Epigenetic mechanisms of memory formation and reconsolidation

EZH2 Methyltransferase Activity Controls Pten Expression and mTOR Signaling during Fear Memory Reconsolidation

Memory retrieval induces a transient period of increased transcriptional and translational regulation in neurons called reconsolidation, which is regulated by the protein kinase B (AKT)-mammalian target of rapamycin (mTOR) pathway. However, it is currently unknown how activation of the AKT-mTOR pathway is regulated during the reconsolidation process. Here, we found that in male rats retrieval of a contextual fear memory transiently increased Enhancer of Zeste Homolog 2 (EZH2) levels along with increased histone H3 lysine 27 trimethylation (H3K27me3) levels, which correlated with decreased levels of phosphatase and tensin homolog (PTEN), a potent inhibitor of AKT-mTOR-dependent signaling in the hippocampus. Further experiments found increased H3K27me3 levels and DNA methylation across the Pten promoter and coding regions, indicating transcriptional silencing of the Pten gene. Pten H3K27me3 levels did not change following training or after the retrieval of a remote (old) fear memory, suggesting that this mechanism of Pten repression was specific to the reconsolidation of a new memory. In vivo siRNA-mediated knockdown of Ezh2 in the hippocampus abolished retrieval-induced increases in H3K27me3 and prevented decreases in PTEN levels. Ezh2 knockdown attenuated increases in the phosphorylation of AKT and mTOR following retrieval, which could be restored by simultaneously reducing Pten, suggesting that H3K27me3 regulates AKT-mTOR phosphorylation via repression of Pten Consistent with these results, knockdown of Ezh2 in area CA1 before retrieval impaired memory on later tests. Collectively, these results suggest that EZH2-mediated H3K27me3 plays a critical role in the repression of Pten transcription necessary for AKT-mTOR activation and memory reconsolidation following retrieval.SIGNIFICANCE STATEMENT Understanding how critical translation pathways, like mTOR-mediated protein synthesis, are regulated during the memory storage process is necessary for improving memory impairments. This study tests whether mTOR activation is coupled to epigenetic mechanisms in the hippocampus following the retrieval of a contextual fear memory. Specifically, this study evaluates the role of epigenetic modifications in the form of histone methylation in downstream mTOR translational control during learning-dependent synaptic plasticity in neurons. Considering the broad implications of transcriptional and translational mechanisms in synaptic plasticity, psychiatric, and neurological and neurodegenerative disorders, these data are of interest to the neuroscience community due to the robust and specific regulation of mTOR signaling we found to be dependent on repressive histone methylation.

Gestational Hypoxia and Developmental Plasticity

Hypoxia is one of the most common and severe challenges to the maintenance of homeostasis. Oxygen sensing is a property of all tissues, and the response to hypoxia is multidimensional involving complicated intracellular networks concerned with the transduction of hypoxia-induced responses. Of all the stresses to which the fetus and newborn infant are subjected, perhaps the most important and clinically relevant is that of hypoxia. Hypoxia during gestation impacts both the mother and fetal development through interactions with an individual's genetic traits acquired over multiple generations by natural selection and changes in gene expression patterns by altering the epigenetic code. Changes in the epigenome determine "genomic plasticity," i.e., the ability of genes to be differentially expressed according to environmental cues. The genomic plasticity defined by epigenomic mechanisms including DNA methylation, histone modifications, and noncoding RNAs during development is the mechanistic substrate for phenotypic programming that determines physiological response and risk for healthy or deleterious outcomes. This review explores the impact of gestational hypoxia on maternal health and fetal development, and epigenetic mechanisms of developmental plasticity with emphasis on the uteroplacental circulation, heart development, cerebral circulation, pulmonary development, and the hypothalamic-pituitary-adrenal axis and adipose tissue. The complex molecular and epigenetic interactions that may impact an individual's physiology and developmental programming of health and disease later in life are discussed.

