Epigenetic mechanisms and memory strength: a comparative study

Annual changes of Neohelice granulata cognitive abilities indicate opposition between short- and long-term memory retention

Neohelice is a long-standing model for memory studies for its strong retention of a reduced escape response when trained to iterative presentations of a visual danger stimulus (VDS). Here we present year-round changes that are related to the memory acquisition, storage, and expression. First, we evaluated exploratory activity and response to the VDS, as necessary for memory acquisition and expression. Both parameters change year-round. Second, short-term memory (STM) and two types of long-term memory (LTM) were assessed throughout the year. STM and long-term context-dependent signal memory (CSM) change between periods of the year, whereas signal memory (SM) does not, indicating that the cognitive abilities of the crab display circannual rhythms. Third, during the reproductive period, STM retention is higher than both CSM and SM, indicating a trade-off between STM and LTM. This is the first report of memory retention abilities changing seasonally as a trade-off between short- and long-term memories.

Contributions of extracellular-signal regulated kinase 1/2 activity to the memory trace

The ability to learn from experience and consequently adapt our behavior is one of the most fundamental capacities enabled by complex and plastic nervous systems. Next to cellular and systems-level changes, learning and memory formation crucially depends on molecular signaling mechanisms. In particular, the extracellular-signal regulated kinase 1/2 (ERK), historically studied in the context of tumor growth and proliferation, has been shown to affect synaptic transmission, regulation of neuronal gene expression and protein synthesis leading to structural synaptic changes. However, to what extent the effects of ERK are specifically related to memory formation and stabilization, or merely the result of general neuronal activation, remains unknown. Here, we review the signals leading to ERK activation in the nervous system, the subcellular ERK targets associated with learning-related plasticity, and how neurons with activated ERK signaling may contribute to the formation of the memory trace.

Memory and plasticity impairment after binge drinking in adolescent rat hippocampus: GluN2A/GluN2B NMDA receptor subunits imbalance through HDAC2

Ethanol (EtOH) induces cognitive impairment through modulation of synaptic plasticity notably in the hippocampus. The cellular mechanism(s) of these EtOH effects may range from synaptic signaling modulation to alterations of the epigenome. Previously, we reported that two binge-like exposures to EtOH (3 g/kg, ip, 9 h apart) in adolescent rats abolished long-term synaptic depression (LTD) in hippocampus slices, induced learning deficits, and increased N-methyl-d-aspartate (NMDA) receptor signaling through its GluN2B subunit after 48 hours. Here, we tested the hypothesis of EtOH-induced epigenetic alterations leading to modulation of GluN2B and GluN2A NMDA receptor subunits. Forty-two days old rats were treated with EtOH or the histone deacetylase inhibitor (HDACi) sodium butyrate (NaB, 600 mg/kg, ip) injected alone or 30 minutes before EtOH. After 48 hours, learning was tested with novel object recognition while synaptic plasticity and the role of GluN2A and GluN2B subunits in NMDA-fEPSP were measured in CA1 field of hippocampus slices. LTD and memory were impaired 48 hours after EtOH and NMDA-fEPSP analysis unraveled changes in the GluN2A/GluN2B balance. These results were associated with an increase in histone deacetylase (HDAC) activity and HDAC2 mRNA and protein while Ac-H4K12 labelling was decreased. EtOH increases expression of HDAC2 and mRNA level for GluN2B subunit (but not GluN2A), while HDAC2 modulates the promoter of the gene encoding GluN2B. Interestingly, NaB pretreatment prevented all the cellular and memory-impairing effects of EtOH. In conclusion, the memory-impairing effects of two binge-like EtOH exposure involve NMDA receptor-dependent LTD deficits due to a GluN2A/GluN2B imbalance resulting from changes in GluN2B expression induced by HDAC2.

Effects of a histone deacetylase 3 inhibitor on extinction and reinstatement of cocaine self-administration in rats

RATIONALE

A challenge in treating substance use disorder is that successful treatment often does not persist, resulting in relapse and continued drug seeking. One approach to persistently weaken drug-seeking behaviors is to pair exposure to drug-associated cues or behaviors with delivery of a compound that may strengthen the inhibition of the association between drug cues and behavior.

