Catherine J. Peña

Epigenetic Effects of Prenatal Stress on 11β-Hydroxysteroid Dehydrogenase-2 in the Placenta and Fetal Brain

Maternal exposure to stress during pregnancy is associated with significant alterations in offspring neurodevelopment and elevated maternal glucocorticoids likely play a central role in mediating these effects. Placental 11β-hydroxysteroid dehydrogenase type 2 (HSD11B2) buffers the impact of maternal glucocorticoid exposure by converting cortisol/corticosterone into inactive metabolites. However, previous studies indicate that maternal adversity during the prenatal period can lead to a down-regulation of this enzyme. In the current study, we examined the impact of prenatal stress (chronic restraint stress during gestational days 14–20) in Long Evans rats on HSD11B2 mRNA in the placenta and fetal brain (E20) and assessed the role of epigenetic mechanisms in these stress-induced effects. In the placenta, prenatal stress was associated with a significant decrease in HSD11B2 mRNA, increased mRNA levels of the DNA methyltransferase DNMT3a, and increased DNA methylation at specific CpG sites within the HSD11B2 gene promoter. Within the fetal hypothalamus, though we find no stress-induced effects on HSD11B2 mRNA levels, prenatal stress induced decreased CpG methylation within the HSD11B2 promoter and increased methylation at sites within exon 1. Within the fetal cortex, HSD11B2 mRNA and DNA methylation levels were not altered by prenatal stress, though we did find stress-induced elevations in DNMT1 mRNA in this brain region. Within individuals, we identified CpG sites within the HSD11B2 gene promoter and exon 1 at which DNA methylation levels were highly correlated between the placenta and fetal cortex. Overall, our findings implicate DNA methylation as a mechanism by which prenatal stress alters HSD11B2 gene expression. These findings highlight the tissue specificity of epigenetic effects, but also raise the intriguing possibility of using the epigenetic status of placenta to predict corresponding changes in the brain.

β-catenin mediates stress resilience through Dicer1/microRNA regulation

β-catenin is a multi-functional protein that has an important role in the mature central nervous system; its dysfunction has been implicated in several neuropsychiatric disorders, including depression. Here we show that in mice β-catenin mediates pro-resilient and anxiolytic effects in the nucleus accumbens, a key brain reward region, an effect mediated by D2-type medium spiny neurons. Using genome-wide β-catenin enrichment mapping, we identify Dicer1—important in small RNA (for example, microRNA) biogenesis—as a β-catenin target gene that mediates resilience. Small RNA profiling after excising β-catenin from nucleus accumbens in the context of chronic stress reveals β-catenin-dependent microRNA regulation associated with resilience. Together, these findings establish β-catenin as a critical regulator in the development of behavioural resilience, activating a network that includes Dicer1 and downstream microRNAs. We thus present a foundation for the development of novel therapeutic targets to promote stress resilience.

Locus-specific epigenetic remodeling controls addiction- and depression-related behaviors

Chronic exposure to drugs of abuse or stress regulates transcription factors, chromatin-modifying enzymes and histone post-translational modifications in discrete brain regions. Given the promiscuity of the enzymes involved, it has not yet been possible to obtain direct causal evidence to implicate the regulation of transcription and consequent behavioral plasticity by chromatin remodeling that occurs at a single gene. We investigated the mechanism linking chromatin dynamics to neurobiological phenomena by applying engineered transcription factors to selectively modify chromatin at a specific mouse gene in vivo. We found that histone methylation or acetylation at the Fosb locus in nucleus accumbens, a brain reward region, was sufficient to control drug- and stress-evoked transcriptional and behavioral responses via interactions with the endogenous transcriptional machinery. This approach allowed us to relate the epigenetic landscape at a given gene directly to regulation of its expression and to its subsequent effects on reward behavior.

Ventral hippocampal afferents to the nucleus accumbens regulate susceptibility to depression.

Enhanced glutamatergic transmission in the nucleus accumbens (NAc), a region critical for reward and motivation, has been implicated in the pathophysiology of depression; however, the afferent source of this increased glutamate tone is not known. The NAc receives glutamatergic inputs from the medial prefrontal cortex (mPFC), ventral hippocampus (vHIP) and basolateral amygdala (AMY). Here, we demonstrate that glutamatergic vHIP afferents to NAc regulate susceptibility to chronic social defeat stress (CSDS). We observe reduced activity in vHIP in mice resilient to CSDS. Furthermore, attenuation of vHIP-NAc transmission by optogenetic induction of long-term depression is pro-resilient, whereas acute enhancement of this input is pro-susceptible. This effect is specific to vHIP afferents to the NAc, as optogenetic stimulation of either mPFC or AMY afferents to the NAc is pro-resilient. These data indicate that vHIP afferents to NAc uniquely regulate susceptibility to CSDS, highlighting an important, novel circuit-specific mechanism in depression.

