Amygdala-dependent fear memory consolidation via miR-34a and Notch signaling

Maternal immune activation alters brain microRNA expression in mouse offspring

Objective

Maternal immune activation (MIA) is associated with an increased risk of autism spectrum disorder (ASD) in offspring. Herein, we investigate the altered expression of microRNAs (miRNA), and that of their target genes, in the brains of MIA mouse offspring.

Methods

To generate MIA model mice, pregnant mice were injected with polyriboinosinic:polyribocytidylic acid on embryonic day 12.5. We performed miRNA microarray and mRNA sequencing in order to determine the differential expression of miRNA and mRNA between MIA mice and controls, at 3 weeks of age. We further identified predicted target genes of dysregulated miRNAs, and miRNA‐target interactions, based on the inverse correlation of their expression levels.

Results

Mice prenatally subjected to MIA exhibited behavioral abnormalities typical of ASD, such as a lack of preference for social novelty and reduced prepulse inhibition. We found 29 differentially expressed miRNAs (8 upregulated and 21 downregulated) and 758 differentially expressed mRNAs (542 upregulated and 216 downregulated) in MIA offspring compared to controls. Based on expression levels of the predicted target genes, 18 downregulated miRNAs (340 target genes) and three upregulated miRNAs (60 target genes) were found to be significantly enriched among the differentially expressed genes. miRNA and target gene interactions were most significant between mmu‐miR‐466i‐3p and Hfm1 (ATP‐dependent DNA helicase homolog), and between mmu‐miR‐877‐3p and Aqp6 (aquaporin 6).

Interpretation

Our results provide novel information regarding miRNA expression changes and their putative targets in the early postnatal period of brain development. Further studies will be needed to evaluate potential pathogenic roles of the dysregulated miRNAs.

Basolateral amygdala calpain is required for extinction of contextual fear-memory

Extinction of fear-memory is essential for emotional and mental changes. However, the mechanisms underlying extinction of fear-memory are largely unknown. Calpain is a type of calcium-dependent protease that plays a critical role in memory consolidation and reconsolidation. Whether calpain functions in extinction of fear-memory is unknown, as are the molecular mechanisms. In this study, we investigated the pivotal role of calpain in extinction of fear-memory in mice, and assessed its mechanism. Conditioned stimulation/unconditioned stimulation-conditioned stimulation paradigms combined with pharmacological methods were employed to evaluate the action of calpain in memory extinction. Our data demonstrated that intraperitoneal or intra-basolateral amygdala (BLA) injection of calpain inhibitors could eliminate extinction of fear-memory in mice. Moreover, extinction of fear-memory paradigm-activated BLA calpain activity, which degraded suprachiasmatic nucleus circadian oscillatory protein (SCOP) and phosphatase and tensin homolog (PTEN), subsequently contributing to activation of a protein kinase B (AKT)-mammalian target of the rapamycin (mTor) signaling pathway. Additionally, cAMP-response element binding protein (CREB) phosphorylation was also augmented following extinction of fear-memory. Calpain inhibitor blocked the signaling pathway activation induced by extinction of fear-memory. Additionally, intra-BLA injection of rapamycin or cycloheximide also blocked the extinction of fear-memory. Conversely, intra-BLA injection of PTEN inhibitor, bpV, reversed the effect of calpeptin on extinction of fear-memory. Together, our data confirmed the function of BLA calpain in extinction of fear-memory, likely via degrading PTEN and activating AKT-mTor-dependent protein synthesis.

Transgenerational inheritance of chronic adolescent stress: Effects of stress response and the amygdala transcriptome

Adolescent stress can impact health and well-being not only during adulthood of the exposed individual but even in future generations. To investigate the molecular mechanisms underlying these long-term effects, we exposed adolescent males to stress and measured anxiety behaviors and gene expression in the amygdala-a critical region in the control of emotional states-in their progeny for two generations, offspring and grandoffspring. Male C57BL/6 mice underwent chronic unpredictable stress (CUS) for 2 weeks during adolescence and were used to produce two generations of offspring. Male and female offspring and grandoffspring were tested in behavioral assays to measure affective behavior and stress reactivity. Remarkably, transgenerational inheritance of paternal stress exposure produced a protective phenotype in the male, but not the female lineage. RNA-seq analysis of the amygdala from male offspring and grandoffspring identified differentially expressed genes (DEGs) in mice derived from fathers exposed to CUS. The DEGSs clustered into numerous pathways, and the "notch signaling" pathway was the most significantly altered in male grandoffspring. Therefore, we show that paternal stress exposure impacts future generations which manifest in behavioral changes and molecular adaptations.

Non-invasive biomarkers of fetal brain development reflecting prenatal stress: An integrative multi-scale multi-species perspective on data collection and analysis

Prenatal stress (PS) impacts early postnatal behavioural and cognitive development. This process of 'fetal programming' is mediated by the effects of the prenatal experience on the developing hypothalamic-pituitary-adrenal (HPA) axis and autonomic nervous system (ANS). We derive a multi-scale multi-species approach to devising preclinical and clinical studies to identify early non-invasively available pre- and postnatal biomarkers of PS. The multiple scales include brain epigenome, metabolome, microbiome and the ANS activity gauged via an array of advanced non-invasively obtainable properties of fetal heart rate fluctuations. The proposed framework has the potential to reveal mechanistic links between maternal stress during pregnancy and changes across these physiological scales. Such biomarkers may hence be useful as early and non-invasive predictors of neurodevelopmental trajectories influenced by the PS as well as follow-up indicators of success of therapeutic interventions to correct such altered neurodevelopmental trajectories. PS studies must be conducted on multiple scales derived from concerted observations in multiple animal models and human cohorts performed in an interactive and iterative manner and deploying machine learning for data synthesis, identification and validation of the best non-invasive detection and follow-up biomarkers, a prerequisite for designing effective therapeutic interventions.

Noncoding RNAs: Stress, Glucocorticoids, and Posttraumatic Stress Disorder

Posttraumatic stress disorder (PTSD) is a pathologic response to trauma that impacts ∼8% of the population and is highly comorbid with other disorders, such as traumatic brain injury. PTSD affects multiple biological systems throughout the body, including the hypothalamic-pituitary-adrenal axis, cortical function, and the immune system, and while the study of the biological underpinnings of PTSD and related disorders are numerous, the roles of noncoding RNAs (ncRNAs) are just emerging. Moreover, deep sequencing has revealed that ncRNAs represent most of the transcribed mammalian genome. Here, we present developing evidence that ncRNAs are involved in critical aspects of PTSD pathophysiology. In that regard, we summarize the roles of three classes of ncRNAs in PTSD and related disorders: microRNAs, long-noncoding RNAs, and retrotransposons. This review evaluates findings from both animal and human studies with a special focus on the role of ncRNAs in hypothalamic-pituitary-adrenal axis abnormalities and glucocorticoid dysfunction in PTSD and traumatic brain injury. We conclude that ncRNAs may prove to be useful biomarkers to facilitate personalized medicines for trauma-related brain disorders.

