Histone deacetylase 5 limits cocaine reward through cAMP-induced nuclear import

Epigenetics of amphetamine-induced sensitization: HDAC5 expression and microRNA in neural remodeling

BACKGROUND

Histone deacetylase (HDAC) activities modify chromatin structure and play a role in learning and memory during developmental processes. Studies of adult mice suggest HDACs are involved in neural network remodeling in brain repair, but its function in drug addiction is less understood. We aimed to examine in vivo HDAC5 expression in a preclinical model of amphetamine-induced sensitization (AIS) of behavior. We generated specific contrast agents to measure HDAC5 levels by in vivo molecular contrast-enhanced (MCE) magnetic resonance imaging (MRI) in amphetamine-naïve mice as well as in mice with AIS. To validate the MRI results we used ex vivo methods including in situ hybridization, RT-PCR, immunohistochemistry, and transmision electron microscopy.

METHODS

We compared the expression of HDAC5 mRNA in an acute exposure paradigm (in which animals experienced a single drug exposure [A1]) and in a chronic-abstinence-challenge paradigm (in which animals were exposed to the drug once every other day for seven doses, then underwent 2 weeks of abstinence followed by a challenge dose [A7WA]). Control groups for each of these exposure paradigms were given saline. To delineate how HDAC5 expression was related to AIS, we compared the expression of HDAC5 mRNA at sequences where no known microRNA (miR) binds (hdac5AS2) and at sequences where miR-2861 is known to bind (miD2861). We synthesized and labeled phosphorothioated oligonucleic acids (sODN) of hdac5AS2 or miD2861 linked to superparamagentic iron oxide nanoparticles (SPION), and generated HDAC5-specific contrast agents (30 ± 20 nm, diameter) for MCE MRI; the same sequences were used for primers for TaqMan® analysis (RT-qPCR) in ex vivo validation. In addition, we used subtraction R2* maps to identify regional HDAC5 expression.

RESULTS

Naïve C57black6 mice that experience acute exposure to amphetamine (4 mg/kg, by injection intraperitoneally) show expression of both total and phosphorylated (S259) HDAC5 antigens in GFAP⁺ and GFAP^- cells, but the appearance of these cells was attenuated in the chronic paradigm. We found that MCE MRI reports HDAC5 mRNA with precision in physiological conditions because the HDAC5 mRNA copy number reported by TaqMan analysis was positively correlated (with a linear coefficient of 1.0) to the ΔR2* values (the frequency of signal reduction above background, 1/s) measured by MRI. We observed SPION-mid2861 as electron dense nanoparticles (EDNs) of less than 30 nm in the nucleus of the neurons, macrophages, and microglia, but not in glia and endothelia. We found no preferential distribution in any particular type of neural cells, but observed scattered EDNs of 60-150 nm (dia) in lysosomes. In the acute paradigm, mice pretreated with miD2861 (1.2 mmol/kg, i.p./icv) exhibited AIS similar to that exibited by mice in the chronic exposure group, which exhibited null response to mid2861 pretreatment. Moreover, SPION-miD2861 identified enhanced HDAC5 expression in the lateral septum and the striatum after amphetamine, where we found neurprogenitor cells coexpressing NeuN and GFAP.

CONCLUSIONS

We conclude that miD2681 targets HDAC5 mRNA with precision similar to that of RT-PCR. Our MCE MRI detects RNA-bound nanoparticles (NPs) in vivo, and ex vivo validation methods confirm that EDNs do not accumulate in any particular cell type. As HDAC5 expression may help nullify AIS and identify progenitor cells, the precise delivery of miD2861 may serve as a vehicle for monitoring network remodeling with target specificity and signal sensitivity after drug exposure that identifies brain repair processes in adult animals.

Dysregulation of Acetylation Enzymes Inanimal Models of Psychostimulant use Disorders: Evolving Stories

Substance use disorders are neuropsychiatric illnesses that have substantial negative biopsychosocial impact. These diseases are defined as compulsive abuse of licit or illicit substances despite adverse medicolegal consequences. Although much research has been conducted to elucidate the pathobiological bases of these disorders, much remains to be done to develop an overarching neurobiological understanding that might be translatable to beneficial pharmacological therapies. Recent advances in epigenetics promise to lead to such an elucidation. Here I provide a brief overview of observations obtained using some models of psychostimulant administration in rodents. The review identifies CREB binding protein (CBP), HDAC1, HDAC2, HADC3, HDAC4, and HDAC5 as important players in the acetylation and deacetylation processes that occur after contingent or non-contingent administration of psychostimulants. These observations are discussed within a framework that suggests a need for better animal models of addiction in order to bring these epigenetic advances to bear on the pharmacological treatment of human addicts.

