Distinct roles for the deacetylase domain of HDAC3 in the hippocampus and medial prefrontal cortex in the formation and extinction of memory

Class 1 histone deacetylases differentially modulate memory and synaptic genes in a spatial and temporal manner in aged and APP/PS1 mice

Epigenetics plays a vital role in aging and Alzheimer's disease (AD); however, whether epigenetic alterations during aging can initiate AD and exacerbate AD progression remains unclear. In this study, using 3-, 12- and 18- month-old APP/PS1 mice and age matched WT littermates, we conducted a series of memory tests, measured synapse-related gene expression, class 1 histone deacetylases (HDACs) abundance, and H3K9ac levels at target gene promoters in the hippocampus and prefrontal cortex (PFC). Our results showed impaired recognition and long-term spatial memory in 18-month-old WT mice and impaired recognition, short-term working, and long-term spatial reference memory in 12-and 18- month-old APP/PS1 mice. These memory impairments are associated with changes of synapse-related gene (nr2a, glur1, glur2, psd95) expression, HDAC abundance, and H3K9ac levels; more specifically, increased HDAC2 was associated with synapse-related gene expression changes through modulation of H3K9ac at the gene promoters during aging and AD progression in the hippocampus. Conversely, increased HDAC3 was associated with synapse-related gene expression changes through modulation of H3K9ac at the gene promoters during AD progression in the PFC. These findings suggest memory impairments in aging and AD may associated with a differential HDAC modulation of synapse-related gene expression in the brain.

Pharmacological HDAC3 inhibition alters memory updating in young and old mice

Long-term memories are not stored in a stable state but must be flexible and dynamic to maintain relevance in response to new information. Existing memories are thought to be updated through the process of reconsolidation, in which memory retrieval initiates destabilization and updating to incorporate new information. Memory updating is impaired in old age, yet little is known about the mechanisms that go awry. One potential mechanism is the repressive histone deacetylase 3 (HDAC3), which is a powerful negative regulator of memory formation that contributes to age-related impairments in memory formation. Here, we tested whether HDAC3 also contributes to age-related impairments in memory updating using the Objects in Updated Locations (OUL) paradigm. We show that blocking HDAC3 immediately after updating with the pharmacological inhibitor RGFP966 ameliorated age-related impairments in memory updating in 18-m.o. mice. Surprisingly, we found that post-update HDAC3 inhibition in young (3-m.o.) mice had no effect on memory updating but instead impaired memory for the original information, suggesting that the original and updated information may compete for expression at test and HDAC3 helps regulate which information is expressed. To test this idea, we next assessed whether HDAC3 inhibition would improve memory updating in young mice given a weak, subthreshold update. Consistent with our hypothesis, we found that HDAC3 blockade strengthened the subthreshold update without impairing memory for the original information, enabling balanced expression of the original and updated information. Together, this research suggests that HDAC3 may contribute to age-related impairments in memory updating and may regulate the strength of a memory update in young mice, shifting the balance between the original and updated information at test.

Impaired extinction of operant cocaine in a genetic mouse model of schizophrenia risk

BACKGROUND

Individuals with schizophrenia have high rates of comorbid substance use problems. One potential explanation for this comorbidity is similar neuropathophysiology in substance use and schizophrenia, which may arise from shared genetic risk factors between the two disorders. Here we investigated if genetic risk for schizophrenia could affect drug reward and reinforcement for cocaine in an established mouse model of genetic risk for schizophrenia, the neuregulin 1 transmembrane domain heterozygous (Nrg1 TM HET) mouse.

METHODS

We examined drug-induced locomotor sensitization and conditioned place preference for several cocaine doses (5, 10, 20, 30 mg/kg) in male adult Nrg1 TM HET and wild-type-like (WT) littermates. We also investigated intravenous self-administration of and motivation for cocaine (doses 0.1, 0.5, 1 mg/kg/infusion), as well as extinction and cue-induced reinstatement of cocaine. In a follow-up experiment, we examined self-administration, extinction and cue-induced reinstatement of a natural reward, oral sucrose.

RESULTS

Cocaine preference was similar between Nrg1 TM HET mice and WT littermates at all doses tested. Locomotor sensitization to cocaine was not affected by Nrg1 genotype at any dose. Although self-administration and motivation for cocaine was unaffected, extinction of cocaine self-administration was impaired in Nrg1 TM HET compared to WT controls, and cue-induced reinstatement was greater in Nrg1 mutants in the middle of the reinstatement session. Sucrose self-administration and extinction thereof was not affected by genotype, but inactive lever responding was elevated during cue-induced reinstatement for operant sucrose in Nrg1 TM HET mice compared to WTs.