The microbiome regulates amygdala-dependent fear recall

The amygdala is a key brain region that is critically involved in the processing and expression of anxiety and fear-related signals. In parallel, a growing number of preclinical and human studies have implicated the microbiome-gut-brain in regulating anxiety and stress-related responses. However, the role of the microbiome in fear-related behaviours is unclear. To this end we investigated the importance of the host microbiome on amygdala-dependent behavioural readouts using the cued fear conditioning paradigm. We also assessed changes in neuronal transcription and post-transcriptional regulation in the amygdala of naive and stimulated germ-free (GF) mice, using a genome-wide transcriptome profiling approach. Our results reveal that GF mice display reduced freezing during the cued memory retention test. Moreover, we demonstrate that under baseline conditions, GF mice display altered transcriptional profile with a marked increase in immediate-early genes (for example, Fos, Egr2, Fosb, Arc) as well as genes implicated in neural activity, synaptic transmission and nervous system development. We also found a predicted interaction between mRNA and specific microRNAs that are differentially regulated in GF mice. Interestingly, colonized GF mice (ex-GF) were behaviourally comparable to conventionally raised (CON) mice. Together, our data demonstrates a unique transcriptional response in GF animals, likely because of already elevated levels of immediate-early gene expression and the potentially underlying neuronal hyperactivity that in turn primes the amygdala for a different transcriptional response. Thus, we demonstrate for what is to our knowledge the first time that the presence of the host microbiome is crucial for the appropriate behavioural response during amygdala-dependent memory retention.

MicroRNA-34 Contributes to the Stress-related Behavior and Affects 5-HT Prefrontal/GABA Amygdalar System through Regulation of Corticotropin-releasing Factor Receptor 1

Recent studies show that microRNA-34 (miR-34) family is critical in the regulation of stress response also suggesting that it may contribute to the individual responsiveness to stress. We have recently demonstrated that mice carrying a genetic deletion of all miR-34 isoforms (triple knockout, TKO) lack the stress-induced serotonin (5-HT) and GABA release in the medial prefrontal cortex (mpFC) and basolateral amygdala (BLA), respectively. Here, we evaluated if the absence of miR-34 was also able to modify the stress-coping strategy in the forced swimming test. We found that the blunted neurochemical response to stress was associated with lower levels of immobility (index of active coping behavior) in TKO compared to WT mice. Interestingly, among the brain regions mostly involved in the stress-related behaviors, the miR-34 displayed the strongest expression in the dorsal raphe nuclei (DRN) of wild-type (WT) mice. In the DRN, the corticotropin-releasing factor receptors (CRFR) 1 and 2, contribute to determine the stress-coping style and the CRFR1 is a target of miR-34. Thus, we hypothesized that the miR-34-dependent modulation of CRFR1 expression may be involved in the DRN regulation of stress-coping strategies. In line with this hypothesis, we found increased CRFR1 levels in the DNR of TKO compared to WT mice. Moreover, infusion of CRFR1 antagonist in the DRN of TKO mice reverted their behavioral and neurochemical phenotype. We propose that miR-34 modulate the mpFC 5-HT/BLA GABA response to stress acting on CRFR1 in the DRN and that this mechanism could contribute to determine individual stress-coping strategy.

Non-coding RNA dysregulation in the amygdala region of schizophrenia patients contributes to the pathogenesis of the disease

Schizophrenia (SCZ) is a neuropsychiatric disorder with a complex genetic etiology. The redundancy of the gene networks underlying SCZ indicates that many gene combinations have the potential to cause a system dysfunction that can manifest as SCZ or a related neurodevelopmental disorder. Recent studies show that small non-coding microRNA (miRNA) and long non-coding RNA (lncRNA) are important factors in shaping these networks and are dynamically regulated by neuronal activation. We investigated the genome-wide transcription profiles of 46 human amygdala samples obtained from 22 SCZ patients and 24 healthy controls. Using RNA sequencing (RNA-seq), we determined lncRNA expression levels in all samples and generated miRNA profiles for 27 individuals (13 cases and 14 controls). Previous studies have identified differentially expressed miRNAs in SCZ, including miR-132, miR-212, and miR-34a/miR-34c. Here we report differential expression of a novel miRNA, miR1307, in SCZ. Notably, miR1307 maps to a locus previously associated with SCZ through GWAS. Additionally, one lncRNA that was overexpressed in SCZ, AC005009.2, also maps to a region previously associated with SCZ based on GWAS and overlapped SCZ-related genes. The results were replicated in a large independent data set of 254 dorsolateral prefrontal cortex samples from the CommonMind consortium. Taken together, these results suggest that miRNA and lncRNAs are important contributors to the pathogenesis of SCZ.

Interplay between TETs and microRNAs in the adult brain for memory formation

5-hydroxymethylation (5-hmC) is an epigenetic modification on DNA that results from the conversion of 5-methylcytosine by Ten-Eleven Translocation (TET) proteins. 5-hmC is widely present in the brain and is subjected to dynamic regulation during development and upon neuronal activity. It was recently shown to be involved in memory processes but currently, little is known about how it is controlled in the brain during memory formation. Here, we show that Tet3 is selectively up-regulated by activity in hippocampal neurons in vitro, and after formation of fear memory in the hippocampus. This is accompanied by a decrease in miR-29b expression that, through complementary sequences, regulates the level of Tet3 by preferential binding to its 3′UTR. We newly reveal that SAM68, a nuclear RNA-binding protein known to regulate splicing, acts upstream of miR-29 by modulating its biogenesis. Together, these findings identify novel players in the adult brain necessary for the regulation of 5-hmC during memory formation.