Epigenetic Signaling in Psychiatric Disorders

Psychiatric disorders are complex multifactorial illnesses involving chronic alterations in neural circuit structure and function. While genetic factors are important in the etiology of disorders such as depression and addiction, relatively high rates of discordance among identical twins clearly indicate the importance of additional mechanisms. Environmental factors such as stress or prior drug exposure are known to play a role in the onset of these illnesses. Such exposure to environmental insults induces stable changes in gene expression, neural circuit function, and ultimately behavior, and these maladaptations appear distinct between developmental and adult exposures. Increasing evidence indicates that these sustained abnormalities are maintained by epigenetic modifications in specific brain regions. Indeed, transcriptional dysregulation and associated aberrant epigenetic regulation is a unifying theme in psychiatric disorders. Aspects of depression and addiction can be modeled in animals by inducing disease-like states through environmental manipulations (e.g., chronic stress, drug administration). Understanding how environmental factors recruit the epigenetic machinery in animal models reveals new insight into disease mechanisms in humans.

Epigenetic Basis of Mental Illness

Psychiatric disorders are complex multifactorial illnesses involving chronic alterations in neural circuit structure and function as well as likely abnormalities in glial cells. While genetic factors are important in the etiology of most mental disorders, the relatively high rates of discordance among identical twins, particularly for depression and other stress-related syndromes, clearly indicate the importance of additional mechanisms. Environmental factors such as stress are known to play a role in the onset of these illnesses. Exposure to such environmental insults induces stable changes in gene expression, neural circuit function, and ultimately behavior, and these maladaptations appear distinct between developmental versus adult exposures. Increasing evidence indicates that these sustained abnormalities are maintained by epigenetic modifications in specific brain regions. Indeed, transcriptional dysregulation and the aberrant epigenetic regulation that underlies this dysregulation is a unifying theme in psychiatric disorders. Here, we provide a progress report of epigenetic studies of the three major psychiatric syndromes, depression, schizophrenia, and bipolar disorder. We review the literature derived from animal models of these disorders as well as from studies of postmortem brain tissue from human patients. While epigenetic studies of mental illness remain at early stages, understanding how environmental factors recruit the epigenetic machinery within specific brain regions to cause lasting changes in disease susceptibility and pathophysiology is revealing new insight into the etiology and treatment of these conditions.

Developmental Timing of the Effects of Maternal Care on Gene Expression and Epigenetic Regulation of Hormone Receptor Levels in Female Rats

Maternal care experienced during postnatal development has enduring effects on neuroendocrine function and behavior. Previous studies in rats have illustrated the effect of maternal licking/grooming (LG) on hormone receptors and maternal behavior of adult female offspring associated with altered DNA methylation. However, the developmental timing of these effects, which provide insight into the cellular and molecular pathways through which early experience alters later behavior, had not been explored. Here, we demonstrate the developmental emergence of these outcomes and use cross-fostering to identify sensitive periods for these effects. Estrogen receptor (ER)α and ERβ mRNA levels within the medial preoptic area (MPOA) of the hypothalamus were increased by postnatal day (PN)21 in female offspring of high LG dams; LG-associated increases in oxytocin receptor mRNA levels were observed beyond the weaning period. Quantification of ERα-immunoreactivity indicated a high degree of neuroanatomical specificity of LG effects within the MPOA that were observed by PN6. Reduced DNA methylation and histone 3 lysine 9 tri-methylation and increased histone 3 lysine 4 tri-methylation at the ERα gene promoter (Esr1) were detected at PN21 in high LG female offspring. Latency to engage in maternal behavior toward donor pups was significantly shorter among high LG females. Cross-fostering revealed that maternal sensitization and MPOA ERα levels are sensitive to maternal care experienced before but not after PN10. Differential windows of plasticity were identified for ERβ and oxytocin receptor mRNA levels. These studies contribute significantly to our understanding of the molecular, neurobiological, and behavioral pathways through which variation in maternal behavior is transmitted from one generation to the next.

