaged in exploring the inside area was still observed. The behavioral phenotypes of the offspring from antibiotic-treated dams at postnatal week 4 could be rescued to a considerable extent through fostering of these offspring by normal dams from postnatal day 1. Although the detailed underlying mechanisms are not fully elucidated, the present results suggest that administration of non-absorbable antibiotics to pregnant dams to perturb the maternal gut microbiota during pregnancy leads to alterations in the behavior of their offspring.

Influence of maternal care on the developing brain: Mechanisms, temporal dynamics and sensitive periods

Variation in maternal care can lead to divergent developmental trajectories in offspring with implications for neuroendocrine function and behavioral phenotypes. Study of the long-term outcomes associated with mother-infant interactions suggests complex mechanisms linking the experience of variation in maternal care and these neurobiological consequences. Through integration of genetic, molecular, cellular, neuroanatomical, and neuroendocrine approaches, significant advances in our understanding of these complex pathways have been achieved. In this review, we will consider the impact of maternal care on male and female offspring development with a particular focus on the issues of timing and mechanism. Identifying the period of sensitivity to maternal care and the temporal dynamics of the molecular and neuroendocrine changes that are a consequence of maternal care represents a critical step in the study of mechanism.

Global and gene-specific DNA methylation alterations in the adolescent amygdala and hippocampus in an animal model of caregiver maltreatment

Epigenetic mechanisms such as DNA methylation are part of an emerging story on how early-life experiences can alter behavioral trajectories and lead to the development of disease and psychological disorders. Previous work from our laboratory has demonstrated alterations in methylation of DNA associated with the brain-derived neurotrophic factor (bdnf) gene within the amygdala and hippocampus of infant and adult rats that were repeatedly exposed to caregiver maltreatment outside the home cage during their first week of life. In the current study we examine changes in global levels of DNA methylation (5-mC) and hydroxymethylation (5-hmC), as well as gene-specific changes in methylation patterns of the candidate gene bdnf (at exons I and IV) within the adolescent amygdala and hippocampus resulting from exposure to maltreatment. While adolescent females exposed to maltreatment showed no significant alterations in global 5-mC or 5-hmC levels, examination of bdnf DNA methylation revealed that maltreated females had greater methylation of exon IV DNA in the amygdala and ventral hippocampus. While adolescent males exposed to maltreatment showed no significant alterations in bdnf DNA methylation, maltreated males had significantly higher 5-mC levels in the dorsal hippocampus and lower 5-hmC levels in the amygdala. These findings demonstrate that the effects of the early caregiving environment are detectable in the adolescent brain at the level of the epigenome, with brain-region specific and sexually-dimorphic epigenetic consequences that could have relevance to adolescent mental health and behavior.

Intergenerational transmission of self-regulation: A multidisciplinary review and integrative conceptual framework

This review examines mechanisms contributing to the intergenerational transmission of self-regulation. To provide an integrated account of how self-regulation is transmitted across generations, we draw from over 75 years of accumulated evidence, spanning case studies to experimental approaches, in literatures covering developmental, social, and clinical psychology, and criminology, physiology, genetics, and human and animal neuroscience (among others). First, we present a taxonomy of what self-regulation is and then examine how it develops--overviews that guide the main foci of the review. Next, studies supporting an association between parent and child self-regulation are reviewed. Subsequently, literature that considers potential social mechanisms of transmission, specifically parenting behavior, interparental (i.e., marital) relationship behaviors, and broader rearing influences (e.g., household chaos) is considered. Finally, evidence that prenatal programming may be the starting point of the intergenerational transmission of self-regulation is covered, along with key findings from the behavioral and molecular genetics literatures. To integrate these literatures, we introduce the self-regulation intergenerational transmission model, a framework that brings together prenatal, social/contextual, and neurobiological mechanisms (spanning endocrine, neural, and genetic levels, including gene-environment interplay and epigenetic processes) to explain the intergenerational transmission of self-regulation. This model also incorporates potential transactional processes between generations (e.g., children's self-regulation and parent-child interaction dynamics that may affect parents' self-regulation) that further influence intergenerational processes. In pointing the way forward, we note key future directions and ways to address limitations in existing work throughout the review and in closing. We also conclude by noting several implications for intervention work.