DISCUSSION

These results suggest impaired response inhibition for cocaine in Nrg1 TM HET mice and suggests Nrg1 mutation may contribute to behaviours which can limit control over cocaine use.

HDAC3 of dorsal hippocampus induces postoperative cognitive dysfunction in aged mice

Postoperative cognitive dysfunction (POCD) affects a substantial number of aged individuals. Although advanced age has been regarded as the only independent risk factor for cognitive decline following anesthesia and surgery, the exact cellular and molecular mechanisms remain poorly understood. Histone deacetylase 3 (HDAC3), an epigenetic regulator of memory plays an important role in age-dependent disease. In this study, we investigated the role of HDAC3 in POCD using a laparotomy mouse model. The results showed that the level of HDAC3 in the dorsal hippocampus (DH) was elevated in aged mice compared with young mice. The surgery impaired the spatial-temporal memory in aged mice, as indicated in the object location memory (OLM) and temporal order memory (TOM) tests. Model mice also exhibited increased expression of HDAC3 protein and decreased levels of dendritic spine density and synaptic plasticity-related proteins in the DH. Selectively blocking HDAC3 in the DH of aged mice reversed spatial-temporal memory impairment induced by surgery and restored dendritic spine density and synaptic plasticity-related proteins in the DH. Overexpression of HDAC3 by adeno-associated virus in the DH of young mice mimicked the behavioral deficits induced by anesthesia and surgery. Our results indicated that HDAC3 negatively regulates spatial-temporal memory in aged mice after anesthesia and surgery. Targeting HDAC3 might represent a potential therapy to avoid POCD.

Adolescent cannabinoid exposure modulates the vulnerability to cocaine-induced conditioned place preference and DNMT3a expression in the prefrontal cortex in Swiss mice

RATIONALE

Cannabis sativa is the most widely used drug by adolescents globally. The recreational use of synthetic cannabinoids by teenagers has also grown in recent years. Despite the wrong perception that exposure to these drugs does not cause harm, repeated exposure to cannabinoids at early stages of life compromises important maturation processes and brain development. Chronic early cannabinoid use has been related to a higher risk of psychiatric outcomes, including cocaine addiction. Evidence suggests that exposure to natural and synthetic cannabinoids during adolescence modifies molecular and behavioral effects of cocaine in adulthood. Responses to cocaine are regulated by epigenetic mechanisms, such as DNA methylation, in the brain's reward regions. However, the involvement of these processes in modulation of the vulnerability to the effects of cocaine induced by prior exposure to cannabinoids remains poorly understood.

OBJECTIVES

Investigate whether exposure to the synthetic cannabinoid WIN55,212-2 during adolescence modulates anxiety- and depression-like behavior, memory, and cocaine reward in adult mice. We also evaluated whether exposure to cannabinoids during adolescence modulates the expression of enzymes that are involved in DNA methylation.

RESULTS

Exposure to WIN55,212-2 during adolescence did not alter anxiety- or depressive-like behavior. However, prior exposure to cannabinoids inhibited cocaine-induced conditioned place preference without modulating cocaine-induced hyperlocomotion, accompanied by an increase in expression of the enzyme DNA methyltransferase 3a (DNMT3a) in the prefrontal cortex.

CONCLUSIONS

Our findings suggest that exposure to WIN55,212-2 during adolescence leads to changes in DNMT3a expression, and this pathway appears to be relevant to modulating the rewarding effects of cocaine.

Systemic HDAC3 inhibition ameliorates impairments in synaptic plasticity caused by simulated galactic cosmic radiation exposure in male mice

Deep space travel presents a number of measurable risks including exposure to a spectrum of radiations of varying qualities, termed galactic cosmic radiation (GCR) that are capable of penetrating the spacecraft, traversing through the body and impacting brain function. Using rodents, studies have reported that exposure to simulated GCR leads to cognitive impairments associated with changes in hippocampus function that can persist as long as one-year post exposure with no sign of recovery. Whether memory can be updated to incorporate new information in mice exposed to GCR is unknown. Further, mechanisms underlying long lasting impairments in cognitive function as a result of GCR exposure have yet to be defined. Here, we examined whether whole body exposure to simulated GCR using 6 ions and doses of 5 or 30 cGy interfered with the ability to update an existing memory or impact hippocampal synaptic plasticity, a cellular mechanism believed to underlie memory processes, by examining long term potentiation (LTP) in acute hippocampal slices from middle aged male mice 3.5-5 months after radiation exposure. Using a modified version of the hippocampus-dependent object location memory task developed by our lab termed "Objects in Updated Locations" (OUL) task we find that GCR exposure impaired hippocampus-dependent memory updating and hippocampal LTP 3.5-5 months after exposure. Further, we find that impairments in LTP are reversed through one-time systemic subcutaneous injection of the histone deacetylase 3 inhibitor RGFP 966 (10 mg/kg), suggesting that long lasting impairments in cognitive function may be mediated at least in part, through epigenetic mechanisms.