MicroRNA-induced silencing in epilepsy: Opportunities and challenges for clinical application

MicroRNAs are master regulators of gene expression. Single microRNAs influence multiple proteins within diverse molecular pathways and networks. Therefore, changes in levels or activity of microRNAs can have profound effects on cellular function. This makes dysregulated microRNA-induced silencing an attractive potential disease mechanism in complex disorders like epilepsy, where numerous cellular pathways and processes are affected simultaneously. Indeed, several years of research in rodent models have provided strong evidence that acute or recurrent seizures change microRNA expression and function. Moreover, altered microRNA expression has been observed in brain and blood from patients with various epilepsy disorders, such as tuberous sclerosis. MicroRNAs can be easily manipulated using sense or antisense oligonucleotides, opening up opportunities for therapeutic intervention. Here, we summarize studies using these techniques to identify microRNAs that modulate seizure susceptibility, describe protein targets mediating some of these effects, and discuss cellular pathways, for example neuroinflammation, that are controlled by epilepsy-associated microRNAs. We critically assess current gaps in knowledge regarding target- and cell-specificity of microRNAs that have to be addressed before clinical application as therapeutic targets or biomarkers. The recent progress in understanding microRNA function in epilepsy has generated strong momentum to encourage in-depth mechanistic studies to develop microRNA-targeted therapies. Developmental Dynamics 247:94-110, 2018. © 2017 Wiley Periodicals, Inc.

Amygdalar MicroRNA-15a Is Essential for Coping with Chronic Stress

MicroRNAs are important regulators of gene expression and associated with stress-related psychiatric disorders. Here, we report that exposing mice to chronic stress led to a specific increase in microRNA-15a levels in the amygdala-Ago2 complex and a concomitant reduction in the levels of its predicted target, FKBP51, which is implicated in stress-related psychiatric disorders. Reciprocally, mice expressing reduced levels of amygdalar microRNA-15a following exposure to chronic stress exhibited increased anxiety-like behaviors. In humans, pharmacological activation of the glucocorticoid receptor, as well as exposure to childhood trauma, was associated with increased microRNA-15a levels in peripheral blood. Taken together, our results support an important role for microRNA-15a in stress adaptation and the pathogenesis of stress-related psychopathologies.

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miR-15a levels are elevated in the amygdala-Ago2 complex following chronic stress

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miR-15a targets FKBP51 and affects behavioral responses to stressful challenges

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miR-15a is elevated in peripheral human blood following dexamethasone exposure

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miR-15a is elevated in peripheral human blood of patients exposed to childhood trauma

Genetic variant rs3750625 in the 3'UTR of ADRA2A affects stress-dependent acute pain severity after trauma and alters a microRNA-34a regulatory site

α2A adrenergic receptor (α2A-AR) activation has been shown in animal models to play an important role in regulating the balance of acute pain inhibition vs facilitation after both physical and psychological stress. To our knowledge, the influence of genetic variants in the gene encoding α2A-AR, ADRA2A, on acute pain outcomes in humans experiencing traumatic stress has not been assessed. In this study, we tested whether a genetic variant in the 3'UTR of ADRA2A, rs3750625, is associated with acute musculoskeletal pain (MSP) severity following motor vehicle collision (MVC, n = 948) and sexual assault (n = 84), and whether this influence was affected by stress severity. We evaluated rs3750625 because it is located in the seed binding region of miR-34a, a microRNA (miRNA) known to regulate pain and stress responses. In both cohorts, the minor allele at rs3750625 was associated with increased musculoskeletal pain in distressed individuals (stress*rs3750625 P = 0.043 for MVC cohort and P = 0.007 for sexual assault cohort). We further found that (1) miR-34a binds the 3'UTR of ADRA2A, (2) the amount of repression is greater when the minor (risk) allele is present, (3) miR-34a in the IMR-32 adrenergic neuroblastoma cell line affects ADRA2A expression, (4) miR-34a and ADRA2A are expressed in tissues known to play a role in pain and stress, (5) following forced swim stress exposure, rat peripheral nerve tissue expression changes are consistent with miR-34a regulation of ADRA2A. Together, these results suggest that ADRA2A rs3750625 contributes to poststress musculoskeletal pain severity by modulating miR-34a regulation.

Deletion of Dlk2 increases the vulnerability to anxiety-like behaviors and impairs the anxiolytic action of alprazolam

The purpose of this study was to evaluate the role of the non-canonical DLK2 NOTCH ligand in the regulation of emotional behavior. To this aim, anxiety and depressive-like behaviors were examined in Dlk2 knock-out (Dlk2^-/-) and its corresponding wild-type (WT) mice. Furthermore, relative gene expression analyses of corticotropin releasing hormone (Crh) in the paraventricular nucleus (PVN), glucocorticoid receptor (NR3C1) and FK506 binding protein 5 (FKBP5) in the hippocampus (HIPP), and the transcription factors Hes1, Hes5 and Hey1 in the PVN, HIPP and amygdala (AMY) were carried out in Dlk2^-/- and WT mice under basal conditions and after exposure to restraint stress. The anxiolytic action of alprazolam and the relative gene expression levels of the GABA-A alpha 2 and gamma 2 receptor subunits (Gabra2 and Gabrg2) were also evaluated in the HIPP and AMY of WT and Dlk2^-/- mice. The results reveal that deletion of Dlk2 increased anxiety and depressive-like behaviors and altered the vulnerability to restraint stress on Crh gene expression in the PVN, Nr3c1 and Fkbp5 gene expression in the HIPP, and Hes1, Hes5 and Hey1 gene expression in the PVN, HIPP and AMY. Interestingly, the administration of alprazolam failed to produce an anxiolytic action in Dlk2^-/- mice. Indeed, Gabra2 and Gabrg2 gene expression levels were significantly affected under basal conditions and after stress exposure in Dlk2^-/- mice compared with WT mice. In conclusion, the results suggest that DLK2 plays an important role in the regulation of emotional behaviors and relevant targets of the stress axis, NOTCH pathway and GABAergic neurotransmission. In addition, the deletion of Dlk2 blocked the anxiolytic response to alprazolam. Future studies are needed to determine the relevance of DLK2 as a potential therapeutic target for the treatment of neuropsychiatric disorders with anxiety or depressive-like behaviors.

Microbial regulation of microRNA expression in the amygdala and prefrontal cortex

BACKGROUND

There is growing evidence for a role of the gut microbiome in shaping behaviour relevant to many psychiatric and neurological disorders. Preclinical studies using germ-free (GF) animals have been essential in contributing to our current understanding of the potential importance of the host microbiome for neurodevelopment and behaviour. In particular, it has been repeatedly demonstrated that manipulation of the gut microbiome modulates anxiety-like behaviours. The neural circuits that underlie anxiety- and fear-related behaviours are complex and heavily depend on functional communication between the amygdala and prefrontal cortex (PFC). Previously, we have shown that the transcriptional networks within the amygdala and PFC of GF mice are altered. MicroRNAs (miRNAs) act through translational repression to control gene translation and have also been implicated in anxiety-like behaviours. However, it is unknown whether these features of host post-transcriptional machinery are also recruited by the gut microbiome to exert control over CNS transcriptional networks.