Evidence from clinical and animal model studies of the long-term and transgenerational impact of stress on DNA methylation

While it is well-known that stress during development and adulthood can confer long-term neurobiological and behavioral consequences, investigators have only recently begun to assess underlying epigenetic modifications. In this review, we highlight clinical research and work from animal models that provide evidence of the impact of stressful experiences either during the perinatal period or adulthood on DNA methylation and behavior. Additionally, we explore the more controversial concept of transgenerational inheritance, including that associated with preconception stress experienced by the mother or father. Finally, we discuss challenges associated with the idea of transgenerational epigenetics and for the field of epigenetics in general.

The long-term impact of adverse caregiving environments on epigenetic modifications and telomeres

Early childhood is a sensitive period in which infant-caregiver experiences have profound effects on brain development and behavior. Clinical studies have demonstrated that infants who experience stress and adversity in the context of caregiving are at an increased risk for the development of psychiatric disorders. Animal models have helped to elucidate some molecular substrates of these risk factors, but a complete picture of the biological basis remains unknown. Studies continue to indicate that environmentally-driven epigenetic modifications may be an important mediator between adverse caregiving environments and psychopathology. Epigenetic modifications such as DNA methylation, which normally represses gene transcription, and microRNA processing, which interferes with both transcription and translation, show long-term changes throughout the brain and body following adverse caregiving. Recent evidence has also shown that telomeres (TTAGGG nucleotide repeats that cap the ends of DNA) exhibit long-term changes in the brain and in the periphery following exposure to adverse caregiving environments. Interestingly, telomeric enzymes and subtelomeric regions are subject to epigenetic modifications—a factor which may play an important role in regulating telomere length and contribute to future mental health. This review will focus on clinical and animal studies that highlight the long-term epigenetic and telomeric changes produced by adverse caregiving in early-life.

Association of brain-derived neurotrophic factor DNA methylation and reduced white matter integrity in the anterior corona radiata in major depression

Considerable evidence suggests a crucial role for the epigenetic regulation of brain-derived neurotrophic factor (BDNF) in the pathophysiology of major depressive disorder (MDD). However, the relationship between BDNF DNA methylation and white matter (WM) integrity in MDD has not yet been investigated. In the current study, we examined the association between the DNA methylation status of the BDNF promoter region and WM integrity in MDD. Sixty patients with MDD and 53 healthy controls underwent T1-weighted structural magnetic resonance imaging (MRI), including diffusion tensor imaging (DTI), to assess their WM integrity. BDNF DNA methylation at 4 CpG sites of the promoter region was also measured. As compared to healthy controls, the MDD group demonstrated reduced fractional anisotropy (FA) in the bilateral anterior and posterior corona radiata (ACR and PCR), genu of the corpus callosum, and the bilateral posterior thalamic radiations. We observed a significant inverse correlation between the DNA methylation of the BDNF promoter region and the FA of the right ACR in MDD patients. Our findings demonstrate a relationship between methylation of the BDNF promoter region and the integrity of the ACR, a key structural component of the emotional and cognitive control network involved in the pathophysiology of MDD. This correlation suggests that BDNF DNA methylation may contribute to structural WM changes in MDD patients.

Long-term effects of early-life caregiving experiences on brain-derived neurotrophic factor histone acetylation in the adult rat mPFC

Infant-caregiver experiences are major contributing factors to neural and behavioral development. Research indicates that epigenetic mechanisms provide a way in which infant-caregiver experiences affect gene activity and other downstream processes in the brain that influence behavioral development. Our laboratory previously demonstrated in a rodent model that exposure to maltreatment alters methylation of DNA associated with the brain-derived neurotrophic factor (bdnf) and reelin genes as well as mRNA of key epigenetic regulatory genes in the medial prefrontal cortex (mPFC). In the current study, we characterized patterns of histone acetylation at bdnf and reelin gene loci after our caregiver manipulations. Using a within-litter design (n = 8-10/group from eight litters), pups were exposed to adverse (maltreatment condition: exposure to a stressed caregiver) or nurturing (cross-foster condition: exposure to a nurturing caregiver) caregiving environments outside the home cage for 30 min daily during the first postnatal week. Remaining pups in a litter were left with the biological mother during each session (providing normal care controls). We then used chromatin immunoprecipitation (ChIP) and quantitative RT-PCR to measure histone 3 lysine 9/14 acetylation associated with bdnf promoters I and IV and the reelin promoter in the adult mPFC. Maltreated females had decreased acetylation at bdnf IV, while neither males nor females exhibited histone acetylation alterations at bdnf I or reelin. These data demonstrate the ability of maltreatment to have long-term consequences on histone acetylation in the mPFC, and provide further evidence of the epigenetic susceptibility of bdnf IV to the quality of infant-caregiver experiences.