SIKs control osteocyte responses to parathyroid hormone

Parathyroid hormone (PTH) activates receptors on osteocytes to orchestrate bone formation and resorption. Here we show that PTH inhibition of SOST (sclerostin), a WNT antagonist, requires HDAC4 and HDAC5, whereas PTH stimulation of RANKL, a stimulator of bone resorption, requires CRTC2. Salt inducible kinases (SIKs) control subcellular localization of HDAC4/5 and CRTC2. PTH regulates both HDAC4/5 and CRTC2 localization via phosphorylation and inhibition of SIK2. Like PTH, new small molecule SIK inhibitors cause decreased phosphorylation and increased nuclear translocation of HDAC4/5 and CRTC2. SIK inhibition mimics many of the effects of PTH in osteocytes as assessed by RNA-seq in cultured osteocytes and following in vivo administration. Once daily treatment with the small molecule SIK inhibitor YKL-05-099 increases bone formation and bone mass. Therefore, a major arm of PTH signalling in osteocytes involves SIK inhibition, and small molecule SIK inhibitors may be applied therapeutically to mimic skeletal effects of PTH.

Parathyroid hormone (PTH) is an endogenous hormone and osteoporosis therapeutic that suppresses sclerostin activity. Here the authors develop SIK inhibitors as potential therapeutic tools and use them to show that PTH-cAMP signalling in osteocytes inhibits SIK2 from driving Hdac4/5 nuclear shuttling to suppress sclerostin.

Metal-dependent Deacetylases: Cancer and Epigenetic Regulators

Epigenetic regulation is a key factor in cellular homeostasis. Post-translational modifications (PTMs) are a central focus of this regulation as they function as signaling markers within the cell. Lysine acetylation is a dynamic, reversible PTM that has garnered recent attention due to alterations in various types of cancer. Acetylation levels are regulated by two opposing enzyme families: lysine acetyltransferases (KATs) and histone deacetylases (HDACs). HDACs are key players in epigenetic regulation and have a role in the silencing of tumor suppressor genes. The dynamic equilibrium of acetylation makes HDACs attractive targets for drug therapy. However, substrate selectivity and biological function of HDAC isozymes is poorly understood. This review outlines the current understanding of the roles and specific epigenetic interactions of the metal-dependent HDACs in addition to their roles in cancer.

Epigenetics and addiction

Addictions are public health menaces. However, despite advances in addiction research, the cellular or molecular mechanisms that cause transition from recreational use to addiction remain to be elucidated. We have recently suggested that addiction may be secondary to long-term epigenetic modifications that determine the clinical course of substance use disorders. A better understanding of epigenetic mechanisms in animal models that mimic human conditions should help to usher in a new area of drug development against addiction.

Assessment of Cocaine-induced Behavioral Sensitization and Conditioned Place Preference in Mice

It is thought that rewarding experiences with drugs create strong contextual associations and encourage repeated intake. In turn, repeated exposures to drugs of abuse make lasting alterations in the brain function of vulnerable individuals, and these persistent alterations likely serve to maintain the maladaptive drug seeking and taking behaviors characteristic of addiction/dependence(2). In rodents, reward experience and contextual associations are frequently measured using the conditioned place preference assay, or CPP, wherein preference for a previously drug-paired context is measured. Behavioral sensitization, on the other hand, is an increase in a drug-induced behavior that develops progressively over repeated exposures. Since sensitized behaviors can often be measured after several months of drug abstinence, depending on the dose and length of initial exposure, they are considered observable correlates of lasting drug-induced plasticity. Researchers have found these assays useful in determining the neurobiological substrates mediating aspects of addiction as well as assessing the potential of different interventions in disrupting these behaviors. This manuscript describes basic, effective protocols for mouse CPP and locomotor behavioral sensitization to cocaine.

