Androgen receptor is a negative regulator of contextual fear memory in male mice

Role of the histone variant H2A.Z.1 in memory, transcription, and alternative splicing is mediated by lysine modification

Creating long-lasting memories requires learning-induced changes in gene expression, which are impacted by epigenetic modifications of DNA and associated histone proteins. Post-translational modifications (PTMs) of histones are key regulators of transcription, with different PTMs producing unique effects on gene activity and behavior. Although recent studies implicate histone variants as novel regulators of memory, effects of PTMs on the function of histone variants are rarely considered. We previously showed that the histone variant H2A.Z suppresses memory, but it is unclear if this role is impacted by H2A.Z acetylation, a PTM that is typically associated with positive effects on transcription and memory. To answer this question, we used a mutation approach to manipulate acetylation on H2A.Z without impacting acetylation of other histone types. Specifically, we used adeno-associated virus (AAV) constructs to overexpress mutated H2A.Z.1 isoforms that either mimic acetylation (acetyl-mimic) by replacing lysines 4, 7 and 11 with glutamine (KQ), or H2A.Z.1 with impaired acetylation (acetyl-defective) by replacing the same lysines with alanine (KA). Expressing the H2A.Z.1 acetyl-mimic (H2A.Z.1KQ) improved memory under weak learning conditions, whereas expressing the acetyl-defective H2A.Z.1KA generally impaired memory, indicating that the effect of H2A.Z.1 on memory depends on its acetylation status. RNA sequencing showed that H2A.Z.1KQ and H2A.Z.1KA uniquely impact the expression of different classes of genes in both females and males. Specifically, H2A.Z.1KA preferentially impacts genes involved in synaptic function, suggesting that acetyl-defective H2A.Z.1 impairs memory by altering synaptic regulation. Finally, we describe, for the first time, that H2A.Z is also involved in alternative splicing of neuronal genes, whereby H2A.Z depletion, as well as expression of H2A.Z.1 lysine mutants influence transcription and splicing of different gene targets, suggesting that H2A.Z.1 can impact behavior through effects on both splicing and gene expression. This is the first study to demonstrate that direct manipulation of H2A.Z post-translational modifications regulates memory, whereby acetylation adds another regulatory layer by which histone variants can fine tune higher brain functions through effects on gene expression and splicing.

Remote blast-related mild traumatic brain injury is associated with differential expression of exosomal microRNAs identified in neurodegenerative and immunological processes

BACKGROUND

Blast traumatic brain injury (TBI) and subconcussive blast exposure have been associated, pathologically, with chronic traumatic encephalopathy (CTE) and, clinically, with cognitive and affective symptoms, but the underlying pathomechanisms of these associations are not well understood. We hypothesized that exosomal microRNA (miRNA) expression, and their relation to neurobehavioral outcomes among Veterans with blunt or blast mild TBI (mTBI) may provide insight into possible mechanisms for these associations and therapeutic targets.

METHODS

This is a subanalysis of a larger Chronic Effects of Neurotrauma Consortium Biomarker Discovery Project. Participants (n = 152) were divided into three groups: Controls (n = 35); Blunt mTBI only (n = 54); and Blast/blast+blunt mTBI (n = 63). Postconcussive and post-traumatic stress symptoms were evaluated using the NSI and PCL-5, respectively. Exosomal levels of 798 miRNA expression were measured.

RESULTS

In the blast mTBI group, 23 differentially regulated miRNAs were observed compared to the blunt mTBI group and 23 compared to controls. From the pathway analysis, significantly dysregulated miRNAs in the blast exposure group correlated with inflammatory, neurodegenerative, and androgen receptor pathways.

DISCUSSION

Our findings suggest that chronic neurobehavioral symptoms after blast TBI may pathomechanistically relate to dysregulated cellular pathways involved with neurodegeneration, inflammation, and central hormonal regulation.