Nuclear receptor corepressors in intellectual disability and autism

Autism spectrum disorder (ASD) is characterized by neurocognitive dysfunctions, such as impaired social interaction and language learning. Gene-environment interactions have a pivotal role in ASD pathogenesis. Nuclear receptor corepressors (NCORs) are transcription co-regulators physically associated with histone deacetylases (HDACs) and many known players in ASD etiology such as transducin β-like 1 X-linked receptor 1 and methyl-CpG binding protein 2. The epigenome-modifying NCOR complex is sensitive to many ASD risk factors, including HDAC inhibitor valproic acid and a variety of endocrine factors, xenobiotic chemicals, or metabolites that can directly bind to multiple nuclear receptors. Here, we review recent studies of NCORs in neurocognition using animal models and human genetics approaches. We discuss functional interplays between NCORs and other known players in ASD etiology. It is conceivable that the NCOR complex may bridge the in utero environmental risk factors of ASD with epigenetic remodeling and can serve as a converging point for many gene-environment interactions in the pathogenesis of ASD and intellectual disability.

Neuroprotection of Radiosensitive Juvenile Mice by Ultra-High Dose Rate FLASH Irradiation

Major advances in high precision treatment delivery and imaging have greatly improved the tolerance of radiotherapy (RT); however, the selective sparing of normal tissue and the reduction of neurocognitive side effects from radiation-induced toxicities remain significant problems for pediatric patients with brain tumors. While the overall survival of pediatric patients afflicted with medulloblastoma (MB), the most common type primary brain cancer in children, remains high (≥80%), lifelong neurotoxic side-effects are commonplace and adversely impact patients’ quality of life. To circumvent these clinical complications, we have investigated the capability of ultra-high dose rate FLASH-radiotherapy (FLASH-RT) to protect the radiosensitive juvenile mouse brain from normal tissue toxicities. Compared to conventional dose rate (CONV) irradiation, FLASH-RT was found to ameliorate radiation-induced cognitive dysfunction in multiple independent behavioral paradigms, preserve developing and mature neurons, minimize microgliosis and limit the reduction of the plasmatic level of growth hormone. The protective “FLASH effect” was pronounced, especially since a similar whole brain dose of 8 Gy delivered with CONV-RT caused marked reductions in multiple indices of behavioral performance (objects in updated location, novel object recognition, fear extinction, light-dark box, social interaction), reductions in the number of immature (doublecortin⁺) and mature (NeuN⁺) neurons and increased neuroinflammation, adverse effects that were not found with FLASH-RT. Our data point to a potentially innovative treatment modality that is able to spare, if not prevent, many of the side effects associated with long-term treatment that disrupt the long-term cognitive and emotional well-being of medulloblastoma survivors.

Neural substrates of habit

Active reward pursuit is supported by the balance between the cognitive and habitual control of behavior. The cognitive, goal-directed strategy relies on the prospective evaluation of anticipated consequences, which allows behavior to readily adapt when circumstances change. Repetition of successful actions promotes less cognitively taxing habits, in which behavior is automatically executed without prospective consideration. Disruption in either of these behavioral regulatory systems contributes to the symptoms that underlie many psychiatric disorders. Here, I review recently identified neural substrates, at multiple neural levels, that contribute to habits and outline gaps in knowledge that must be addressed to fully understand the neural mechanisms of behavioral control.

The medial prefrontal cortex - hippocampus circuit that integrates information of object, place and time to construct episodic memory in rodents: Behavioral, anatomical and neurochemical properties

Rats and mice have been demonstrated to show episodic-like memory, a prototype of episodic memory, as defined by an integrated memory of the experience of an object or event, in a particular place and time. Such memory can be assessed via the use of spontaneous object exploration paradigms, variably designed to measure memory for object, place, temporal order and object-location inter-relationships. We review the methodological properties of these tests, the neurobiology about time and discuss the evidence for the involvement of the medial prefrontal cortex (mPFC), entorhinal cortex (EC) and hippocampus, with respect to their anatomy, neurotransmitter systems and functional circuits. The systematic analysis suggests that a specific circuit between the mPFC, lateral EC and hippocampus encodes the information for event, place and time of occurrence into the complex episodic-like memory, as a top-down regulation from the mPFC onto the hippocampus. This circuit can be distinguished from the neuronal component memory systems for processing the individual information of object, time and place.