RESULTS

We conducted Illumina® next-generation sequencing (NGS) in the amygdala and PFC of conventional, GF and germ-free colonized mice (exGF). We found a large proportion of miRNAs to be dysregulated in GF animals in both brain regions (103 in the amygdala and 31 in the PFC). Additionally, colonization of GF mice normalized some of the noted alterations. Next, we used a complementary approach to GF by manipulating the adult rat microbiome with an antibiotic cocktail to deplete the gut microbiota and found that this strategy also impacted the expression of relevant miRNAs.

CONCLUSION

These results suggest that the microbiome is necessary for appropriate regulation of miRNA expression in brain regions implicated in anxiety-like behaviours.

miR-12 and miR-124 contribute to defined early phases of long-lasting and transient memory

MicroRNAs (miRNAs) are important epigenetic regulators of mRNA translation implicated in long-lasting synaptic plasticity and long-term memory (LTM). Since recent findings demonstrated a role of epigenetic regulation of gene expression in early memory phases we investigated whether epigenetic regulation by miRNAs also contributes to early memory phases. We used the olfactory associative learning paradigm in honeybees and addressed the contribution of miRNAs depending on the conditioning strength. We selected miR-12, miR-124, and miR-125 that have been implicated in processes of neuronal plasticity and analysed their contribution to non-associative and associative learning using miRNA inhibitors. Blocking miR-12, miR-124, or miR125 neither affects gustatory sensitivity nor habituation nor sensitization. Blocking the function of miR-12 and miR-124 during and shortly after 3-trial conditioning impairs different early memory phases. Although different, the function of miR-12 and miR-124 is also required for early phases of transient memory that is induced by 1-trial conditioning. Blocking miR-125 has no effect on early memory independent of the conditioning strength. These findings demonstrate that distinct miRNAs contribute to early phases of both, transient memories as well as long-lasting memories.

Amygdalar expression of the microRNA miR-101a and its target Ezh2 contribute to rodent anxiety-like behaviour

A greater understanding of neural mechanisms contributing to anxiety is needed in order to develop better therapeutic interventions. This study interrogates a novel molecular mechanism that shapes anxiety-like behaviour, demonstrating that the microRNA miR-101a-3p and its target, enhancer of zeste homolog 2 (Ezh2) in the amygdala, contribute to rodent anxiety-like behaviour. We utilized rats that were selectively bred for differences in emotionality and stress reactivity, showing that high-novelty-responding (HR) rats, which display low trait anxiety, have lower miR-101a-3p levels in the amygdala compared to low-novelty-responding (LR) rats that characteristically display high trait anxiety. To determine whether there is a causal relationship between amygdalar miR-101a-3p and anxiety behaviour, we used a viral approach to overexpress miR-101a-3p in the amygdala of HR rats and test whether it would increase their typically low levels of anxiety-like behaviour. We found that increasing miR-101a-3p in the amygdala increased HRs' anxiety-like behaviour in the open-field test and elevated plus maze. Viral-mediated miR-101a-3p overexpression also reduced expression of the histone methyltransferase Ezh2, which mediates gene silencing via trimethylation of histone 3 at lysine 27 (H3K27me3). Knockdown of Ezh2 with short-interfering RNA (siRNA) also increased HRs' anxiety-like behaviour, but to a lesser degree than miR-101a-3p overexpression. Overall, our data demonstrate that increasing miR-101a-3p expression in the amygdala increases anxiety-like behaviour and that this effect is at least partially mediated via repression of Ezh2. This work adds to the growing body of evidence implicating miRNAs and epigenetic regulation as molecular mediators of anxiety behaviour.

Central Ghrelin Resistance Permits the Overconsolidation of Fear Memory

BACKGROUND

There are many contradictory findings about the role of the hormone ghrelin in aversive processing, with studies suggesting that ghrelin signaling can both inhibit and enhance aversion. Here, we characterize and reconcile the paradoxical role of ghrelin in the acquisition of fearful memories.

METHODS

We used enzyme-linked immunosorbent assay to measure endogenous acyl-ghrelin and corticosterone at time points surrounding auditory fear learning. We used pharmacological (systemic and intra-amygdala) manipulations of ghrelin signaling and examined several aversive and appetitive behaviors. We also used biotin-labeled ghrelin to visualize ghrelin binding sites in coronal brain sections of amygdala. All work was performed in rats.

RESULTS

In unstressed rodents, endogenous peripheral acyl-ghrelin robustly inhibits fear memory consolidation through actions in the amygdala and accounts for virtually all interindividual variability in long-term fear memory strength. Higher levels of endogenous ghrelin after fear learning were associated with weaker long-term fear memories, and pharmacological agonism of the ghrelin receptor during the memory consolidation period reduced fear memory strength. These fear-inhibitory effects cannot be explained by changes in appetitive behavior. In contrast, we show that chronic stress, which increases both circulating endogenous acyl-ghrelin and fear memory formation, promotes profound loss of ghrelin binding sites in the amygdala and behavioral insensitivity to ghrelin receptor agonism.

CONCLUSIONS

These studies provide a new link between stress, a novel type of metabolic resistance, and vulnerability to excessive fear memory formation and reveal that ghrelin can regulate negative emotionality in unstressed animals without altering appetite.

MicroRNA-Mediated Rescue of Fear Extinction Memory by miR-144-3p in Extinction-Impaired Mice

BACKGROUND

MicroRNA (miRNA)-mediated control of gene expression suggests that miRNAs are interesting targets and/or biomarkers in the treatment of anxiety- and trauma-related disorders, where often memory-associated gene expression is adversely affected.

METHODS

The role of miRNAs in the rescue of impaired fear extinction was assessed using the 129S1/SvlmJ (S1) mouse model of impaired fear extinction. miRNA microarray analysis, reverse transcription polymerase chain reaction, fluorescent in situ hybridization, lentiviral overexpression, and Luciferase reporter assays were used to gain insight into the mechanisms underlying miRNA-mediated normalization of deficient fear extinction.