Genetic stress-reactivity, sex, and conditioning intensity affect stress-enhanced fear learning

The Stress-Enhanced Fear Learning (SEFL) model of posttraumatic stress disorder (PTSD) reveals increased fear memory in animals exposed to stress prior to contextual fear conditioning (CFC), similar to the increased likelihood of developing PTSD in humans after prior stress. The present study utilized the SEFL model by exposing animals to restraint stress as the first stressor, followed by CFC using foot-shocks with 0.6 mA or 0.8 mA intensity. Adult males and females from the two nearly isogenic rat strains, the genetically more stress-reactive Wistar Kyoto (WKY) More Immobile (WMI), and the less stress-reactive WKY Less Immobile (WLI) were employed. Percent time spent freezing at acquisition and at recall differed between these strains in both prior stress and no stress conditions. The significant correlations between percent freezing at acquisition and at recall suggest that fear memory differences represent a true phenotype related to the stress-reactivity differences between the strains. This assumption is further substantiated by the lack of effect of either conditioning intensity on percent freezing in WLI males, while WMI males were affected by both intensities albeit with opposite directional changes after prior stress. Differences between the sexes in sensitivity to the two conditioning intensities became apparent by the opposite directional and inverse relationship between fear memory and the intensity of conditioning in WMI males and females. The present data also illustrate that although corticosterone (CORT) responses to prior stress are known to be necessary for SEFL, plasma CORT and percent freezing were positively correlated only in the stress less-reactive WLI strain. These differences in baseline fear acquisition, fear memory, and the percent freezing responses to the SEFL paradigm in the two genetically close inbred WMI and WLI strains provide a unique opportunity to study the genetic contribution to the variation in these phenotypes.

Histone H2A.Z is required for androgen receptor-mediated effects on fear memory

Epigenetic factors translate environmental signals into stable outcomes, but how they are influenced by regulators of plasticity remain unclear. We previously showed that androgen receptor overexpression inhibited fear memory in male mice and increased expression of the histone variant H2A.Z, a novel epigenetic regulator of memory. Here, we used conditional-inducible H2A.Z knockout mice to investigate how H2A.Z deletion influences androgenic regulation of fear memory. We showed that conditional inducible H2A.Z deletion blocked memory-enhancing effects of androgen depletion (induced by gonadectomy), and of pharmacological inhibition of the androgen receptor with flutamide. Similarly, H2A.Z deletion blocked the memory-reducing effects of DHT, and DHT treatment in cultured hippocampal neurons altered H2A.Z binding, suggesting that AR is an H2A.Z regulator in neurons. Overall, these data show that fear memory formation is regulated by interactions between sex hormones and epigenetic factors, which has implications for sex differences in fear-related disorders.

The neurobiological basis of sex differences in learned fear and its inhibition

Learning that certain cues or environments predict threat enhances survival by promoting appropriate fear and the resulting defensive responses. Adapting to changing stimulus contingencies by learning that such cues no longer predict threat, or distinguishing between these threat-related and other innocuous stimuli, also enhances survival by limiting fear responding in an appropriate manner to conserve resources. Importantly, a failure to inhibit fear in response to harmless stimuli is a feature of certain anxiety and trauma-related disorders, which are also associated with dysfunction of the neural circuitry underlying learned fear and its inhibition. Interestingly, these disorders are up to twice as common in women, compared to men. Despite this striking sex difference in disease prevalence, the neurobiological factors involved remain poorly understood. This is due in part to the majority of relevant preclinical studies having neglected to include female subjects alongside males, which has greatly hindered progress in this field. However, more recent studies have begun to redress this imbalance and emerging evidence indicates that there are significant sex differences in the inhibition of learned fear and associated neural circuit function. This paper provides a narrative review on sex differences in learned fear and its inhibition through extinction and discrimination, along with the key gonadal hormone and brain mechanisms involved. Understanding the endocrine and neural basis of sex differences in learned fear inhibition may lead to novel insights on the neurobiological mechanisms underlying the enhanced vulnerability to develop anxiety-related disorders that are observed in women.

Inhibition of steroid receptor coactivator-1 in the hippocampus impairs the consolidation and reconsolidation of contextual fear memory in mice

AIMS

Steroid receptor coactivator-1 (SRC-1) is a key coactivator for the efficient transcriptional activity of steroids in the regulation of hippocampal functions. However, the effect of SRC-1 on hippocampal memory processes remains unknown. Our aim was to investigate the roles of hippocampal SRC-1 in the consolidation and reconsolidation of contextual fear memory in mice.

MAIN METHODS

Contextual fear conditioning paradigm was constructed in adult male C57BL/6 mice to examine the fear learning and memory processes. Adeno-associated virus (AAV) vector-mediated RNA interference (RNAi) was infused into hippocampus to block hippocampal SRC-1 level. Immunofluorescent staining was used to detect the efficiency of transfection. High plus maze and open field test were used to determine anxiety and locomotor activity. Western blot analyses were used to detect the expression of SRC-1 and synaptic proteins in the hippocampus.