Dysregulation of histone acetylation pathways in hippocampus and frontal cortex of Alzheimer's disease patients

Memory impairment is the main feature of Alzheimer's disease (AD). Initial impairments originate in the temporal lobe area and propagate throughout the brain in a sequential manner. Epigenetic mechanisms, especially histone acetylation, regulate plasticity and memory processes. These may be dismantled during the disease. The aim of this work was to establish changes in the acetylation-associated pathway in two key brain regions affected in AD: the hippocampus and the F2 area of frontal cortex in end-stage AD patients and age-matched controls. We found that the F2 area was more affected than the hippocampus. Indeed, CREB-Binding Protein (CBP), P300/CBP-associated protein (PCAF), Histone Deacetylase 1 (HDAC1) and HDAC2 (but not HDAC3) levels were strongly decreased in F2 area of AD compared to controls patients, whereas only HDAC1 was decreased and CBP showed a downward trend in the hippocampus. At the histone level, we detected a substantial increase in total (H3 and H2B) histone levels in the frontal cortex, but these were decreased in nuclear extracts, pointing to a dysregulation in histone trafficking/catabolism in this brain region. Histone H3 acetylation levels were increased in cell nuclei mainly in the frontal cortex. These findings provide evidence for acetylation dysfunctions at the level of associated enzymes and of histones in AD brains, which may underlie transcriptional dysregulations and AD-related cognitive impairments. They further point to stronger dysregulations in the F2 area of the frontal cortex than in the hippocampus at an end-stage of the disease, suggesting a differential vulnerability and/or compensatory mechanisms efficiency towards epigenetic alterations.

A neuroscientist's guide to transgenic mice and other genetic tools

The past decade has produced an explosion in the number and variety of genetic tools available to neuroscientists, resulting in an unprecedented ability to precisely manipulate the genome and epigenome in behaving animals. However, no single resource exists that describes all of the tools available to neuroscientists. Here, we review the genetic, transgenic, and viral techniques that are currently available to probe the complex relationship between genes and cognition. Topics covered include types of traditional transgenic mouse models (knockout, knock-in, reporter lines), inducible systems (Cre-loxP, Tet-On, Tet-Off) and cell- and circuit-specific systems (TetTag, TRAP, DIO-DREADD). Additionally, we provide details on virus-mediated and siRNA/shRNA approaches, as well as a comprehensive discussion of the myriad manipulations that can be made using the CRISPR-Cas9 system, including single base pair editing and spatially- and temporally-regulated gene-specific transcriptional control. Collectively, this review will serve as a guide to assist neuroscientists in identifying and choosing the appropriate genetic tools available to study the complex relationship between the brain and behavior.

Inhibition of histone deacetylase 3 by MiR-494 alleviates neuronal loss and improves neurological recovery in experimental stroke

HDAC3 is an essential negative regulator of neuronal plasticity and memory formation. Although a chemical inhibitor has been invented, little is known about its endogenous modulators. We explored whether miR-494 affects HDAC3-mediated neuronal injury following acute ischemic stroke. A substantial increase in plasma miR-494 was detected in AIS patients and was positively associated with the mRS at one year after symptom onset. The miR-494 levels were transiently increased in the infarcted brain tissue of mice. In contrast, miR-494 levels were reduced in neurons but increased in the medium after OGD. Intracerebroventricular injection of miR-494 agomir reduced neuronal apoptosis and infarct volume at the acute stage of MCAO, promoted axonal plasticity and long-term outcomes at the recovery stage, suppressed neuronal ataxin-3 and HDAC3 expression and increased acetyl-H3K9 levels in the ipsilateral hemisphere. In vitro studies confirmed that miR-494 posttranslationally inhibited HDAC3 in neurons and prevented OGD-induced neuronal axonal injury. The HDAC3 inhibitor increased acetyl-H3K9 levels and reversed miR-494 antagomir-aggravated acute cerebral ischemic injury, as well as brain atrophy and long-term functional recovery. These results suggest that miR-494 may serve as a predictive biomarker of functional outcomes in AIS patients and a potential therapeutic target for the treatment of ischemic stroke.