The influence of infant-caregiver experiences on amygdala Bdnf, OXTr, and NPY expression in developing and adult male and female rats

Previous work with various animal models has demonstrated that alterations in the caregiving environment produce long-term changes in anxiety-related and social behaviors, as well as amygdala gene expression. We previously introduced a rodent model in which the timing and duration of exposure to maltreatment or nurturing care outside the home cage can be controlled to assess neurobiological outcomes. Here we sought to determine whether our brief experimental conditions produce changes in gene expression within the developing and adult amygdala. Using a candidate gene approach, we examined fold mRNA changes for the Brain-derived neurotrophic factor (Bdnf), Oxytocin receptor (OXTr), and Neuropeptide Y (NPY) genes, which are all highly expressed in the amygdala and play important roles in anxiety-related and social behaviors. In adults, significant group differences were detected for only Bdnf, with higher levels of Bdnf mRNA for females that had been exposed to maltreatment and males exposed to nurturing care outside the home cage relative to littermate controls. For pups, significant group differences were detected for only OXTr, with lower levels of OXTr mRNA in females exposed to maltreatment. Finally, for adolescents, maltreated-females showed significant changes in Bdnf (decreased), OXTr (decreased), and NPY (increased) mRNA relative to controls. These data illustrate the ability of brief, but repeated exposure to different caregiving environments during the first postnatal week to have long-term effects on gene expression within the developing and adult amygdala, especially for females.

Infant-caregiver experiences alter telomere length in the brain

Following adverse childhood experiences, high quality maternal care can protect against accelerated telomere shortening in peripheral cells. It is less clear, however, how telomere length in the brain is influenced by early caregiving experiences. Using rats, we investigated if quality of care (i.e., aversive or nurturing care outside of the homecage) during the first seven days of postnatal (PN) life affected telomere length in the adult brain (PN90) of male and female rats. At PN90, we found that nurturing care outside of the homecage was associated with longer telomeres in the medial prefrontal cortex relative to nurturing care inside the homecage (i.e., normal maternal care) and aversive care outside of the homecage. Further, pups exposed to aversive care outside of the homecage demonstrated longer telomeres in the amygdala relative to pups exposed to nurturing care inside the homecage. These effects were specific to females. No differences in telomere length between caregiving conditions were observed in the ventral hippocampus. Thus, positive and negative early-life experiences result in long-term, sex-specific changes of telomeres in the brain.

Bdnf DNA methylation modifications in the hippocampus and amygdala of male and female rats exposed to different caregiving environments outside the homecage

We have previously shown in infant rats that brief and repeated experiences with a stressed dam outside the homecage (maltreatment) alters methylation of DNA associated with the brain-derived neurotrophic factor (bdnf) gene within the developing and adult prefrontal cortex. BDNF is a key mediator of activity-dependent processes that have a profound influence on neural development and plasticity. Here we examined whether maltreatment also alters bdnf DNA methylation in two additional regions known to be prominently affected by diverse forms of early life adversity in humans- the hippocampus and amygdala. We found significant bdnf DNA methylation modifications present within the adult hippocampus (dorsal and ventral) and amygdala (central/basolateral complex). We observed that the nature of change differed between sexes, gene locus (bdnf I vs. IV), and brain region. Furthermore, a manipulation that did not produce any obvious behavior difference in infants (brief and repeated experiences with a nurturing foster dam) also had long-term effects on methylation. These data provide further empirical support of DNA methylation modifications as biological consequences of caregiving environments.

Sex-dependent effects of an early life treatment in rats that increases maternal care: vulnerability or resilience?

Early life stress (ELS) in rodents has profound long-term effects that are partially mediated by changes in maternal care. ELS not only induces “detrimental” effects in adulthood, increasing psychopathology, but also promotes resilience to further stressors. In Long-Evans rats, we evaluated a combination of two procedures as a model of ELS: restriction of bedding during the first post-natal days and exposure to a “substitute” mother. The maternal care of biological and “substitute” mothers was measured. The male and female offspring were evaluated during adulthood in several contexts. Anxiety was measured by the elevated plus-maze (EPM), acoustic startle response (ASR) and forced swim test (FST). In other group of animals, novelty-seeking was measured (activity in an inescapable novel environment, preference for novel environments and exploration of novel objects). Plasmatic ACTH and corticosterone in basal conditions and in response to stress were also measured. Cognitive impulsivity was assessed by a delay-discounting paradigm, and impulsive action, attention and compulsive-like behavior by a five choice serial reaction time task (5CSRTT). ELS decreased pup body weight and increased the care of the biological mother; however, the “substitute” mother did not exhibit overt maltreatment. A mixture of “detrimental” and “beneficial” effects was shown. In the 5CSRTT, attention was impaired in both genders, and in females, ELS increased compulsive-like behavior. Novel object exploration was only increased by ELS in males, but the preference for novel spaces decreased in both genders. Baseline anxiety (EPM and ASR) and recognition memory were not affected. Unexpectedly, ELS decreased the ACTH response to novelty and swim stress and increased active coping in the FST in both genders. Cognitive impulsivity was decreased only in females, but impulsive action was not affected. The enhancement in maternal care may “buffer” the effects of ELS in a context-dependent manner.