Neuroepigenetics of Post-Traumatic Stress Disorder

While diagnosis of PTSD is based on behavioral symptom clusters that are most directly associated with brain function, epigenetic studies of PTSD in humans to date have been limited to peripheral tissues. Animal models of PTSD have been key for understanding the epigenetic alterations in the brain most directly relevant to endophenotypes of PTSD, in particular those pertaining to fear memory and stress response. This chapter provides an overview of neuroepigenetic studies based on animal models of PTSD, with an emphasis on the effect of stress on fear memory. Where relevant, we also describe human-based studies with relevance to neuroepigenetic insights gleaned from animal work and suggest promising directions for future studies of PTSD neuroepigenetics in living humans that combine peripheral epigenetic measures with measures of central nervous system activity, structure and function.

Hypothesis: The Psychedelic Ayahuasca Heals Traumatic Memories via a Sigma 1 Receptor-Mediated Epigenetic-Mnemonic Process

Ayahuasca ingestion modulates brain activity, neurotransmission, gene expression and epigenetic regulation. N,N-Dimethyltryptamine (DMT, one of the alkaloids in Ayahuasca) activates sigma 1 receptor (SIGMAR1) and others. SIGMAR1 is a multi-faceted stress-responsive receptor which promotes cell survival, neuroprotection, neuroplasticity, and neuroimmunomodulation. Simultaneously, monoamine oxidase inhibitors (MAOIs) also present in Ayahuasca prevent the degradation of DMT. One peculiarity of SIGMAR1 activation and MAOI activity is the reversal of mnemonic deficits in pre-clinical models. Since traumatic memories in post-traumatic stress disorder (PTSD) are often characterised by “repression” and PTSD patients ingesting Ayahuasca report the retrieval of such memories, it cannot be excluded that DMT-mediated SIGMAR1 activation and the concomitant MAOIs effects during Ayahuasca ingestion might mediate such “anti-amnesic” process. Here I hypothesise that Ayahuasca, via hyperactivation of trauma and emotional memory-related centres, and via its concomitant SIGMAR1- and MAOIs- induced anti-amnesic effects, facilitates the retrieval of traumatic memories, in turn making them labile (destabilised). As Ayahuasca alkaloids enhance synaptic plasticity, increase neurogenesis and boost dopaminergic neurotransmission, and those processes are involved in memory reconsolidation and fear extinction, the fear response triggered by the memory can be reprogramed and/or extinguished. Subsequently, the memory is stored with this updated significance. To date, it is unclear if new memories replace, co-exist with or bypass old ones. Although the mechanisms involved in memory are still debated, they seem to require the involvement of cellular and molecular events, such as reorganisation of homo and heteroreceptor complexes at the synapse, synaptic plasticity, and epigenetic re-modulation of gene expression. Since SIGMAR1 mobilises synaptic receptor, boosts synaptic plasticity and modulates epigenetic processes, such effects might be involved in the reported healing of traumatic memories in PTSD patients. If this theory proves to be true, Ayahuasca could come to represent the only standing pharmacological treatment which targets traumatic memories in PTSD. Lastly, since SIGMAR1 activation triggers both epigenetic and immunomodulatory programmes, the mechanism here presented could help understanding and treating other conditions in which the cellular memory is dysregulated, such as cancer, diabetes, autoimmune and neurodegenerative pathologies and substance addiction.

The Ubiquitin-Proteasome System and Memory: Moving Beyond Protein Degradation

Cellular models of memory formation have focused on the need for protein synthesis. Recently, evidence has emerged that protein degradation mediated by the ubiquitin-proteasome system (UPS) is also important for this process. This has led to revised cellular models of memory formation that focus on a balance between protein degradation and synthesis. However, protein degradation is only one function of the UPS. Studies using single-celled organisms have shown that non-proteolytic ubiquitin-proteasome signaling is involved in histone modifications and DNA methylation, suggesting that ubiquitin and the proteasome can regulate chromatin remodeling independent of protein degradation. Despite this evidence, the idea that the UPS is more than a protein degradation pathway has not been examined in the context of memory formation. In this article, we summarize recent findings implicating protein degradation in memory formation and discuss various ways in which both ubiquitin signaling and the proteasome could act independently to regulate epigenetic-mediated transcriptional processes necessary for learning-dependent synaptic plasticity. We conclude by proposing comprehensive models of how non-proteolytic functions of the UPS could work in concert to control epigenetic regulation of the cellular memory consolidation process, which will serve as a framework for future studies examining the role of the UPS in memory formation.