Epigenetic pharmacotherapy for substance use disorder

Identifying novel therapeutics for the treatment of substance use disorder (SUD) is an area of intensive investigation. Prior strategies that have attempted to modify one or a few neurotransmitter receptors have had limited success, and currently there are no FDA-approved medications for the treatment of cocaine, methamphetamine, and marijuana use disorders. Because drugs of abuse are known to alter the expression of numerous genes in reward-related brain regions, epigenetic-based therapies have emerged as intriguing targets for therapeutic innovation. Here, I evaluate potential therapeutic approaches and challenges in targeting epigenetic factors for the treatment of SUD and highlight examples of promising strategies and future directions.

Regulation of HDAC1 and HDAC2 during consolidation and extinction of fear memory

Histone deacetylases (HDACs) regulate gene expression epigenetically through synchronized removal of acetyl groups from histones required towards memory consolidation. Moreover, dysregulated epigenetic machinery during fear or extinction learning may result in altered expression of some of these genes and result in Post Traumatic Stress Disorder (PTSD). In the present study, region-specific expression of Histone deacetylase 1 (HDAC1) and Histone deacetylase 2 (HDAC2) was correlated to the acetylation of histones H3 and H4 and the resultant conditioned response, in rats undergone fear and extinction learning. The neuronal activation, histone acetylation at H3/H4 and expression of HDAC1/HDAC2 in centrolateral amygdala (CeL) and centromedial amygdala (CeM) of central Amygdala (CeA) and prelimbic (PL) and infralimbic (IL) of Prefrontal cortex (PFC) were found to be associated in a differential manner following fear and extinction learning. Moreover in CeM, the main output of the fear circuitry, the level of HDAC1 was down-regulated following conditioning and up-regulated following extinction as opposed to which HDAC2 was down-regulated in CeM following conditioning but not following extinction. Furthermore, in CeL the HDAC1 was upregulated and HDAC2 was downregulated following conditioning and extinction. This has important implications in speculating of the role of HDACs in fear memory consolidation and its extinction.

HDAC3-Mediated Repression of the Nr4a Family Contributes to Age-Related Impairments in Long-Term Memory

Aging is accompanied by cognitive deficits, including impairments in long-term memory formation. Understanding the molecular mechanisms that support preserved cognitive function in aged animals is a critical step toward identifying novel therapeutic targets that could improve memory in aging individuals. One potential mechanism is the Nr4a family of genes, a group of CREB-dependent nuclear orphan receptors that have previously been shown to be important for hippocampal memory formation. Here, using a cross-species approach, we tested the role of Nr4a1 and Nr4a2 in age-related memory impairments. Using a rat model designed to identify individual differences in age-related memory impairments, we first identified Nr4a2 as a key gene that fails to be induced by learning in cognitively impaired male aged rats. Next, using a mouse model that allows for genetic manipulations, we determined that histone deacetylase 3 (HDAC3) negatively regulates Nr4a2 in the aged male and female hippocampus. Finally, we show that overexpression of Nr4a1, Nr4a2, or both transcripts in the male mouse dorsal hippocampus can ameliorate age-related impairments in object location memory. Together, our results suggest that Nr4a2 may be a key mechanism that promotes preserved cognitive function in old age, with HDAC3-mediated repression of Nr4a2 contributing to age-related cognitive decline. More broadly, these results indicate that therapeutic strategies to promote Nr4a gene expression or function may be an effective strategy to improve cognitive function in old age.SIGNIFICANCE STATEMENT Aging is accompanied by memory impairments, although there is a great deal of variability in the severity of these impairments. Identifying molecular mechanisms that promote preserved memory or participate in cognitive reserve in old age is important to develop strategies that promote healthy cognitive aging. Here, we show that learning-induced expression of the CREB-regulated nuclear receptor gene Nr4a2 is selectively impaired in aged rats with memory impairments. Further, we show that Nr4a2 is regulated by histone deacetylase HDAC3 in the aged mouse hippocampus. Finally, we demonstrate that hippocampal overexpression of either Nr4a2 or its family member, Nr4a1, can ameliorate age-related memory impairments. This suggests that promoting Nr4a expression may be a novel strategy to improve memory in aging individuals.

