Jennifer Blaze

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

Quality of maternal care in infancy is an important contributing factor in the development of behavior and psychopathology. One way maternal care could affect behavioral trajectories is through environmentally induced epigenetic alterations within brain regions known to play prominent roles in cognition, emotion regulation, and stress responsivity. Whereas such research has largely focused on the hippocampus or hypothalamus, the prefrontal cortex (PFC) has only just begun to receive attention. The current study was designed to determine whether exposure to maltreatment or nurturing care is associated with differential methylation of candidate gene loci (bdnf and reelin) within the medial PFC (mPFC) of developing and adult rats. Using a within-litter design, infant male and female rats were exposed to an adverse or nurturing caregiving environment outside their home cage for 30 min per day during the first postnatal week. Additional littermates remained with their biological caregiver within the home cage during the manipulations. We observed that infant rats subjected to caregiver maltreatment emitted more audible and ultrasonic vocalizations than littermates subjected to nurturing care either within or outside of the home cage. While we found no maltreatment-induced changes in bdnf DNA methylation present in infancy, sex-specific alterations were present in the mPFC of adolescents and adults that had been exposed to maltreatment. Furthermore, while maltreated females showed differences in reelin DNA methylation that were transient, males exposed to maltreatment and both males and females exposed to nurturing care outside the home cage showed differences in reelin methylation that emerged by adulthood. Our results demonstrate the ability of infant-caregiver interactions to epigenetically mark genes known to play a prominent role in cognition and psychiatric disorders within the mPFC. Furthermore, our data indicate the remarkable complexity of alterations that can occur, with both transient and later emerging DNA methylation differences that could shape developmental trajectories and underlie gender differences in outcomes.

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.

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.

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

Abstract 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.

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.

Epigenetic mechanisms in learning and memory

Recent discoveries have associated epigenetic mechanisms, including DNA methylation, histone modifications, and microRNA (miRNA) processing, with activity-dependent changes in gene expression necessary to drive long-term memory formation. Here, we discuss the current interpretation of epigenetic mechanisms in the context of memory and sustained behavioral change. One of the two emerging viewpoints is that epigenetic mechanisms subserve information storage in the central nervous system (CNS), a notion supported by rodent studies of fear, recognition and spatial memories, and stress. The second viewpoint is that epigenetics serves as a mechanism for passing on acquired information across generations, a provocative notion now supported by several lines of work using developing and adult rodents. Continued research on such mechanisms promises to advance our understanding of biological pathways linking experiences to long-term and even multigenerational trajectories in neurobiology and behavior. WIREs Cogn Sci 2013, 4:105-115. doi: 10.1002/wcs.1205 For further resources related to this article, please visit the WIREs website.

Caregiver maltreatment causes altered neuronal DNA methylation in female rodents

Negative experiences with a caregiver during infancy can result in long-term changes in brain function and behavior, but underlying mechanisms are not well understood. It is our central hypothesis that brain and behavior changes are conferred by early childhood adversity through epigenetic changes involving DNA methylation. Using a rodent model of early-life caregiver maltreatment (involving exposure to an adverse caregiving environment for postnatal days 1-7), we have previously demonstrated abnormal methylation of DNA associated with the brain-derived neurotrophic factor (Bdnf) gene in the medial prefrontal cortex (mPFC) of adult rats. The aim of the current study was to characterize Bdnf DNA methylation in specific cell populations within the mPFC. In the prefrontal cortex, there is approximately twice as many neurons as glia, and studies have recently shown differential and distinctive DNA methylation patterns in neurons versus nonneurons. Here, we extracted nuclei from the mPFC of adult animals that had experienced maltreatment and used fluorescence-activated cell sorting to isolate cell types before performing bisulfite sequencing to estimate methylation of cytosine-guanine sites. Our data indicate that early-life stress induced methylation of DNA associated with Bdnf IV in a cell-type and sex-specific manner. Specifically, females that experienced early-life maltreatment exhibited greater neuronal cytosine-guanine methylation compared to controls, while no changes were detected in Bdnf methylation in males regardless of cell type. These changes localize the specificity of our previous findings to mPFC neurons and highlight the capacity of maltreatment to cause methylation changes that are likely to have functional consequences for neuronal function.

Intrauterine exposure to maternal stress alters Bdnf IV DNA methylation and telomere length in the brain of adult rat offspring

DNA methylation (addition of methyl groups to cytosines) and changes in telomere length (TTAGGG repeats on the ends of chromosomes) are two molecular modifications that result from stress and could contribute to the long‐term effects of intrauterine exposure to maternal stress on offspring behavior. Here, we measured methylation of DNA associated with the Brain‐derived neurotrophic factor (Bdnf) gene, a gene important in development and plasticity, and telomere length in the brains of adult rat male and female offspring whose mothers were exposed to unpredictable and variable stressors throughout gestation. Males exposed to prenatal stress had greater methylation (Bdnf IV) in the medial prefrontal cortex (mPFC) compared to non‐stressed male controls and stressed females. Further, prenatally‐stressed animals had shorter telomeres than controls in the mPFC. Together findings indicate a long‐term impact of prenatal stress on brain DNA methylation and telomere biology with relevance for behavioral and health outcomes, and contribute to a growing literature linking stress to intergenerational molecular changes.

Changes in dam and pup behavior following repeated postnatal exposure to a predator odor (TMT): A preliminary investigation in Long-Evans rats.

The present study investigated whether repeated early postnatal exposure to the predator odor 2,5-dihydro-2,4,5-trimethylthiazoline (TMT) alters behavioral responses to the stimulus later in life, at postnatal day (PN30). Long-Evans rat pups with their mothers were exposed for 20 min daily to TMT, water, or a noxious odor, butyric acid (BTA), during the first three weeks of life. Mothers exposed to TMT displayed more crouching and nursing behavior than those exposed to BTA, and TMT exposed pups emitted more ultrasonic vocalizations than BTA exposed pups. At PN30, rats were tested for freezing to TMT, water, or BTA. Rats exposed to TMT during the postnatal period displayed less freezing to TMT than rats exposed postnatally to water or BTA. Our data indicate that early-life experience with a predator cue has a significant impact on later fear responses to that same cue, highlighting the programming capacity of the postnatal environment on the development of behavior.