Epigenetic regulation of Fgf1 transcription by CRTC1 and memory enhancement

Recent evidence demonstrates that epigenetic regulation of gene transcription is critically involved in learning and memory. Here, we discuss the role of histone acetylation and DNA methylation, which are two best understood epigenetic processes in memory processes. More specifically, we focus on learning-strength-dependent changes in chromatin on the fibroblast growth factor 1 (Fgf1) gene and on the molecular events that modulate regulation of Fgf1 transcription, required for memory enhancement, with the specific focus on CREB-regulated transcription coactivator 1 (CRTC1).

Circadian Regulation of Hippocampal-Dependent Memory: Circuits, Synapses, and Molecular Mechanisms

Circadian modulation of learning and memory efficiency is an evolutionarily conserved phenomenon, occurring in organisms ranging from invertebrates to higher mammalian species, including humans. While the suprachiasmatic nucleus (SCN) of the hypothalamus functions as the master mammalian pacemaker, recent evidence suggests that forebrain regions, including the hippocampus, exhibit oscillatory capacity. This finding, as well as work on the cellular signaling events that underlie learning and memory, has opened promising new avenues of investigation into the precise cellular, molecular, and circuit-based mechanisms by which clock timing impacts plasticity and cognition. In this review, we examine the complex molecular relationship between clock timing and memory, with a focus on hippocampal-dependent tasks. We evaluate how the dysregulation of circadian timing, both at the level of the SCN and at the level of ancillary forebrain clocks, affects learning and memory. Further, we discuss experimentally validated intracellular signaling pathways (e.g., ERK/MAPK and GSK3β) and potential cellular signaling mechanisms by which the clock affects learning and memory formation. Finally, we examine how long-term potentiation (LTP), a synaptic process critical to the establishment of several forms of memory, is regulated by clock-gated processes.

Rapid actions of oestrogens and their receptors on memory acquisition and consolidation in females

Increased attention has been paid in recent years to the ways in which oestrogens and oestrogen receptors rapidly affect learning and memory. These rapid effects occur within a timeframe that is too narrow for the classical genomic mode of action of oestrogen, thus suggesting nonclassical effects as underlying mechanisms. The present review examines recent developments in the study of the rapid effects of 17β-oestradiol and oestrogen receptor (ER) agonists on learning and memory tasks in female rodents, including social recognition, object recognition, object placement (spatial memory) and social learning. By comparing studies utilising systemic or intracranial treatments, as well as pre- and post-acquisition administration of oestradiol or ER agonists, the respective contributions of individual ERs within specific brain regions to various forms of learning and memory can be determined. The first part of this review explores the effects of systemic administration of 17β-oestradiol and ER agonists on memory when administered either pre- or post-acquisition. The second part not only focuses on the effects of pre- and post-acquisition infusions of 17β-oestradiol or ER agonists into the dorsal hippocampus on memory, but also discusses the contributions of other brain regions, including the medial amygdala, medial prefrontal cortex and paraventricular nucleus of the hypothalamus. The cellular mechanisms mediating the rapid effects of 17β-oestradiol on memory, including activation of intracellular signalling cascades and epigenetic processes, are discussed. Finally, the review concludes by comparing pre- and post-acquisition findings and effects of 17β-oestradiol and ER agonists in different brain regions.

Retrieval-Induced Upregulation of Tet3 in Pyramidal Neurons of the Dorsal Hippocampus Mediates Cocaine-Associated Memory Reconsolidation

BACKGROUND

Memory retrieval refers to reexposure to information previously encoded and stored in the brain. Following retrieval, a once-consolidated memory destabilizes and undergoes reconsolidation, during which gene expression changes to restabilize memory. Investigating epigenetic regulation during reconsolidation could provide insights into normal memory formation and pathological memory associated with psychiatric disorders.

METHODS

We used cocaine-induced conditioned place preference to assess the cocaine-associated memory of mice and used chemogenetic methods to manipulate the activity of the pyramidal neurons in the dorsal hippocampus. We isolated the ribosome-associated transcripts from the excitatory neurons in the dorsal hippocampus by RiboTag purification to identify the potential epigenetic regulators, and we specifically knocked down gene expression in pyramidal neurons with a Cre-dependent lentivirus.