Epigenetic regulation of immediate-early gene Nr4a2/Nurr1 in the medial habenula during reinstatement of cocaine-associated behavior

Propensity to relapse following long periods of abstinence is a key feature of substance use disorder. Drugs of abuse, such as cocaine, cause long-term changes in the neural circuitry regulating reward, motivation, and memory processes through dysregulation of various molecular mechanisms, including epigenetic regulation of activity-dependent gene expression. Underlying drug-induced changes to neural circuit function are the molecular mechanisms regulating activity-dependent gene expression. Of note, histone acetyltransferases and histone deacetylases (HDACs), powerful epigenetic regulators of gene expression, are dysregulated following both acute and chronic cocaine exposure and are linked to cocaine-induced changes in neural circuit function. To better understand the effect of drug-induced changes on epigenetic function and behavior, we investigated HDAC3-mediated regulation of Nr4a2/Nurr1 in the medial habenula, an understudied pathway in cocaine-associated behaviors. Nr4a2, a transcription factor critical in cocaine-associated behaviors and necessary for MHb development, is enriched in the cholinergic cell-population of the MHb; yet, the role of NR4A2 within the MHb in the adult brain remains elusive. Here, we evaluated whether epigenetic regulation of Nr4a2 in the MHb has a role in reinstatement of cocaine-associated behaviors. We found that HDAC3 disengages from Nr4a2 in the MHb in response to cocaine-primed reinstatement. Whereas enhancing HDAC3 function in the MHb had no effect on reinstatement, we found, using a dominant-negative splice variant (NURR2C), that loss of NR4A2 function in the MHb blocked reinstatement behaviors. These results show for the first time that regulation of NR4A2 function in the MHb is critical in relapse-like behaviors.

Enhancement of BDNF Expression and Memory by HDAC Inhibition Requires BET Bromodomain Reader Proteins

Histone deacetylase (HDAC) inhibitors may have therapeutic utility in multiple neurological and psychiatric disorders, but the underlying mechanisms remain unclear. Here, we identify BRD4, a BET bromodomain reader of acetyl-lysine histones, as an essential component involved in potentiated expression of brain-derived neurotrophic factor (BDNF) and memory following HDAC inhibition. In in vitro studies, we reveal that pharmacological inhibition of BRD4 reversed the increase in BDNF mRNA induced by the class I/IIb HDAC inhibitor suberoylanilide hydroxamic acid (SAHA). Knock-down of HDAC2 and HDAC3, but not other HDACs, increased BDNF mRNA expression, whereas knock-down of BRD4 blocked these effects. Using dCas9-BRD4, locus-specific targeting of BRD4 to the BDNF promoter increased BDNF mRNA. In additional studies, RGFP966, a pharmacological inhibitor of HDAC3, elevated BDNF expression and BRD4 binding to the BDNF promoter, effects that were abrogated by JQ1 (an inhibitor of BRD4). Examining known epigenetic targets of BRD4 and HDAC3, we show that H4K5ac and H4K8ac modifications and H4K5ac enrichment at the BDNF promoter were elevated following RGFP966 treatment. In electrophysiological studies, JQ1 reversed RGFP966-induced enhancement of LTP in hippocampal slice preparations. Last, in behavioral studies, RGFP966 increased subthreshold novel object recognition memory and cocaine place preference in male C57BL/6 mice, effects that were reversed by cotreatment with JQ1. Together, these data reveal that BRD4 plays a key role in HDAC3 inhibitor-induced potentiation of BDNF expression, neuroplasticity, and memory.SIGNIFICANCE STATEMENT Some histone deacetylase (HDAC) inhibitors are known to have neuroprotective and cognition-enhancing properties, but the underlying mechanisms have yet to be fully elucidated. In the current study, we reveal that BRD4, an epigenetic reader of histone acetylation marks, is necessary for enhancing brain-derived neurotrophic factor (BDNF) expression and improved memory following HDAC inhibition. Therefore, by identifying novel epigenetic regulators of BDNF expression, these data may lead to new therapeutic targets for the treatment of neuropsychiatric disorders.

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.

Brain and Cognition for Addiction Medicine: From Prevention to Recovery Neural Substrates for Treatment of Psychostimulant-Induced Cognitive Deficits

Addiction to psychostimulants like cocaine, methamphetamine, and nicotine poses a continuing medical and social challenge both in the United States and all over the world. Despite a desire to quit drug use, return to drug use after a period of abstinence is a common problem among individuals dependent on psychostimulants. Recovery for psychostimulant drug-dependent individuals is particularly challenging because psychostimulant drugs induce significant changes in brain regions associated with cognitive functions leading to cognitive deficits. These cognitive deficits include impairments in learning/memory, poor decision making, and impaired control of behavioral output. Importantly, these drug-induced cognitive deficits often impact adherence to addiction treatment programs and predispose abstinent addicts to drug use relapse. Additionally, these cognitive deficits impact effective social and professional rehabilitation of abstinent addicts. The goal of this paper is to review neural substrates based on animal studies that could be pharmacologically targeted to reverse psychostimulant-induced cognitive deficits such as impulsivity and impairment in learning and memory. Further, the review will discuss neural substrates that could be used to facilitate extinction learning and thus reduce emotional and behavioral responses to drug-associated cues. Moreover, the review will discuss some non-pharmacological approaches that could be used either alone or in combination with pharmacological compounds to treat the above-mentioned cognitive deficits. Psychostimulant addiction treatment, which includes treatment for cognitive deficits, will help promote abstinence and allow for better rehabilitation and integration of abstinent individuals into society.