RESULTS

Rescuing impaired fear extinction via dietary zinc restriction was associated with differential expression of miRNAs in the amygdala. One candidate, miR-144-3p, robustly expressed in the basolateral amygdala, showed specific extinction-induced, but not fear-induced, increased expression in both extinction-rescued S1 mice and extinction-intact C57BL/6 (BL6) mice. miR-144-3p upregulation and effects on subsequent behavioral adaption was assessed in S1 and BL6 mice. miR-144-3p overexpression in the basolateral amygdala rescued impaired fear extinction in S1 mice, led to enhanced fear extinction acquisition in BL6 mice, and furthermore protected against fear renewal in BL6 mice. miR-144-3p targets a number of genes implicated in the control of plasticity-associated signaling cascades, including Pten, Spred1, and Notch1. In functional interaction studies, we revealed that the miR-144-3p target, PTEN, colocalized with miR-144-3p in the basolateral amygdala and showed functional downregulation following successful fear extinction in S1 mice.

CONCLUSIONS

These findings identify a fundamental role of miR-144-3p in the rescue of impaired fear extinction and suggest this miRNA as a viable target in developing novel treatments for posttraumatic stress disorder and related disorders.

miR-135a Regulates Synaptic Transmission and Anxiety-Like Behavior in Amygdala

MicroRNAs are a class of non-coding RNAs with a growing relevance in the regulation of gene expression related to brain function and plasticity. They have the potential to orchestrate complex phenomena, such as the neuronal response to homeostatic challenges. We previously demonstrated the involvement of miR-135a in the regulation of early stress response. In the present study, we examine the role of miR-135a in stress-related behavior. We show that the knockdown (KD) of miR-135a in the mouse amygdala induces an increase in anxiety-like behavior. Consistently with behavioral studies, electrophysiological experiments in acute brain slices indicate an increase of amygdala spontaneous excitatory postsynaptic currents, as a result of miR-135a KD. Furthermore, we presented direct evidences, by in vitro assays and in vivo miRNA overexpression in the amygdala, that two key regulators of synaptic vesicle fusion, complexin-1 and complexin-2, are direct targets of miR-135a. In vitro analysis of miniature excitatory postsynaptic currents on miR-135a KD primary neurons indicates unpaired quantal excitatory neurotransmission. Finally, increased levels of complexin-1 and complexin-2 proteins were detected in the mouse amygdala after acute stress, accordingly to the previously observed stress-induced miR-135a downregulation. Overall, our results unravel a previously unknown miRNA-dependent mechanism in the amygdala for regulating anxiety-like behavior, providing evidences of a physiological role of miR-135a in the modulation of presynaptic mechanisms of glutamatergic neurotransmission.

MicroRNAs underlying memory deficits in neurodegenerative disorders

Neurodegenerative disorders are defined by neuronal loss and often associated with dementia. Understanding the multifactorial nature of cognitive decline is of particular interest. Cell loss is certainly a possibility but also an early imbalance in the complex gene networks involved in learning and memory. The small (~22nt) non-coding microRNAs play a major role in gene expression regulation and have been linked to neuronal survival and cognition. Interestingly, changes in microRNA signatures are associated with neurodegenerative disorders. In this review, we explore the role of three microRNAs, namely miR-132, miR-124 and miR-34, which are dysregulated in major neurodegenerative diseases such as Alzheimer's disease, Parkinson's disease and Huntington's disease. Interestingly, these microRNAs have been associated with both memory impairment and neuronal survival, providing a potential common molecular mechanism contributing to dementia.

Newer insights into the role of miRNA a tiny genetic tool in psychiatric disorders: focus on post-traumatic stress disorder

Post-traumatic stress disorder (PTSD) is a mental disorder occurring in about 2-9% of individuals after their exposure to life-threatening events, such as severe accidents, sexual abuse, combat or a natural catastrophe. Because PTSD patients are exposed to trauma, it is likely that epigenetic modifications have an important role in disease development and prognosis. For the past two decades, abnormal expression of the epigenetic regulators microRNAs (miRs) and miR-mediated gene regulation have been given importance in a variety of human diseases, such as cancer, heart disease and viral infection. Emerging evidence supports a role for miR dysregulation in psychiatric and neurological disorders, including schizophrenia, bipolar disorder, anxiety, major depressive disorder, autism spectrum disorder and Tourette's syndrome. Recently mounting of evidence supports the role of miR both in preclinical and clinical settings of psychiatric disorders. Abnormalities in miR expression can fine-tune the expression of multiple genes within a biological network, suggesting that miR dysregulation may underlie many of the molecular changes observed in PTSD pathogenesis. This provides strong evidence that miR not only has a critical role in PTSD pathogenesis, but can also open up new avenues for the development of diagnostic tools and therapeutic targets for the PTSD phenotype. In this review, we revisit some of the recent evidence associated with miR and PTSD in preclinical and clinical settings. We also discuss the possible clinical applications and future use of miRs in PTSD therapy.

MicroRNAs and psychiatric disorders: From aetiology to treatment

The emergence of psychiatric disorders relies on the interaction between genetic vulnerability and environmental adversities. Several studies have demonstrated a crucial role for epigenetics (e.g. DNA methylation, post-translational histone modifications and microRNA-mediated post-transcriptional regulation) in the translation of environmental cues into adult behavioural outcome, which can prove to be harmful thus increasing the risk to develop psychopathology. Within this frame, non-coding RNAs, especially microRNAs, came to light as pivotal regulators of many biological processes occurring in the Central Nervous System, both during the neuronal development as well as in the regulation of adult function, including learning, memory and neuronal plasticity. On these basis, in recent years it has been hypothesised a central role for microRNA modulation and expression regulation in many brain disorders, including neurodegenerative disorders and mental illnesses. Indeed, the aim of the present review is to present the most recent state of the art regarding microRNA involvement in psychiatric disorders. We will first describe the mechanisms that regulate microRNA biogenesis and we will report evidences of microRNA dysregulation in peripheral body fluids, in postmortem brain tissues from patients suffering from psychopathology as well as in animal models. Last, we will discuss the potential to consider microRNAs as putative target for pharmacological intervention, using common psychotropic drugs or more specific tools, with the aim to normalize functions that are disrupted in different psychiatric conditions.