RESULTS

Chemogenetically silencing the activity of the pyramidal neurons in the dorsal hippocampus immediately after memory retrieval markedly impaired memory reconsolidation, and the ribosome-associated mRNA level of the ten-eleven translocation (Tet) family methylcytosine dioxygenase Tet3, but not Tet1 or Tet2, was dramatically upregulated 10 minutes after memory retrieval. The protein level of Tet3 in the dorsal hippocampus but not in the anterior cingulate cortex was dramatically increased 1 hour after memory retrieval. Specifically, knockdown of Tet3 in pyramidal neurons in the dorsal hippocampus decreased the activation of pyramidal neurons and impaired the reconsolidation of cocaine-associated memory.

CONCLUSIONS

Our findings highlight the new function of the DNA demethylation regulator Tet3 in pyramidal neurons of the dorsal hippocampus in regulating the reconsolidation of cocaine-associated memory.

Epigenetic regulation by G9a/GLP complex ameliorates amyloid-beta 1-42 induced deficits in long-term plasticity and synaptic tagging/capture in hippocampal pyramidal neurons

Altered epigenetic mechanisms are implicated in the cognitive decline associated with neurodegenerative diseases such as in Alzheimer's disease (AD). AD is the most prevalent form of dementia worldwide; amyloid plaques and neurofibrillary tangles are the histopathological hallmarks of AD. We have recently reported that the inhibition of G9a/GLP complex promotes long-term potentiation (LTP) and its associative mechanisms such as synaptic tagging and capture (STC). However, the role of this complex in plasticity impairments remains elusive. Here, we investigated the involvement of G9a/GLP complex in alleviating the effects of soluble Amyloid-β 1-42 oligomers (oAβ) on neuronal plasticity and associativity in the CA1 region of acute hippocampal slices from 5- to 7-week-old male Wistar rats. Our findings demonstrate that the regulation of G9a/GLP complex by inhibiting its catalytic activity reverses the amyloid-β oligomer-induced deficits in late-LTP and STC. This is achieved by releasing the transcription repression of the brain-derived neurotrophic factor (Bdnf) gene. The catalytic inhibition of G9a/GLP complex leads to the upregulation of Bdnf expression in the slices treated with oAβ. This further ensures the availability of BDNF that subsequently binds its receptor tyrosine kinase B (TrkB) and maintains the late-LTP. Furthermore, the capture of BDNF by weakly activated synapses re-establishes STC. Our findings regarding the reinstatement of functional plasticity and associativity in AD-like conditions provide the first evidence for the role of G9a/GLP complex in AD. We propose G9a/GLP complex as the possible target for preventing oAβ-induced plasticity deficits in hippocampal neurons.

DNA methylation in demyelinated multiple sclerosis hippocampus

Multiple Sclerosis (MS) is an immune-mediated demyelinating disease of the human central nervous system (CNS). Memory impairments and hippocampal demyelination are common features in MS patients. Our previous data have shown that demyelination alters neuronal gene expression in the hippocampus. DNA methylation is a common epigenetic modifier of gene expression. In this study, we investigated whether DNA methylation is altered in MS hippocampus following demyelination. Our results show that mRNA levels of DNA methyltransferase were increased in demyelinated MS hippocampus, while de-methylation enzymes were decreased. Comparative methylation profiling identify hypo-methylation within upstream sequences of 6 genes and hyper-methylation of 10 genes in demyelinated MS hippocampus. Genes identified in the current study were also validated in an independent microarray dataset generated from MS hippocampus. Independent validation using RT-PCR revealed that DNA methylation inversely correlated with mRNA levels of the candidate genes. Queries across cell-specific databases revealed that a majority of the candidate genes are expressed by astrocytes and neurons in mouse and human CNS. Taken together, our results expands the list of genes previously identified in MS hippocampus and establish DNA methylation as a mechanism of altered gene expression in MS hippocampus.

Neuroprotective effects of artemisinin against isoflurane-induced cognitive impairments and neuronal cell death involve JNK/ERK1/2 signalling and improved hippocampal histone acetylation in neonatal rats

OBJECTIVE

This study was performed to assess the effect of artemisinin against isoflurane-induced neuronal apoptosis and cognitive impairment in neonatal rats.

METHODS

Artemisinin (50, 100 or 200 mg/kg b.wt/day; oral gavage) was administered to separate groups of neonatal rats starting from postnatal day 3 (P3) to postnatal day 21 (P21). On postnatal day 7 (P7), animals were exposed to inhalation anaesthetic isoflurane (0.75%) for 6 h.