Regulation of class IIa HDAC activities: it is not only matter of subcellular localization

In response to environmental cues, enzymes that influence the functions of proteins, through reversible post-translational modifications supervise the coordination of cell behavior like orchestral conductors. Class IIa histone deacetylases (HDACs) belong to this category. Even though in vertebrates these deacetylases have discarded the core enzymatic activity, class IIa HDACs can assemble into multiprotein complexes devoted to transcriptional reprogramming, including but not limited to epigenetic changes. Class IIa HDACs are subjected to variegated and interconnected layers of regulation, which reflect the wide range of biological responses under the scrutiny of this gene family. Here, we discuss about the key mechanisms that fine tune class IIa HDACs activities.

Erythropoietin and carbamylated erythropoietin promote histone deacetylase 5 phosphorylation and nuclear export in rat hippocampal neurons

Erythropoietin (EPO) produces neurotrophic effects in animal model of neurodegeneration. However, clinical use of EPO is limited due to thrombotic risk. Carbamylated EPO (cEPO), devoid of thrombotic risk, has been proposed as a novel neuroprotective and neurotrophic agent although the molecular mechanisms of cEPO remain incomplete. Here, we show a previously unidentified role of histone deacetylase 5 (HDAC5) in the actions of EPO and cEPO. EPO and cEPO regulate the HDAC5 phosphorylation at two critical sites, Ser259 and Ser498 through a protein kinase D (PKD) dependent pathway. In addition, EPO and cEPO rapidly stimulates nuclear export of HDAC5 in rat hippocampal neurons which expressing HDAC5-GFP. Consequently, EPO and cEPO enhanced the myocyte enhancer factor-2 (MEF2) target gene expression. Taken together, our results reveal that EPO and cEPO mediate MEF2 target gene expression via the regulation of HDAC5 phosphorylation at Ser259/498, and suggest that HDAC5 could be a potential mechanism contributing to the therapeutic actions of EPO and cEPO.

Approaches for Studying the Subcellular Localization, Interactions, and Regulation of Histone Deacetylase 5 (HDAC5)

As a member of the class IIa family of histone deacetylases, the histone deacetylase 5 (HDAC5) is known to undergo nuclear-cytoplasmic shuttling and to be a critical transcriptional regulator. Its misregulation has been linked to prominent human diseases, including cardiac diseases and tumorigenesis. In this chapter, we describe several experimental methods that have proven effective for studying the functions and regulatory features of HDAC5. We present methods for assessing the subcellular localization, protein interactions, posttranslational modifications (PTMs), and activity of HDAC5 from the standpoint of investigating either the endogenous protein or tagged protein forms in human cells. Specifically, given that at the heart of HDAC5 regulation lie its dynamic localization, interactions, and PTMs, we present methods for assessing HDAC5 localization in fixed and live cells, for isolating HDAC5-containing protein complexes to identify its interactions and modifications, and for determining how these PTMs map to predicted HDAC5 structural motifs. Lastly, we provide examples of approaches for studying HDAC5 functions with a focus on its regulation during cell-cycle progression. These methods can readily be adapted for the study of other HDACs or non-HDAC-proteins of interest. Individually, these techniques capture temporal and spatial snapshots of HDAC5 functions; yet together, these approaches provide powerful tools for investigating both the regulation and regulatory roles of HDAC5 in different cell contexts relevant to health and disease.

Epigenetics of Stress, Addiction, and Resilience: Therapeutic Implications

Substance use disorders (SUDs) are highly prevalent. SUDs involve vicious cycles of binges followed by occasional periods of abstinence with recurrent relapses despite treatment and adverse medical and psychosocial consequences. There is convincing evidence that early and adult stressful life events are risks factors for the development of addiction and serve as cues that trigger relapses. Nevertheless, the fact that not all individuals who face traumatic events develop addiction to licit or illicit drugs suggests the existence of individual and/or familial resilient factors that protect these mentally healthy individuals. Here, I give a brief overview of the epigenetic bases of responses to stressful events and of epigenetic changes associated with the administration of drugs of abuse. I also discuss the psychobiology of resilience and alterations in epigenetic markers that have been observed in models of resilience. Finally, I suggest the possibility that treatment of addiction should involve cognitive and pharmacological approaches that enhance resilience in at risk individuals. Similar approaches should also be used with patients who have already succumbed to the nefarious effects of addictive substances.