Effects of maternal care on the development of midbrain dopamine pathways and reward‐directed behavior in female offspring

Variation within mesolimbic dopamine (DA) pathways has significant implications for behavioral responses to rewards, and previous studies have indicated long‐term programming effects of early life stress on these pathways. In the current study, we examined the impact of natural variations in maternal care in Long Evans rats on the development of DA pathways in female offspring and the consequences for reward‐directed behaviors. We found that tyrosine hydroxylase (TH) immunoreactivity in the ventral tegmental area was elevated by postnatal day 6 in response to maternal licking/grooming (LG), and that these effects were sustained into adulthood. Increased TH immunoreactivity was not found to be associated with altered epigenetic regulation or transcriptional activation of Th, but probably involved LG‐associated changes in the differentiation of postnatal DA neurons through increased expression of Cdkn1c, and enhanced survival of DA projections through LG‐associated increases in Lmx1b and brain‐derived neurotrophic factor. At weaning, high‐LG offspring had elevated DA receptor mRNA levels within the nucleus accumbens and increased conditioned place preference for a high‐fat diet. In contrast, high‐LG, as compared with low‐LG, juvenile offspring had a reduced preference for social interactions with siblings, and haloperidol administration abolished group differences in conditioned place preference through a shift towards increased social preferences in high‐LG offspring. The effects of maternal care on developing DA pathways and reward‐directed behavior of female offspring that we have observed may play a critical role in the behavioral transmission of maternal LG from mother to daughter, and account for individual differences in the mesolimbic DA system.

In vivo imaging identifies temporal signature of D1 and D2 medium spiny neurons in cocaine reward

Significance Strong associations between cocaine and the environmental contexts where cocaine is administered are thought to drive relapse. The nucleus accumbens (NAc) encodes these cue–reward associations, and here we determined how cocaine alters the ability of cells in NAc to respond to drug-associated environmental stimuli to drive drug seeking. Using fiber photometry calcium imaging we define the specific population of cells, dopamine D1 receptor-expressing neurons, that encodes information about drug associations and show that these cells can be manipulated to attenuate the strength of drug associations and prevent relapse. Together, these data define a basic circuit mechanism underlying drug–context associations and suggest that pharmacotherapeutic agents aimed at D1-type neurons may help to promote sustained abstinence in cocaine abusers. The reinforcing and rewarding properties of cocaine are attributed to its ability to increase dopaminergic transmission in nucleus accumbens (NAc). This action reinforces drug taking and seeking and leads to potent and long-lasting associations between the rewarding effects of the drug and the cues associated with its availability. The inability to extinguish these associations is a key factor contributing to relapse. Dopamine produces these effects by controlling the activity of two subpopulations of NAc medium spiny neurons (MSNs) that are defined by their predominant expression of either dopamine D1 or D2 receptors. Previous work has demonstrated that optogenetically stimulating D1 MSNs promotes reward, whereas stimulating D2 MSNs produces aversion. However, we still lack a clear understanding of how the endogenous activity of these cell types is affected by cocaine and encodes information that drives drug-associated behaviors. Using fiber photometry calcium imaging we define D1 MSNs as the specific population of cells in NAc that encodes information about drug associations and elucidate the temporal profile with which D1 activity is increased to drive drug seeking in response to contextual cues. Chronic cocaine exposure dysregulates these D1 signals to both prevent extinction and facilitate reinstatement of drug seeking to drive relapse. Directly manipulating these D1 signals using designer receptors exclusively activated by designer drugs prevents contextual associations. Together, these data elucidate the responses of D1- and D2-type MSNs in NAc to acute cocaine and during the formation of context–reward associations and define how prior cocaine exposure selectively dysregulates D1 signaling to drive relapse.

Early life stress confers lifelong stress susceptibility in mice via ventral tegmental area OTX2

An early window of stress susceptibility defines a mouse’s response to stress in adulthood. Early life stress in depression susceptibility The linkage between stress early in life and behavioral depression in adulthood is complex. Peña et al. were able to define a time period in early development when mice are especially susceptible to stress. Mice subjected to stress during this time period were less resilient to stress in adulthood. Genes regulated by the transcription factor orthodenticle homeobox 2 (OTX2) primed the response toward depression in adulthood. Although early stress could establish the groundwork for later depression, that priming could be undone by intervention at the right moment. Science, this issue p. 1185 Early life stress increases risk for depression. Here we establish a “two-hit” stress model in mice wherein stress at a specific postnatal period increases susceptibility to adult social defeat stress and causes long-lasting transcriptional alterations that prime the ventral tegmental area (VTA)—a brain reward region—to be in a depression-like state. We identify a role for the developmental transcription factor orthodenticle homeobox 2 (Otx2) as an upstream mediator of these enduring effects. Transient juvenile—but not adult—knockdown of Otx2 in VTA mimics early life stress by increasing stress susceptibility, whereas its overexpression reverses the effects of early life stress. This work establishes a mechanism by which early life stress encodes lifelong susceptibility to stress via long-lasting transcriptional programming in VTA mediated by Otx2.