KEY FINDINGS

Neuronal apoptosis following anaesthetic exposure was significantly reduced by artemisinin. Isoflurane-induced upregulated cleaved caspase-3, Bax and Bad expression were downregulated. Western blotting analysis revealed that treatment with artemisinin significantly enhanced the expression of anti-apoptotic proteins (Bcl-2, Bcl-xL, c-IAP-1, c-IAP-2, xIAP and survivin). Artemisinin increased the acetylation of H3K9 and H4K12 while reducing the expression of histone deacetlyases (HDACs) - HDAC-2 and HDAC-3. Isoflurane-induced activation of JNK signalling and downregulated ERK1/2 expression was effectively modulated by artemisinin. General behaviour of the animals in open-field and T-maze test were improved. Morris water maze test and object recognition test revealed better learning, working memory and also better memory retention on artemisinin treatment.

CONCLUSIONS

Artemisinin effectively inhibited neuronal apoptosis and improved cognition and memory via regulating histone acetylation and JNK/ERK1/2 signalling.

The proteasome and epigenetics: zooming in on histone modifications

The proteasome is a structural complex of many proteins that degrades substrates marked by covalent linkage to ubiquitin. Many years of research has shown a role for ubiquitin-proteasome-mediated proteolysis in synaptic plasticity and memory mainly in degrading synaptic, cytoplasmic and nuclear proteins. Recent work indicates that the proteasome has wider proteolytic and non-proteolytic roles in processes such as histone modifications that affect synaptic plasticity and memory. In this review, we assess the evidence gathered from neuronal as well as non-neuronal cell types regarding the function of the proteasome in positive or negative regulation of posttranslational modifications of histones, such as acetylation, methylation and ubiquitination. We discuss the critical roles of the proteasome in clearing excess histone proteins in various cellular contexts and the possible non-proteolytic functions in regulating transcription of target genes. In addition, we summarize the current literature on diverse chromatin-remodeling machineries, such as histone acetyltransferases, deacetylates, methyltransferases and demethylases, as targets for proteasomal degradation across experimental models. Lastly, we provide a perspective on how proteasomal regulation of histone modifications may modulate synaptic plasticity in the nervous system.

Reconsolidation and extinction: Using epigenetic signatures to challenge conventional wisdom

Epigenetic mechanisms have the potential to give rise to lasting changes in cell function that ultimately can affect behavior persistently. This concept is especially interesting with respect to fear reconsolidation and fear memory extinction. These two behavioral approaches are used in the laboratory to investigate how fear memory can be attenuated, which becomes important when searching for therapeutic intervention to treat anxiety disorders and post-traumatic stress disorder. Here we review the role of several key epigenetic mechanisms in reconsolidation and extinction of learned fear and their potential to persistently alter behavioral responses to conditioned cues. We also briefly discuss how epigenetic mechanisms may establish persistent behaviors that challenge our definitions of extinction and reconsolidation.

H3K9me3 Inhibition Improves Memory, Promotes Spine Formation, and Increases BDNF Levels in the Aged Hippocampus

An increasing number of studies show that an altered epigenetic landscape may cause impairments in regulation of learning and memory-related genes within the aged hippocampus, eventually resulting in cognitive deficits in the aged brain. One such epigenetic repressive mark is trimethylation of H3K9 (H3K9me3), which is typically implicated in gene silencing. Here, we identify, for the first time, an essential role for H3K9me3 and its histone methyl transferase (SUV39H1) in mediating hippocampal memory functions. Pharmacological inhibition of SUV39H1 using a novel and selective inhibitor decreased levels of H3K9me3 in the hippocampus of aged mice, and improved performance in the objection location memory and fear conditioning tasks and in a complex spatial environment learning task. The inhibition of SUV39H1 induced an increase in spine density of thin and stubby but not mushroom spines in the hippocampus of aged animals and increased surface GluR1 levels in hippocampal synaptosomes, a key index of spine plasticity. Furthermore, there were changes at BDNF exon I gene promoter, in concert with overall BDNF levels in the hippocampus of drug-treated animals compared with control animals. Together, these data demonstrate that SUV39H1 inhibition and the concomitant H3K9me3 downregulation mediate gene transcription in the hippocampus and reverse age-dependent deficits in hippocampal memory.