OBJECTIVES

We evaluated whether a selective histone deacetylase 3 (HDAC3) inhibitor could promote extinction and weaken contextual control of operant drug seeking after intravenous cocaine self-administration.

METHODS

Male Long-Evans rats received a systemic injection of the HDAC3 inhibitor RGFP966 either before or immediately after the first extinction session. Persistence of extinction was tested over subsequent extinction sessions, as well as tests of reinstatement that included cue-induced reinstatement, contextual renewal, and cocaine-primed reinstatement. Additional extinction sessions occurred between each reinstatement test. We also evaluated effects of RGFP966 on performance and motivation during stable fixed ratio operant responding for cocaine and during a progressive ratio of reinforcement.

RESULTS

RGFP966 administered before the first extinction session led to significantly less responding during subsequent extinction and reinstatement tests compared to vehicle-injected rats. Follow-up studies found that these effects were not likely due to a performance deficit or a change in motivation to self-administer cocaine, as injections of RGFP966 had no effect on stable responding during a fixed or progressive ratio schedule. In addition, RGFP966 administered just after the first extinction session had no effect during early extinction and reinstatement tests, but weakened long-term responding during later extinction sessions.

CONCLUSIONS

These results suggest that a systemic injection of a selective HDAC3 inhibitor can enhance extinction and suppress reinstatement after cocaine self-administration. The finding that behavioral and pharmacological manipulations can be combined to decrease drug seeking provides further potential for treatment by epigenetic modulation.

CaMKII Isoforms in Learning and Memory: Localization and Function

Calcium/calmodulin-dependent protein kinase II (CaMKII) is a key protein kinase in neural plasticity and memory, as have been shown in several studies since the first evidence in long-term potentiation (LTP) 30 years ago. However, most of the studies were focused mainly in one of the four isoforms of this protein kinase, the CaMKIIα. Here we review the characteristics and the role of each of the four isoforms in learning, memory and neural plasticity, considering the well known local role of α and β isoforms in dendritic terminals as well as recent findings about the γ isoform as calcium signals transducers from synapse to nucleus and δ isoform as a kinase required for a more persistent memory trace.

HDAC3 Inhibitor RGFP966 Modulates Neuronal Memory for Vocal Communication Signals in a Songbird Model

Epigenetic mechanisms that modify chromatin conformation have recently been under investigation for their contributions to learning and the formation of memory. For example, the role of enzymes involved in histone acetylation are studied in the formation of long-lasting memories because memory consolidation requires gene expression events that are facilitated by an open state of chromatin. We recently proposed that epigenetic events may control the entry of specific sensory features into long-term memory by enabling transcription-mediated neuronal plasticity in sensory brain areas. Histone deacetylases, like HDAC3, may thereby regulate the specific sensory information that is captured for entry into long-term memory stores (Phan and Bieszczad, 2016). To test this hypothesis, we used an HDAC3-selective inhibitor (RGFP966) to determine whether its application after an experience with a sound stimulus with unique acoustic features could contribute to the formation of a memory that would assist in mediating its later recognition. We gave adult male zebra finches limited exposure to unique conspecific songs (20 repetitions each, well below the normal threshold to form long-term memory), followed by treatment with RGFP966 or vehicle. In different groups, we either made multi-electrode recordings in the higher auditory area NCM (caudal medial nidopallidum), or determined expression of an immediate early gene, zenk (also identified as zif268, egr-1, ngfi-a and krox24), known to participate in neuronal memory in this system. We found that birds treated with RGFP966 showed neuronal memory after only limited exposure, while birds treated with vehicle did not. Strikingly, evidence of neuronal memory in NCM induced by HDAC3-inhibition was lateralized to the left-hemisphere, consistent with our finding that RGFP966-treatment also elevated zenk expression only in the left hemisphere. The present findings show feasibility for epigenetic mechanisms to control neural plasticity underlying the formation of specific memories for conspecific communication sounds. This is the first evidence in zebra finches that epigenetic mechanisms may contribute to gene expression events for memory of acoustically-rich sensory cues.