Ketamine produces antidepressant-like effects through phosphorylation-dependent nuclear export of histone deacetylase 5 (HDAC5) in rats

Ketamine produces rapid antidepressant-like effects in animal assays for depression, although the molecular mechanisms underlying these behavioral actions remain incomplete. Here, we demonstrate that ketamine rapidly stimulates histone deacetylase 5 (HDAC5) phosphorylation and nuclear export in rat hippocampal neurons through calcium/calmodulin kinase II- and protein kinase D-dependent pathways. Consequently, ketamine enhanced the transcriptional activity of myocyte enhancer factor 2 (MEF2), which leads to regulation of MEF2 target genes. Transfection of a HDAC5 phosphorylation-defective mutant (Ser259/Ser498 replaced by Ala259/Ala498, HDAC5-S/A), resulted in resistance to ketamine-induced nuclear export, suppression of ketamine-mediated MEF2 transcriptional activity, and decreased expression of MEF2 target genes. Behaviorally, viral-mediated hippocampal knockdown of HDAC5 blocked or occluded the antidepressant effects of ketamine both in unstressed and stressed animals. Taken together, our results reveal a novel role of HDAC5 in the actions of ketamine and suggest that HDAC5 could be a potential mechanism contributing to the therapeutic actions of ketamine.

RGS9-2--controlled adaptations in the striatum determine the onset of action and efficacy of antidepressants in neuropathic pain states

The striatal protein Regulator of G-protein signaling 9-2 (RGS9-2) plays a key modulatory role in opioid, monoamine, and other G-protein-coupled receptor responses. Here, we use the murine spared-nerve injury model of neuropathic pain to investigate the mechanism by which RGS9-2 in the nucleus accumbens (NAc), a brain region involved in mood, reward, and motivation, modulates the actions of tricyclic antidepressants (TCAs). Prevention of RGS9-2 action in the NAc increases the efficacy of the TCA desipramine and dramatically accelerates its onset of action. By controlling the activation of effector molecules by G protein α and βγ subunits, RGS9-2 affects several protein interactions, phosphoprotein levels, and the function of the epigenetic modifier histone deacetylase 5, which are important for TCA responsiveness. Furthermore, information from RNA-sequencing analysis reveals that RGS9-2 in the NAc affects the expression of many genes known to be involved in nociception, analgesia, and antidepressant drug actions. Our findings provide novel information on NAc-specific cellular mechanisms that mediate the actions of TCAs in neuropathic pain states.

Incubation of methamphetamine craving is associated with selective increases in expression of Bdnf and trkb, glutamate receptors, and epigenetic enzymes in cue-activated fos-expressing dorsal striatal neurons

Cue-induced methamphetamine seeking progressively increases after withdrawal (incubation of methamphetamine craving), but the underlying mechanisms are largely unknown. We determined whether this incubation is associated with alterations in candidate genes in dorsal striatum (DS), a brain area implicated in cue- and context-induced drug relapse. We first measured mRNA expression of 24 candidate genes in whole DS extracts after short (2 d) or prolonged (1 month) withdrawal in rats following extended-access methamphetamine or saline (control condition) self-administration (9 h/d, 10 d). We found minimal changes. Next, using fluorescence-activated cell sorting, we compared gene expression in Fos-positive dorsal striatal neurons, which were activated during "incubated" cue-induced drug-seeking tests after prolonged withdrawal, with nonactivated Fos-negative neurons. We found significant increases in mRNA expression of immediate early genes (Arc, Egr1), Bdnf and its receptor (Trkb), glutamate receptor subunits (Gria1, Gria3, Grm1), and epigenetic enzymes (Hdac3, Hdac4, Hdac5, GLP, Dnmt3a, Kdm1a) in the Fos-positive neurons only. Using RNAscope to determine striatal subregion and cell-type specificity of the activated neurons, we measured colabeling of Fos with Drd1 and Drd2 in three DS subregions. Fos expression was neither subregion nor cell-type specific (52.5 and 39.2% of Fos expression colabeled with Drd1 and Drd2, respectively). Finally, we found that DS injections of SCH23390 (C17H18ClNO), a D1-family receptor antagonist known to block cue-induced Fos induction, decreased incubated cue-induced methamphetamine seeking after prolonged withdrawal. Results demonstrate a critical role of DS in incubation of methamphetamine craving and that this incubation is associated with selective gene-expression alterations in cue-activated D1- and D2-expressing DS neurons.