KEY FINDINGS

We first showed that the expression of SRC-1 was regulated by fear conditioning training in a time-dependent manner, and knockdown of SRC-1 impaired contextual fear memory consolidation without affecting innate anxiety or locomotor activity. In addition, hippocampal SRC-1 was also regulated by the retrieval of contextual fear memory, and downregulation of SRC-1 disrupted fear memory reconsolidation. Moreover, knockdown of SRC-1 reversed the increased GluR1 and PSD-95 levels induced by contextual fear memory retrieval.

SIGNIFICANCE

Our data indicate that hippocampal SRC-1 is required for the consolidation and reconsolidation of contextual fear memory, and SRC-1 may be a potential therapeutic target for mental disorders that are involved in hippocampal memory dysfunction.

Hormone-epigenome interactions in behavioural regulation

Interactions between hormones and epigenetic factors are key regulators of behaviour, but the mechanisms that underlie their effects are complex. Epigenetic factors can modify sensitivity to hormones by altering hormone receptor expression, and hormones can regulate epigenetic factors by recruiting epigenetic regulators to DNA. The bidirectional nature of this relationship is becoming increasingly evident and suggests that the ability of hormones to regulate certain forms of behaviour may depend on their ability to induce changes in the epigenome. Moreover, sex differences have been reported for several epigenetic modifications, and epigenetic factors are thought to regulate sexual differentiation of behaviour, although specific mechanisms remain to be understood. Indeed, hormone-epigenome interactions are highly complex and involve both canonical and non-canonical regulatory pathways that may permit for highly specific gene regulation to promote variable forms of behavioural adaptation.

Translational studies of estradiol and progesterone in fear and PTSD

Translational models of fear have greatly informed our understanding of PTSD and its underlying fear circuitry. One of the most replicated findings in the field is the two-fold higher PTSD incidence in females compared to males. While sociocultural factors play a role, the most robust biological influencers to date are gonadal hormones, such as estradiol and progesterone, which fluctuate across the menstrual cycle. Among studies that account for these hormones, most do so in isolation or collect both and only report one. Variation in study findings suggests that the ratio between these two hormones (the P/E ratio) may be an important and missing variable to further understand gonadal hormone influences on fear. Here we review cross-species examinations of fear and PTSD, within the contexts of estradiol and progesterone as well as P/E ratios that were calculated based on extant literature. We then provide recommendations for best practices in assay methods and reporting to improve research on the P/E ratio in fear and PTSD. Ultimately, greater understanding of this important variable will advance efforts to characterize gonadal hormone influences on fear learning processes in humans and animals.

Centella Asiatica Improves Memory and Promotes Antioxidative Signaling in 5XFAD Mice

Centella asiatica (CA) herb is a traditional medicine, long reputed to provide cognitive benefits. We have reported that CA water extract (CAW) treatment improves cognitive function of aged Alzheimer’s disease (AD) model Tg2576 and wild-type (WT) mice, and induces an NRF2-regulated antioxidant response in aged WT mice. Here, CAW was administered to AD model 5XFAD female and male mice and WT littermates (age: 7.6 +/− 0.6 months), and object recall and contextual fear memory were tested after three weeks treatment. CAW’s impact on amyloid-β plaque burden, and markers of neuronal oxidative stress and synaptic density, was assessed after five weeks treatment. CAW antioxidant activity was evaluated via nuclear transcription factor (erythroid-derived 2)-like 2 (NRF2) and NRF2-regulated antioxidant response element gene expression. Memory improvement in both genders and genotypes was associated with dose-dependent CAW treatment without affecting plaque burden, and marginally increased synaptic density markers in the hippocampus and prefrontal cortex. CAW treatment increased Nrf2 in hippocampus and other NRF2 targets (heme oxygenase-1, NAD(P)H quinone dehydrogenase 1, glutamate-cysteine ligase catalytic subunit). Reduced plaque-associated SOD1, an indicator of oxidative stress, was observed in the hippocampi and cortices of CAW-treated 5XFAD mice. We postulate that CAW treatment leads to reduced oxidative stress, contributing to improved neuronal health and cognition.