SIGNIFICANCE STATEMENT

Cognitive decline is a debilitating condition associated with not only neurodegenerative diseases but also aging in general. However, effective treatments have been slow to emerge so far. In this study, we demonstrate that epigenetic regulation of key synaptic proteins may be an underlying, yet reversible, cause of this decline. Our findings suggest that histone 3 trimethylation is a probable target for pharmacological intervention that can counteract cognitive decline in the aging brain. Finally, we provide support to the hypothesis that, by manipulating the enzyme that regulates H3K9me3 (using a newly developed specific inhibitor of SUV39H1), it is possible to alter the chromatin state of subjects and restore memory and synaptic function in the aging brain.

Histone Acetylation Regulation in Sleep Deprivation-Induced Spatial Memory Impairment

Sleep disorders negatively affect cognition and health. Recent evidence has indicated that chromatin remodeling via histone acetylation regulates cognitive function. This study aimed to investigate the possible roles of histone acetylation in sleep deprivation (SD)-induced cognitive impairment. Results of the Morris water maze test showed that 3 days of SD can cause spatial memory impairment in Wistar rats. SD can also decrease histone acetylation levels, increase histone deacetylase 2 (HDAC2) expression, and decrease histone acetyltransferase (CBP) expression. Furthermore, SD can reduce H3 and H4 acetylation levels in the promoters of the brain-derived neurotrophic factor (Bdnf) gene and thus significantly downregulate BDNF expression and impair the activity of key BDNF signaling pathways (pCaMKII, pErk2, and pCREB). However, treatment with the HDAC inhibitor trichostatin A attenuated all the negative effects induced by SD. Therefore, BDNF and its histone acetylation regulation may play important roles in SD-induced spatial memory impairment, whereas HDAC inhibition possibly confers protection against SD-induced impairment in spatial memory and hippocampal functions.

Role of histone acetylation in long-term neurobehavioral effects of neonatal Exposure to sevoflurane in rats

Human studies, and especially laboratory studies, provide evidence that early life exposure to general anesthesia may affect neurocognitive development via largely unknown mechanisms. We explored whether hippocampal histone acetylation had a role in neurodevelopmental effects of sevoflurane administered to neonatal rats. Male Sprague-Dawley rats were exposed to 3% sevoflurane or were subjected to maternal separation only for 2h daily at postnatal days 6, 7, and 8. The histone deacetylase inhibitor, sodium butyrate (250mg/kg, intraperitoneally), or saline was administered starting 2h prior to anesthesia or maternal separation and continued daily until the end of behavioral tests, which were performed between postnatal days 33 and 50. Upon completion of the behavioral tests, the brain tissues were harvested for further analysis. Rats neonatally exposed to sevoflurane exhibited decreased freezing time in the fear conditioning contextual test and increased escape latency, decreased time in target quadrant, and number of platform crossings in the Morris water maze test. The sevoflurane-exposed rats had lower hippocampal density of dendritic spines, reduced levels of the brain-derived neurotrophic factor, c-fos protein, microtubule-associated protein 2, synapsin1, postsynaptic density protein 95, pCREB/CREB, CREB binding protein, and acetylated histones H3 and H4, and increased levels of histone deacetylases 3 and 8. These neurobehavioral abnormalities were normalized in the sevoflurane-exposed rats treated with sodium butyrate. Our findings provide evidence that neonatal exposure to sevoflurane induces neurobehavioral abnormalities and long-lasting alterations in histone acetylation; normalization of histone acetylation may alleviate the neurodevelopmental side effects of the anesthetic.