A novel approach to PTSD modeling in rats reveals alternating patterns of limbic activity in different types of stress reaction

Human reactions to trauma exposure are extremely diverse, with some individuals exhibiting only time-limited distress and others qualifying for posttraumatic stress disorder diagnosis (PTSD). Furthermore, whereas most PTSD patients mainly display fear-based symptoms, a minority of patients display a co-morbid anhedonic phenotype. We employed an individual profiling approach to model these intriguing facets of the psychiatric condition in underwater-trauma exposed rats. Based on long-term assessments of anxiety-like and anhedonic behaviors, our analysis uncovered three separate phenotypes of stress response; an anxious, fear-based (38%), a co-morbid, fear-anhedonic (15%), and an exposed-unaffected group (47%). Immunohistochemical assessments for cellular activation (c-Fos) and activation of inhibition (c-Fos+GAD67) revealed a differential involvement of limbic regions and distinct co-activity patterns for each of these phenotypes, validating the behavioral categorization. In accordance with recent neurocognitive hypotheses for posttraumatic depression, we show that enhanced pretrauma anxiety predicts the progression of posttraumatic anhedonia only in the fear-anhedonic phenotype.

Effects of lack of microRNA-34 on the neural circuitry underlying the stress response and anxiety

Stress-related psychiatric disorders, including anxiety, are complex diseases that have genetic, and environmental causes. Stressful experiences increase the release of prefrontal amygdala neurotransmitters, a response that is relevant to cognitive, emotional, and behavioral coping. Moreover, exposure to stress elicits anxiety-like behavior and dendritic remodeling in the amygdala. Members of the miR-34 family have been suggested to regulate synaptic plasticity and neurotransmission processes, which mediate stress-related disorders. Using mice that harbored targeted deletions of all 3 members of the miR-34-family (miR-34-TKO), we evaluated acute stress-induced basolateral amygdala (BLA)-GABAergic and medial prefrontal cortex (mpFC) aminergic outflow by intracerebral in vivo microdialysis. Moreover, we also examined fear conditioning/extinction, stress-induced anxiety, and dendritic remodeling in the BLA of stress-exposed TKO mice.

We found that TKO mice showed resilience to stress-induced anxiety and facilitation in fear extinction. Accordingly, no significant increase was evident in aminergic prefrontal or amygdala GABA release, and no significant acute stress-induced amygdalar dendritic remodeling was observed in TKO mice. Differential GRM7, 5-HT2C, and CRFR1 mRNA expression was noted in the mpFC and BLA between TKO and WT mice. Our data demonstrate that the miR-34 has a critical function in regulating the behavioral and neurochemical response to acute stress and in inducing stress-related amygdala neuroplasticity.

Mechanism of Notch Pathway Activation and Its Role in the Regulation of Olfactory Plasticity in Drosophila melanogaster

The neural plasticity of sensory systems is being increasingly recognized as playing a role in learning and memory. We have previously shown that Notch, part of an evolutionarily conserved intercellular signaling pathway, is required in adult Drosophila melanogaster olfactory receptor neurons (ORNs) for the structural and functional plasticity of olfactory glomeruli that is induced by chronic odor exposure. In this paper we address how long-term exposure to odor activates Notch and how Notch in conjunction with chronic odor mediates olfactory plasticity. We show that upon chronic odor exposure a non-canonical Notch pathway mediates an increase in the volume of glomeruli by a mechanism that is autonomous to ORNs. In addition to activating a pathway that is autonomous to ORNs, chronic odor exposure also activates the Notch ligand Delta in second order projection neurons (PNs), but this does not appear to require acetylcholine receptor activation in PNs. Delta on PNs then feeds back to activate canonical Notch signaling in ORNs, which restricts the extent of the odor induced increase in glomerular volume. Surprisingly, even though the pathway that mediates the increase in glomerular volume is autonomous to ORNs, nonproductive transsynaptic Delta/Notch interactions that do not activate the canonical pathway can block the increase in volume. In conjunction with chronic odor, the canonical Notch pathway also enhances cholinergic activation of PNs. We present evidence suggesting that this is due to increased acetylcholine release from ORNs. In regulating physiological plasticity, Notch functions solely by the canonical pathway, suggesting that there is no direct connection between morphological and physiological plasticity.

Late Arc/Arg3.1 expression in the basolateral amygdala is essential for persistence of newly-acquired and reactivated contextual fear memories

A feature of fear memory is its persistence, which could be a factor for affective disorders. Memory retrieval destabilizes consolidated memories, and then rapid molecular cascades contribute to early stabilization of reactivated memories. However, persistence of reactivated memories has been poorly understood. Here, we discover that late Arc (also known as Arg3.1) expression in the mouse basolateral amygdala (BLA) is involved in persistence of newly-acquired and reactivated fear memories. After both fear learning and retrieval, Arc levels increased at 2 h, returned to basal levels at 6 h but increased again at 12 h. Inhibiting late Arc expression impaired memory retention 7 d, but not 2 d, after fear learning and retrieval. Moreover, blockade of NR2B-containing N-methyl-D-aspartate receptors (NMDARs) prevented memory destabilization and inhibited late Arc expression. These findings indicate that NR2B-NMDAR and late Arc expression plays a critical role in the destabilization and persistence of reactivated memories.

Micro-RNAs in cognition and cognitive disorders: Potential for novel biomarkers and therapeutics

Micro-RNAs (miRNAs) are small regulatory non-coding RNAs involved in the regulation of many biological functions. In the brain, they have distinct expression patterns depending on region, cell-type and developmental stage. Their expression profile is altered by neuronal activation in response to behavioral training or chemical/electrical stimulation. The dynamic changes in miRNA level regulate the expression of genes required for cognitive processes such as learning and memory. In addition, in cognitive dysfunctions such as dementias, expression levels of many miRNAs are perturbed, not only in brain areas affected by the pathology, but also in peripheral body fluids such as serum and cerebrospinal fluid. This presents an opportunity to utilize miRNAs as biomarkers for early detection and assessment of cognitive dysfunctions. Further, since miRNAs target many genes and pathways, they may represent key molecular signatures that can help understand the mechanisms of cognitive disorders and the development of potential therapeutic agents.

Multiple repressive mechanisms in the hippocampus during memory formation

Memory stabilization after learning requires translational and transcriptional regulations in the brain, yet the temporal molecular changes that occur after learning have not been explored at the genomic scale. We used ribosome profiling and RNA sequencing to quantify the translational status and transcript levels in the mouse hippocampus after contextual fear conditioning. We revealed three types of repressive regulations: translational suppression of ribosomal protein-coding genes in the hippocampus, learning-induced early translational repression of specific genes, and late persistent suppression of a subset of genes via inhibition of estrogen receptor 1 (ESR1/ERα) signaling. In behavioral analyses, overexpressing Nrsn1, one of the newly identified genes undergoing rapid translational repression, or activating ESR1 in the hippocampus impaired memory formation. Collectively, this study unveils the yet-unappreciated importance of gene repression mechanisms for memory formation.