Late-Life Environmental Enrichment Induces Acetylation Events and Nuclear Factor κB-Dependent Regulations in the Hippocampus of Aged Rats Showing Improved Plasticity and Learning

Aging weakens memory functions. Exposing healthy rodents or pathological rodent models to environmental enrichment (EE) housing improves their cognitive functions by changing neuronal levels of excitation, cellular signaling, and plasticity, notably in the hippocampus. At the molecular level, brain derived-neurotrophic factor (BDNF) represents an important player that supports EE-associated changes. EE facilitation of learning was also shown to correlate with chromatin acetylation in the hippocampus. It is not known, however, whether such mechanisms are still into play during aging. In this study, we exposed a cohort of aged rats (18-month-old) to either a 6 month period of EE or standard housing conditions and investigated chromatin acetylation-associated events [histone acetyltranferase activity, gene expression, and histone 3 (H3) acetylation] and epigenetic modulation of the Bdnf gene under rest conditions and during learning. We show that EE leads to upregulation of acetylation-dependent mechanisms in aged rats, whether at rest or following a learning challenge. We found an increased expression of Bdnf through Exon-I-dependent transcription, associated with an enrichment of acetylated H3 at several sites of Bdnf promoter I, more particularly on a proximal nuclear factor κB (NF-κB) site under learning conditions. We further evidenced p65/NF-κB binding to chromatin at promoters of genes important for plasticity and hippocampus-dependent learning (e.g., Bdnf, CamK2D). Altogether, our findings demonstrate that aged rats respond to a belated period of EE by increasing hippocampal plasticity, together with activating sustained acetylation-associated mechanisms recruiting NF-κB and promoting related gene transcription. These responses are likely to trigger beneficial effects associated with EE during aging.

SIGNIFICANCE STATEMENT

Aging weakens memory functions. Optimizing the neuronal circuitry required for normal brain function can be achieved by increasing sensory, motor, and cognitive stimuli resulting from interactions with the environment (behavioral therapy). This can be experimentally modeled by exposing rodents to environmental enrichment (EE), as with large cages, numerous and varied toys, and interaction with other rodents. However, EE effects in aged rodents has been poorly studied, and it is not known whether beneficial mechanisms evidenced in the young adults can still be recruited during aging. Our study shows that aged rats respond to a belated period of EE by activating specific epigenetic and transcriptional signaling that promotes gene expression likely to facilitate plasticity and learning behaviors.

Induction of Lumen Formation in a Three-dimensional Model of Mammary Morphogenesis by Transcriptional Regulator ID4: ROLE OF CaMK2D IN THE EPIGENETIC REGULATION OF ID4 GENE EXPRESSION

Concomitant loss of lumen formation and cell adhesion protein CEACAM1 is a hallmark feature of breast cancer. In a three-dimensional culture model, transfection of CEACAM1 into MCF7 breast cells can restore lumen formation by an unknown mechanism. ID4, a transcriptional regulator lacking a DNA binding domain, is highly up-regulated in CEACAM1-transfected MCF7 cells, and when down-regulated with RNAi, abrogates lumen formation. Conversely, when MCF7 cells, which fail to form lumena in a three-dimensional culture, are transfected with ID4, lumen formation is restored, demonstrating that ID4 may substitute for CEACAM1. After showing the ID4 promoter is hypermethylated in MCF7 cells but hypomethylated in MCF/CEACAM1 cells, ID4 expression was induced in MCF7 cells by agents affecting chromatin remodeling and methylation. Mechanistically, CaMK2D was up-regulated in CEACAM1-transfected cells, effecting phosphorylation of HDAC4 and its sequestration in the cytoplasm by the adaptor protein 14-3-3. CaMK2D also phosphorylates CEACAM1 on its cytoplasmic domain and mutation of these phosphorylation sites abrogates lumen formation. Thus, CEACAM1 is able to maintain the active transcription of ID4 by an epigenetic mechanism involving HDAC4 and CaMK2D, and the same kinase enables lumen formation by CEACAM1. Because ID4 can replace CEACAM1 in parental MCF7 cells, it must act downstream from CEACAM1 by inhibiting the activity of other transcription factors that would otherwise prevent lumen formation. This overall mechanism may be operative in other cancers, such as colon and prostate, where the down-regulation of CEACAM1 is observed.