Integrating early life experience, gene expression, brain development, and emergent phenotypes: unraveling the thread of nature via nurture

Adaptation to environmental changes is based on the perpetual generation of new phenotypes. Modern biology has focused on the role of epigenetic mechanisms in facilitating the adaptation of organisms to changing environments through alterations in gene expression. Inherited and/or acquired epigenetic factors are relatively stable and have regulatory roles in numerous genomic activities that translate into phenotypic outcomes. Evidence that dietary and pharmacological interventions have the potential to reverse environment-induced modification of epigenetic states (e.g., early life experience, nutrition, medication, infection) has provided an additional stimulus for understanding the biological basis of individual differences in cognitive abilities and disorders of the brain. It has been suggested that accurate quantification of the relative contribution of heritable genetic and epigenetic variation is essential for understanding phenotypic divergence and adaptation in changing environments, a process requiring stable modulation of gene expression. The main challenge for epigenetics in psychology and psychiatry is to determine how experiences and environmental cues, including the nature of our nurture, influence the expression of neuronal genes to produce long-term individual differences in behavior, cognition, personality, and mental health. To this end, focusing on DNA and histone modifications and their initiators, mediators and readers may provide new inroads for understanding the molecular basis of phenotypic plasticity and disorders of the brain. In this chapter, we review recent discoveries highlighting epigenetic aspects of normal brain development and mental illness, as well as discuss some future directions in the field of behavioral epigenetics.

Early life trauma and attachment: immediate and enduring effects on neurobehavioral and stress axis development

Over half a century of converging clinical and animal research indicates that early life experiences induce enduring neuroplasticity of the HPA-axis and the developing brain. This experience-induced neuroplasticity is due to alterations in the frequency and intensity of stimulation of pups' sensory systems (i.e., olfactory, somatosensory, gustatory) embedded in mother-infant interactions. This stimulation provides "hidden regulators" of pups' behavioral, physiological, and neural responses that have both immediate and enduring consequences, including those involving the stress response. While variation in stimulation can produce individual differences and adaptive behaviors, pathological early life experiences can induce maladaptive behaviors, initiate a pathway to pathology, and increase risk for later-life psychopathologies, such as mood and affective disorders, suggesting that infant-attachment relationships program later-life neurobehavioral function. Recent evidence suggests that the effects of maternal presence or absence during this sensory stimulation provide a major modulatory role in neural and endocrine system responses, which have minimal impact on pups' immediate neurobehavior but a robust impact on neurobehavioral development. This concept is reviewed here using two complementary rodent models of infant trauma within attachment: infant paired-odor-shock conditioning (mimicking maternal odor attachment learning) and rearing with an abusive mother that converge in producing a similar behavioral phenotype in later-life including depressive-like behavior as well as disrupted HPA-axis and amygdala function. The importance of maternal social presence on pups' immediate and enduring brain and behavior suggests unique processing of sensory stimuli in early life that could provide insight into the development of novel strategies for prevention and therapeutic interventions for trauma experienced with the abusive caregiver.

Exposure to caregiver maltreatment alters expression levels of epigenetic regulators in the medial prefrontal cortex

Quality of maternal care experienced during infancy is a key factor that can confer vulnerability or resilience to psychiatric disorders later in life. Research continues to indicate that early-life experiences can affect developmental trajectories through epigenetic alterations capable of affecting gene regulation and neural plasticity. Previously, our lab has shown that experiences within an adverse caregiving environment (i.e. maltreatment) produce aberrant DNA methylation patterns at various gene loci in the medial prefrontal cortex (mPFC) of developing and adult rats. This study aimed to determine whether caregiver maltreatment likewise affects expression levels of several genes important in regulating DNA methylation patterns (Dnmt1, Dnmt3a, MeCP2, Gadd45b, and Hdac1). While we observed minimal changes in gene expression within the mPFC of developing rats, we observed expression changes for all genes in adult animals. Specifically, exposure to maltreatment produced a significant decrease in mRNA levels of all epigenetic regulators in adult males and a significant decrease in Gadd45b in adult females. Our results here provide further empirical support for the long-term and sex-specific epigenetic consequences of caregiver maltreatment on the mPFC.