Plasticity-related genes in brain development and amygdala-dependent learning

Learning about motivationally important stimuli involves plasticity in the amygdala, a temporal lobe structure. Amygdala-dependent learning involves a growing number of plasticity-related signaling pathways also implicated in brain development, suggesting that learning-related signaling in juveniles may simultaneously influence development. Here, we review the pleiotropic functions in nervous system development and amygdala-dependent learning of a signaling pathway that includes brain-derived neurotrophic factor (BDNF), extracellular signaling-related kinases (ERKs) and cyclic AMP-response element binding protein (CREB). Using these canonical, plasticity-related genes as an example, we discuss the intersection of learning-related and developmental plasticity in the immature amygdala, when aversive and appetitive learning may influence the developmental trajectory of amygdala function. We propose that learning-dependent activation of BDNF, ERK and CREB signaling in the immature amygdala exaggerates and accelerates neural development, promoting amygdala excitability and environmental sensitivity later in life.

Infralimbic EphB2 Modulates Fear Extinction in Adolescent Rats

Adolescent rats are prone to impaired fear extinction, suggesting that mechanistic differences in extinction could exist in adolescent and adult rats. Since the infralimbic cortex (IL) is critical for fear extinction, we used PCR array technology to identify gene expression changes in IL induced by fear extinction in adolescent rats. Interestingly, the ephrin type B receptor 2 (EphB2), a tyrosine kinase receptor associated with synaptic development, was downregulated in IL after fear extinction. Consistent with the PCR array results, EphB2 levels of mRNA and protein were reduced in IL after fear extinction compared with fear conditioning, suggesting that EphB2 signaling in IL regulates fear extinction memory in adolescents. Finally, reducing EphB2 synthesis in IL with shRNA accelerated fear extinction learning in adolescent rats, but not in adult rats. These findings identify EphB2 in IL as a key regulator of fear extinction during adolescence, perhaps due to the increase in synaptic remodeling occurring during this developmental phase.

microRNAs Modulate Spatial Memory in the Hippocampus and in the Ventral Striatum in a Region-Specific Manner

MicroRNAs are endogenous, noncoding RNAs crucial for the post-transcriptional regulation of gene expression. Their role in spatial memory formation, however, is poorly explored. In this study, we analyzed learning-induced microRNA expression in the hippocampus and in the ventral striatum. Among miRNAs specifically downregulated by spatial training, we focused on the hippocampus-specific miR-324-5p and the ventral striatum-specific miR-24. In vivo overexpression of the two miRNAs demonstrated that miR-324-5p is able to impair memory if administered in the hippocampus but not in the ventral striatum, while the opposite is true for miR-24. Overall, these findings demonstrate a causal relationship between miRNA expression changes and spatial memory formation. Furthermore, they provide support for a regional dissociation in the post-transcriptional processes underlying spatial memory in the two brain structures analyzed.

miRNAs and other non-coding RNAs in posttraumatic stress disorder: A systematic review of clinical and animal studies

In the last couple of years, non-coding (nc) RNAs like micro-RNAs (miRNAs), small interference RNAs (siRNAs) and long ncRNAs (lncRNAs) have emerged as promising candidates for biomarkers and drug-targets in a variety of psychiatric disorders. In contrast to reports on ncRNAs in affective disorders, schizophrenia and anxiety disorders, manuscripts on ncRNAs in posttraumatic stress disorder (PTSD) and associated animal models are scarce. Aiming to stimulate ncRNA research in PTSD and to identify the hitherto most promising ncRNA candidates and associated pathways for psychotrauma research, we conducted the first review on ncRNAs in PTSD. We aimed to identify studies reporting on the expression, function and regulation of ncRNAs in PTSD patients and in animals exhibiting a PTSD-like syndrome. Following the PRISMA guidelines for systematic reviews, we systematically screened the PubMed database for clinical and animal studies on ncRNAs in PTSD, animal models for PTSD and animal models employing a classical fear conditioning paradigm. Using 112 different combinations of search terms, we retrieved 523 articles of which we finally included and evaluated three clinical and 12 animal studies. In addition, using the web-based tool DIANA miRPath v2.0, we searched for molecular pathways shared by the predicted targets of the here-evaluated miRNA candidates. Our findings suggest that mir-132, which has been found to be regulated in three of the here included studies, as well as miRNAs with an already established role in Alzheimer's disease (AD) seem to be particularly promising candidates for future miRNA studies in PTSD. These results are limited by the low number of human trials and by the heterogeneity of included animal studies.

Noncoding RNAs and neurobehavioral mechanisms in psychiatric disease

The human genome project has revolutionized our understanding of the underlying mechanisms in psychiatric disease. It is now abundantly clear that neurobehavioral phenotypes are epigenetically controlled by noncoding RNAs (ncRNAs). The microRNA (miRNA) class of ncRNAs are ubiquitously expressed throughout the brain and govern all major neuronal pathways. The attractive therapeutic potential of miRNAs is underscored by their pleiotropic capacities, putatively targeting multiple pathways within a single neuron. Many psychiatric diseases stem from a multifactorial origin, thus conventional drug targeting of single proteins may not prove most effective. In this exciting post-genome sequencing era, many new epigenetic targets are emerging for therapeutic investigation. Here we review the reported roles of miRNAs, as well as other ncRNA classes, in the pathology of psychiatric disorders; there are both common and unique ncRNA mechanisms that influence the various diagnoses. Collectively, these potent epigenetic regulators may clarify the disrupted signaling networks in psychiatric phenotypes.

Notch is required in adult Drosophila sensory neurons for morphological and functional plasticity of the olfactory circuit

Olfactory receptor neurons (ORNs) convey odor information to the central brain, but like other sensory neurons were thought to play a passive role in memory formation and storage. Here we show that Notch, part of an evolutionarily conserved intercellular signaling pathway, is required in adult Drosophila ORNs for the structural and functional plasticity of olfactory glomeruli that is induced by chronic odor exposure. Specifically, we show that Notch activity in ORNs is necessary for the odor specific increase in the volume of glomeruli that occurs as a consequence of prolonged odor exposure. Calcium imaging experiments indicate that Notch in ORNs is also required for the chronic odor induced changes in the physiology of ORNs and the ensuing changes in the physiological response of their second order projection neurons (PNs). We further show that Notch in ORNs acts by both canonical cleavage-dependent and non-canonical cleavage-independent pathways. The Notch ligand Delta (Dl) in PNs switches the balance between the pathways. These data define a circuit whereby, in conjunction with odor, N activity in the periphery regulates the activity of neurons in the central brain and Dl in the central brain regulates N activity in the periphery. Our work highlights the importance of experience dependent plasticity at the first olfactory synapse.