CREST in the Nucleus Accumbens Core Regulates Cocaine Conditioned Place Preference, Cocaine-Seeking Behavior, and Synaptic Plasticity

Epigenetic mechanisms result in persistent changes at the cellular level that can lead to long-lasting behavioral adaptations. Nucleosome remodeling is a major epigenetic mechanism that has not been well explored with regards to drug-seeking behaviors. Nucleosome remodeling is performed by multi-subunit complexes that interact with DNA or chromatin structure and possess an ATP-dependent enzyme to disrupt nucleosome-DNA contacts and ultimately regulate gene expression. Calcium responsive transactivator (CREST) is a transcriptional activator that interacts with enzymes involved in both histone acetylation and nucleosome remodeling. Here, we examined the effects of knocking down CREST in the nucleus accumbens (NAc) core on drug-seeking behavior and synaptic plasticity in male mice as well as drug-seeking in male rats. Knocking down CREST in the NAc core results in impaired cocaine-induced conditioned place preference (CPP) as well as theta-induced long-term potentiation in the NAc core. Further, similar to the CPP findings, using a self-administration procedure, we found that CREST knockdown in the NAc core of male rats had no effect on instrumental responding for cocaine itself on a first-order schedule, but did significantly attenuate responding on a second-order chain schedule, in which responding has a weaker association with cocaine. Together, these results suggest that CREST in the NAc core is required for cocaine-induced CPP, synaptic plasticity, as well as cocaine-seeking behavior.SIGNIFICANCE STATEMENT This study demonstrates a key role for the role of Calcium responsive transactivator (CREST), a transcriptional activator, in the nucleus accumbens (NAc) core with regard to cocaine-induced conditioned place preference (CPP), self-administration (SA), and synaptic plasticity. CREST is a unique transcriptional regulator that can recruit enzymes from two different major epigenetic mechanisms: histone acetylation and nucleosome remodeling. In this study we also found that the level of potentiation in the NAc core correlated with whether or not animals formed a CPP. Together the results indicate that CREST is a key downstream regulator of cocaine action in the NAc.

Epigenetic regulation of the circadian gene Per1 contributes to age-related changes in hippocampal memory

Aging is accompanied by impairments in both circadian rhythmicity and long-term memory. Although it is clear that memory performance is affected by circadian cycling, it is unknown whether age-related disruption of the circadian clock causes impaired hippocampal memory. Here, we show that the repressive histone deacetylase HDAC3 restricts long-term memory, synaptic plasticity, and experience-induced expression of the circadian gene Per1 in the aging hippocampus without affecting rhythmic circadian activity patterns. We also demonstrate that hippocampal Per1 is critical for long-term memory formation. Together, our data challenge the traditional idea that alterations in the core circadian clock drive circadian-related changes in memory formation and instead argue for a more autonomous role for circadian clock gene function in hippocampal cells to gate the likelihood of long-term memory formation.

Circadian rhythms are known to modulate memory, but it’s not known whether clock genes in the hippocampus are required for memory consolidation. Here, the authors show that epigenetic regulation of clock gene Period1 in the hippocampus regulates memory and contributes to age-related memory decline, independent of circadian rhythms.

The Process and Strategy for Developing Selective Histone Deacetylase 3 Inhibitors

Histone deacetylases (HDACs) are epigenetic drug targets that have gained major scientific attention. Inhibition of these important regulatory enzymes is used to treat cancer, and has the potential to treat a host of other diseases. However, currently marketed HDAC inhibitors lack selectivity for the various HDAC isoenzymes. Several studies have shown that HDAC3, in particular, plays an important role in inflammation and degenerative neurological diseases, but the development of selective HDAC3 inhibitors has been challenging. This review provides an up-to-date overview of selective HDAC3 inhibitors, and aims to support the development of novel HDAC3 inhibitors in the future.