Multigenerational effects of fetal-neonatal iron deficiency on hippocampal BDNF signaling

Fetal-neonatal iron deficiency induces adult learning impairments concomitant with changes in expression of key genes underlying hippocampal learning and memory in spite of neonatal iron replenishment. Notably, expression of brain-derived neurotrophic factor (BDNF), a gene critical for neuronal maturation and synaptic plasticity, is lowered both acutely and in adulthood following early-life iron deficiency. Although the mechanism behind its long-term downregulation remains unclear, epigenetic modification in BDNF, as seen in other models of early-life adversity, may play a role. Given that early iron deficiency occurs during critical periods in both hippocampal and gonadal development, we hypothesized that the iron-sufficient offspring (F2 IS) of formerly iron-deficient (F1 FID) rats would show a similar suppression of the BDNF gene as their parents. We compared hippocampal mRNA levels of BDNF and functionally related genes among F1 IS, F1 ID, and F2 IS male rats at postnatal day (P) 15 and P65 using RT-qPCR. As expected, the F1 ID group showed a downregulation of BDNF and associated genes acutely at P15 and chronically at P65. However, the F2 IS group showed an upregulation of these genes at P15, returning to control levels at P65. These results demonstrate that adverse effects of early iron deficiency on hippocampal gene expression observed in the F1 are not present in the F2 generation, suggesting differential effects of nutritionally induced epigenetic programing during the critical periods of hippocampal and gonadal development.

Epigenomic Mechanisms of Early Adversity and HPA Dysfunction: Considerations for PTSD Research

Childhood adversity can have life-long consequences for the response to stressful events later in life. Abuse or severe neglect are well-known risk factors for post-traumatic stress disorder (PTSD), at least in part via changes in neural systems mediating the endocrine response to stress. Determining the biological signatures of risk for stress-related mental disorders such as PTSD is important for identifying homogenous subgroups and improving treatment options. This review will focus on epigenetic regulation in early life by adversity and parental care - prime mediators of offspring neurodevelopment - in order to address several questions: (1) what have studies of humans and analogous animal models taught us about molecular mechanisms underlying changes in stress-sensitive physiological systems in response to early life trauma? (2) What are the considerations for studies relating early adversity and PTSD risk, going forward? I will summarize studies in animals and humans that address the epigenetic response to early adversity in the brain and in peripheral tissues. In so doing, I will describe work on the glucocorticoid receptor and other well-characterized genes within the stress response pathway and then turn to genomic studies to illustrate the use of increasingly powerful high-throughput approaches to the study of epigenomic mechanisms.

Neuroimaging, a new tool for investigating the effects of early diet on cognitive and brain development

Nutrition is crucial to the initial development of the central nervous system (CNS), and then to its maintenance, because both depend on dietary intake to supply the elements required to develop and fuel the system. Diet in early life is often seen in the context of "programming" where a stimulus occurring during a vulnerable period can have long-lasting or even lifetime effects on some aspect of the organism's structure or function. Nutrition was first shown to be a programming stimulus for growth, and then for cognitive behavior, in animal studies that were able to employ methods that allowed the demonstration of neural effects of early nutrition. Such research raised the question of whether nutrition could also programme cognition/brain structure in humans. Initial studies of cognitive effects were observational, usually conducted in developing countries where the presence of confounding factors made it difficult to interpret the role of nutrition in the cognitive deficits that were seen. Attributing causality to nutrition required randomized controlled trials (RCTs) and these, often in developed countries, started to appear around 30 years ago. Most demonstrated convincingly that early nutrition could affect subsequent cognition. Until the advent of neuroimaging techniques that allowed in vivo examination of the brain, however, we could determine very little about the neural effects of early diet in humans. The combination of well-designed trials with neuroimaging tools means that we are now able to pose and answer questions that would have seemed impossible only recently. This review discusses various neuroimaging methods that are suitable for use in nutrition studies, while pointing out some of the limitations that they may have. The existing literature is small, but examples of studies that have used these methods are presented. Finally, some considerations that have arisen from previous studies, as well as suggestions for future research, are discussed.