Early malnutrition results in long-lasting impairments in pattern-separation for overlapping novel object and novel location memories and reduced hippocampal neurogenesis

Numerous epidemiological studies indicate that malnutrition during in utero development and/or childhood induces long-lasting learning disabilities and enhanced susceptibility to develop psychiatric disorders. However, animal studies aimed to address this question have yielded inconsistent results due to the use of learning tasks involving negative or positive reinforces that interfere with the enduring changes in emotional reactivity and motivation produced by in utero and neonatal malnutrition. Consequently, the mechanisms underlying the learning deficits associated with malnutrition in early life remain unknown. Here we implemented a behavioural paradigm based on the combination of the novel object recognition and the novel object location tasks to define the impact of early protein-restriction on the behavioural, cellular and molecular basis of memory processing. Adult rats born to dams fed a low-protein diet during pregnancy and lactation, exhibited impaired encoding and consolidation of memory resulting from impaired pattern separation. This learning deficit was associated with reduced production of newly born hippocampal neurons and down regulation of BDNF gene expression. These data sustain the existence of a causal relationship between early malnutrition and impaired learning in adulthood and show that decreased adult neurogenesis is associated to the cognitive deficits induced by childhood exposure to poor nutrition.

CaMKII regulates proteasome phosphorylation and activity and promotes memory destabilization following retrieval

Numerous studies have suggested that memories "destabilize" and require de novo protein synthesis in order to reconsolidate following retrieval, but very little is known about how this destabilization process is regulated. Recently, ubiquitin-proteasome mediated protein degradation has been identified as a critical regulator of memory trace destabilization following retrieval, though the specific mechanisms controlling retrieval-induced changes in ubiquitin-proteasome activity remain equivocal. Here, we found that proteasome activity is increased in the amygdala in a CaMKII-dependent manner following the retrieval of a contextual fear memory. We show that in vitro inhibition of CaMKII reversed retrieval-induced increases in proteasome activity. Additionally, in vivo pharmacological blockade of CaMKII abolished increases in proteolytic activity and activity related regulatory phosphorylation in the amygdala following retrieval, suggesting that CaMKII was "upstream" of protein degradation during the memory reconsolidation process. Consistent with this, while inhibiting CaMKII in the amygdala did not impair memory following retrieval, it completely attenuated the memory impairments that resulted from post-retrieval protein synthesis blockade. Collectively, these results suggest that CaMKII controls the initiation of the memory reconsolidation process through regulation of the proteasome.

NF-κB mediates Gadd45β expression and DNA demethylation in the hippocampus during fear memory formation

Gadd45-mediated DNA demethylation mechanisms have been implicated in the process of memory formation. However, the transcriptional mechanisms involved in the regulation of Gadd45 gene expression during memory formation remain unexplored. NF-κB (nuclear factor kappa-light-chain-enhancer of activated B cells) controls transcription of genes in neurons and is a critical regulator of synaptic plasticity and memory formation. In silico analysis revealed several NF-κB (p65/RelA and cRel) consensus sequences within the Gadd45β gene promoter. Whether NF-κB activity regulates Gadd45 expression and associated DNA demethylation in neurons during memory formation is unknown. Here, we found that learning in a fear conditioning paradigm increased Gadd45β gene expression and brain-derivedneurotrophic factor (BDNF) DNA demethylation in area CA1 of the hippocampus, both of which were prevented with pharmacological inhibition of NF-κB activity. Further experiments found that conditional mutations in p65/RelA impaired fear memory formation but did not alter changes in Gadd45β expression. The learning-induced increases in Gadd45β mRNA levels, Gadd45β binding at the BDNF gene and BDNF DNA demethylation were blocked in area CA1 of the c-rel knockout mice. Additionally, local siRNA-mediated knockdown of c-rel in area CA1 prevented fear conditioning-induced increases in Gadd45β expression and BDNF DNA demethylation, suggesting that c-Rel containing NF-κB transcription factor complex is responsible for Gadd45β regulation during memory formation. Together, these results support a novel transcriptional role for NF-κB in regulation of Gadd45β expression and DNA demethylation in hippocampal neurons during fear memory.

Regulatory RNAs and control of epigenetic mechanisms: expectations for cognition and cognitive dysfunction

The diverse functions of noncoding RNAs (ncRNAs) can influence virtually every aspect of the transcriptional process including epigenetic regulation of genes. In the CNS, regulatory RNA networks and epigenetic mechanisms have broad relevance to gene transcription changes involved in long-term memory formation and cognition. Thus, it is becoming increasingly clear that multiple classes of ncRNAs impact neuronal development, neuroplasticity, and cognition. Currently, a large gap exists in our knowledge of how ncRNAs facilitate epigenetic processes, and how this phenomenon affects cognitive function. In this review, we discuss recent findings highlighting a provocative role for ncRNAs including lncRNAs and piRNAs in the control of epigenetic mechanisms involved in cognitive function. Furthermore, we discuss the putative roles for these ncRNAs in cognitive disorders such as schizophrenia and Alzheimer's disease.