The discovery of novel HDAC3 inhibitors via virtual screening and in vitro bioassay

Histone deacetylase 3 (HDAC3) is a potential target for the treatment of human diseases such as cancers, diabetes, chronic inflammation and neurodegenerative diseases. Previously, we proposed a virtual screening (VS) pipeline named “Hypo1_FRED_SAHA-3” for the discovery of HDAC3 inhibitors (HDAC3Is) and had thoroughly validated it by theoretical calculations. In this study, we attempted to explore its practical utility in a large-scale VS campaign. To this end, we used the VS pipeline to hierarchically screen the Specs chemical library. In order to facilitate compound cherry-picking, we then developed a knowledge-based pose filter (PF) by using our in-house quantitative structure activity relationship- (QSAR-) modelling approach and coupled it with FRED and Autodock Vina. Afterward, we purchased and tested 11 diverse compounds for their HDAC3 inhibitory activity in vitro. The bioassay has identified compound 2 (Specs ID: AN-979/41971160) as a HDAC3I (IC₅₀ = 6.1 μM), which proved the efficacy of our workflow. As a medicinal chemistry study, we performed a follow-up substructure search and identified two more hit compounds of the same chemical type, i.e. 2–1 (AQ-390/42122119, IC₅₀ = 1.3 μM) and 2–2 (AN-329/43450111, IC₅₀ = 12.5 μM). Based on the chemical structures and activities, we have demonstrated the essential role of the capping group in maintaining the activity for this class of HDAC3Is. In addition, we tested the hit compounds for their in vitro activities on other HDACs, including HDAC1, HDAC2, HDAC8, HDAC4 and HDAC6. We have identified these compounds are HDAC1/2/3 selective inhibitors, of which compound 2 show the best selectivity profile. Taken together, the present study is an experimental validation and an update to our earlier VS strategy. The identified hits could be used as starting structures for the development of highly potent and selective HDAC3Is.

Extinction of Contextual Cocaine Memories Requires Cav1.2 within D1R-Expressing Cells and Recruits Hippocampal Cav1.2-Dependent Signaling Mechanisms

Exposure to cocaine-associated contextual cues contributes significantly to relapse. Extinction of these contextual associations, which involves a new form of learning, reduces cocaine-seeking behavior; however, the molecular mechanisms underlying this process remain largely unknown. We report that extinction, but not acquisition, of cocaine conditioned place preference (CPP) in male mice increased Ca_v1.2 L-type Ca²⁺ channel mRNA and protein in postsynaptic density (PSD) fractions of the hippocampus, a brain region involved in drug-context associations. Moreover, viral-mediated deletion of Ca_v1.2 in the dorsal hippocampus attenuated extinction of cocaine CPP. Molecular studies examining downstream Ca_v1.2 targets revealed that extinction recruited calcium/calmodulin (Ca²⁺/CaMK)-dependent protein kinase II (CaMKII) to the hippocampal PSD. This occurred in parallel with an increase in phosphorylation of the AMPA GluA1 receptor subunit at serine 831 (S831), a CaMKII site, along with an increase in total PSD GluA1. The necessity of S831 GluA1 was further demonstrated by the lack of extinction in S831A GluA1 phosphomutant mice. Of note hippocampal GluA1 levels remained unaltered at the PSD, but were reduced near the PSD and at perisynaptic sites of dendritic spines in extinction-resistant S831A mutant mice. Finally, conditional knock-out of Ca_v1.2 in dopamine D1 receptor (D1R)-expressing cells resulted in attenuation of cocaine CPP extinction and lack of extinction-dependent changes in hippocampal PSD CaMKII expression and S831 GluA1 phosphorylation. In summary, we demonstrate an essential role for the hippocampal Ca_v1.2/CaMKII/S831 GluA1 pathway in cocaine CPP extinction, with data supporting contribution of hippocampal D1R-expressing cells in this process. These findings demonstrate a novel role for Ca_v1.2 channels in extinction of contextual cocaine-associated memories.SIGNIFICANCE STATEMENT Continued drug-seeking behavior, a defining characteristic of cocaine addiction, can be precipitated by contextual cues, yet the molecular mechanisms required for extinction of these context-specific memories remain poorly understood. Here, we have uncovered a novel and selective role of the Ca_v1.2 L-type Ca²⁺ channel and its downstream signaling pathway in the hippocampus that mediate extinction of cocaine conditioned place preference (CPP). We additionally provide evidence that supports a role of Ca_v1.2 within dopamine D1 receptor-expressing cells of the hippocampus for extinction of cocaine CPP. Therefore, these findings reveal a previously unknown role of Ca_v1.2 channels within the hippocampus and in D1 receptor-expressing cells in extinction of cocaine-associated memories, providing a framework for further exploration of mechanisms underlying extinction of cocaine-seeking behavior.