Appropriate behavioral responses to changing environmental signals, such as odors, are essential for an organism’s survival. In Drosophila odors are detected by olfactory receptor neurons (ORNs) that synapse with second order projection neurons (PNs) and local interneurons in morphologically identifiable neuropils in the antennal lobe called glomeruli. Chronic odor exposure leads to changes in animal behavior as well as to changes in the activity of neurons in the olfactory circuit and increases in the volume of glomeruli. Here, we establish that Notch, an evolutionarily conserved transmembrane receptor that plays profound and pervasive roles in animal development, is required in adult Drosophila ORNs for functional and morphological plasticity in response to chronic odor exposure. These findings are significant because they point to a role for Notch in regulating activity dependent plasticity. Furthermore, we show that in regulating the odor dependent change in glomerular volume, Notch acts by both non-canonical, cleavage-independent and canonical, cleavage-dependent mechanisms, with the Notch ligand Delta in PNs switching the balance between the pathways. Because both the Notch pathway and the processing of olfactory information are highly conserved between flies and vertebrates these findings are likely to be of general relevance.

Determining the role of microRNAs in psychiatric disorders

Recent studies have revealed that patients with psychiatric disorders have altered microRNA (miRNA) expression profiles in the circulation and brain. Furthermore, animal studies have shown that manipulating the levels of particular miRNAs in the brain can alter behaviour. Here, we review recent studies in humans, animal models, cellular systems and bioinformatics that have advanced our understanding of the contribution of brain miRNAs to the regulation of behaviour in the context of psychiatric conditions. These studies highlight the potential of miRNA levels to be used in the diagnosis of psychiatric disorders and suggest that brain miRNAs could become novel treatment targets for psychiatric disorders.

Notch in memories: Points to remember

Memory is a temporally evolving molecular and structural process, which involves changes from local synapses to complex neural networks. There is increasing evidence for an involvement of developmental pathways in regulating synaptic communication in the adult nervous system. Notch signaling has been implicated in memory formation in a variety of species. Nevertheless, the mechanism of Notch underlying memory consolidation remains poorly understood. In this commentary, besides offering an overview of the advances in the field of Notch in memory, we highlight some of the weaknesses of the studies and attempt to cast light on the apparent discrepancies on the role of Notch in memory. We believe that future studies, employing high-throughput technologies and targeted Notch loss and gain of function animal models, will reveal the mechanisms of Notch dependent plasticity and resolve whether this signaling pathway is implicated in the cognitive deficit associated with dementia.

Notch in memories: Points to remember

Memory is a temporally evolving molecular and structural process, which involves changes from local synapses to complex neural networks. There is increasing evidence for an involvement of developmental pathways in regulating synaptic communication in the adult nervous system. Notch signaling has been implicated in memory formation in a variety of species. Nevertheless, the mechanism of Notch in memory consolidation remains poorly understood. In this commentary, besides offering an overview of the advances in the field of Notch in memory, we highlight some of the weaknesses of the studies and attempt to cast light on some of the apparent discrepancies on the role of Notch in memory. We believe that future studies, employing high-throughput technologies and targeted Notch loss and gain of function animal models, will reveal the mechanisms of Notch-dependent plasticity and resolve whether this signaling pathway is implicated in the cognitive deficit associated with dementia.

Notch Intracellular Domain (NICD) Suppresses Long-Term Memory Formation in Adult Drosophila Flies

Notch receptor signaling is evolutionarily conserved and well known for its roles in animal development. Many studies in Drosophila have shown that Notch also performs important functions in memory formation in adult flies. An intriguing observation is that increased expression of the full-length Notch receptor (Nfull) triggers long-term memory (LTM) formation even after very weak training (single training). Canonical Notch signaling is mediated by Notch intracellular domain (NICD), but it is not known whether increased expression of NICD recapitulates the LTM enhancement induced by increased Nfull expression. Here, we report that increased NICD expression either has no impact on LTM formation or suppresses it. Furthermore, it either has no impact or decreases both the levels and activity of cAMP response element binding protein, a key factor supporting LTM. These results indicate that NICD signaling is not sufficient to explain Nfull-induced LTM enhancement. Our findings may also shed light on the molecular mechanisms of memory loss in neurological diseases associated with increased NICD expression and canonical Notch signaling.

Manipulating miRNA Expression: A Novel Approach for Colon Cancer Prevention and Chemotherapy

Small non-coding RNA has been implicated in the control of various cellular processes such as proliferation, apoptosis, and differentiation. About 50% of the miRNA genes are positioned in cancer-associated genomic regions. Several studies have shown that miRNA expression is deregulated in cancer and modulating their expression has reversed the cancer phenotype. Therefore, mechanisms to modulate microRNA (miRNA) activity have provided a novel opportunity for cancer prevention and therapy. In addition, a common cause for development of colorectal cancers is environmental and lifestyle factors. One such factor, diet has been shown to modulate miRNA expression in colorectal cancer patients. In this chapter, we will summarize the work demonstrating that miRNAs are novel promising drug targets for cancer chemoprevention and therapy. Improved delivery, increased stability and enhanced regulation of off-target effects will overcome the current challenges of this exciting approach in the field of cancer prevention and therapy.

Epigenetic mechanisms underlying learning and the inheritance of learned behaviors

Gene expression and regulation is an important sculptor of the behavior of organisms. Epigenetic mechanisms regulate gene expression not by altering the genetic alphabet but rather by the addition of chemical modifications to proteins associated with the alphabet or of methyl marks to the alphabet itself. Being dynamic, epigenetic mechanisms of gene regulation serve as an important bridge between environmental stimuli and genotype. In this review, we outline epigenetic mechanisms by which gene expression is regulated in animals and humans. Using fear learning as a framework, we then delineate how such mechanisms underlie learning and stress responsiveness. Finally, we discuss how epigenetic mechanisms might inform us about the transgenerational inheritance of behavioral traits that are being increasingly reported.