Sex steroid hormones matter for learning and memory: estrogenic regulation of hippocampal function in male and female rodents

Ample evidence has demonstrated that sex steroid hormones, such as the potent estrogen 17β-estradiol (E2), affect hippocampal morphology, plasticity, and memory in male and female rodents. Yet relatively few investigators who work with male subjects consider the effects of these hormones on learning and memory. This review describes the effects of E2 on hippocampal spinogenesis, neurogenesis, physiology, and memory, with particular attention paid to the effects of E2 in male rodents. The estrogen receptors, cell-signaling pathways, and epigenetic processes necessary for E2 to enhance memory in female rodents are also discussed in detail. Finally, practical considerations for working with female rodents are described for those investigators thinking of adding females to their experimental designs.

Molecular mechanisms underlying the memory-enhancing effects of estradiol

This article is part of a Special Issue "Estradiol and cognition". Since the publication of the 1998 special issue of Hormones and Behavior on estrogens and cognition, substantial progress has been made towards understanding the molecular mechanisms through which 17β-estradiol (E2) regulates hippocampal plasticity and memory. Recent research has demonstrated that rapid effects of E2 on hippocampal cell signaling, epigenetic processes, and local protein synthesis are necessary for E2 to facilitate the consolidation of object recognition and spatial memories in ovariectomized female rodents. These effects appear to be mediated by non-classical actions of the intracellular estrogen receptors ERα and ERβ, and possibly by membrane-bound ERs such as the G-protein-coupled estrogen receptor (GPER). New findings also suggest a key role of hippocampally-synthesized E2 in regulating hippocampal memory formation. The present review discusses these findings in detail and suggests avenues for future study.

Environmental enrichment reverses histone methylation changes in the aged hippocampus and restores age-related memory deficits

A decline in long-term memory (LTM) formation is a common feature of the normal aging process, which corresponds with abnormal expression of memory-related genes in the aged hippocampus. Epigenetic modulation of chromatin structure is required for proper transcriptional control of genes, such as the brain-derived neurotrophic factor (Bdnf) and Zif268 in the hippocampus during the consolidation of new memories. Recently, the view has emerged that aberrant transcriptional regulation of memory-related genes may be reflective of an altered epigenetic landscape within the aged hippocampus, resulting in memory deficits with aging. Here, we found that baseline resting levels for tri-methylation of histone H3 at lysine 4 (H3K4me3) and acetylation of histone H3 at lysine 9 and 14 (H3K9,K14ac) were altered in the aged hippocampus as compared to levels in the hippocampus of young adult rats. Interestingly, object learning failed to increase activity-dependent H3K4me3 and di-methylation of histone H3 at lysine 9 (H3K9me2) levels in the hippocampus of aged adults as compared to young adults. Treatment with the LSD-1 histone demethylase inhibitor, t-PCP, increased baseline resting H3K4me3 and H3K9,K14ac levels in the young adult hippocampus, while young adult rats exhibited similar memory deficits as observed in aged rats. After environmental enrichment (EE), we found that object learning induced increases in H3K4me3 levels around the Bdnf, but not the Zif268, gene region in the aged hippocampus and rescued memory deficits in aged adults. Collectively, these results suggest that histone lysine methylation levels are abnormally regulated in the aged hippocampus and identify histone lysine methylation as a transcriptional mechanism by which EE may serve to restore memory formation with aging.

The role of nutrient-based epigenetic changes in buffering against stress, aging, and Alzheimer's disease

Converging evidence identifies stress-related disorders as putative risk factors for Alzheimer Disease (AD). This article reviews evidence on the complex interplay of stress, aging, and genes-epigenetics interactions. The recent classification of AD into preclinical, mild cognitive impairment, and AD offers a window for intervention to prevent, delay, or modify the course of AD. Evidence in support of the cognitive effects of epigenetics-diet, and nutraceuticals is reviewed. A proactive epigenetics diet and nutraceuticals program holds promise as potential buffer against the negative impact of aging and stress responses on cognition, and can optimize vascular, metabolic, and brain health in the community.