Vitamin D receptor and epigenetics in HIV infection and drug abuse

Illicit drug abuse is highly prevalent and serves as a powerful co-factor for HIV exacerbation. Epigenetic alterations in drug abuse and HIV infection determine expression of several critical genes such as vitamin D receptor (VDR), which participates in proliferation, differentiation, cell death under both physiological and pathological conditions. On that account, active vitamin D, the ligand of VDR, is used as an adjuvant therapy to control infection, slow down progression of chronic kidney diseases, and cancer chemotherapy. Interestingly, vitamin D may not be able to augment VDR expression optimally in several instances where epigenetic contributes to down regulation of VDR; however, reversal of epigenetic corruption either by demethylating agents (DACs) or histone deacetylase (HDAC) inhibitors would be able to maximize expression of VDR in these instances.

Cocaine decreases saccharin preference without altering sweet taste sensitivity

In rodents, saccharin consumption is suppressed when the sweet taste stimulus is paired with moderate doses of cocaine. Several hypotheses have been used to explain the seemingly contradictory effect of decreased consumption of a normally preferred substance following a highly rewarding drug. A common theme across these hypotheses is that saccharin is interpreted as less rewarding after cocaine pairing. We considered the alternative possibility that suppression is caused not by a change in reward circuitry, but rather by a change in taste detection, for instance by altering the afferent taste response and decreasing sensitivity to sweet taste stimuli. To evaluate this possibility, we measured saccharin taste sensitivity of mice before and after a standard cocaine-pairing paradigm. We measured taste sensitivity using a brief-access lickometer equipped with multiple concentrations of saccharin solution and established concentration-response curves before and after saccharin-cocaine pairing. Our results indicate that the EC50 for saccharin was unaltered following pairing. Instead, the avidity of licking saccharin, an indicator of motivation, was depressed. Latency to first-lick, a negative indicator of motivation, was also dramatically increased. Thus, our findings are consistent with the interpretation that saccharin-cocaine pairing results in devaluing of the sweet taste reward.

Class IIa HDACs - new insights into their functions in physiology and pathology

HDAC4, 5, 7 and 9 constitute the class IIa histone deacetylases (HDACs) within the large family of protein deacetylases. Class IIa HDACs have unique features that distinguish them from other HDACs. They contain an N-terminal domain that is required for their interaction with tissue-specific transcription factors and recruitment to their target genes. The N-terminal domain on class IIa HDACs also bears conserved serine residues that undergo signal-dependent phosphorylation, which brings about nuclear export of the enzymes and de-repression of their targets. One of the most important aspects of class IIa HDACs is their expression in specific tissues and organs within the organism, where they have crucial roles in development and differentiation processes. This review brings up to date our knowledge of the physiological and pathological functions of class IIa HDACs, focusing in particular on the most recent discoveries from in vivo studies of mouse model systems.

Retrieval of morphine-associated context induces cFos in dentate gyrus neurons

Addiction has been proposed to emerge from associations between the drug and the reward-associated contexts. This associative learning has a cellular correlate, as there are more cFos+ neurons in the hippocampal dentate gyrus (DG) after psychostimulant conditioned place preference (CPP) versus saline controls. However, it is unknown whether morphine CPP leads to a similar DG activation, or whether DG activation is due to locomotion, handling, pharmacological effects, or-as data from contextual fear learning suggests-exposure to the drug-associated context. To explore this, we employed an unbiased, counterbalanced, and shortened CPP design that led to place preference and more DG cFos+ cells. Next, mice underwent morphine CPP but were then sequestered into the morphine-paired (conditioned stimulus+ [CS+]) or saline-paired (CS-) context on test day. Morphine-paired mice sequestered to CS+ had ∼30% more DG cFos+ cells than saline-paired mice. Furthermore, Bregma analysis revealed morphine-paired mice had more cFos+ cells in CS+ compared to CS- controls. Notably, there was no significant difference in DG cFos+ cell number after handling alone or after receiving morphine in home cage. Thus, retrieval of morphine-associated context is accompanied by activation of hippocampal DG granule cell neurons.

Post-translational modifications regulate class IIa histone deacetylase (HDAC) function in health and disease

Class IIa histone deacetylases (HDACs4, -5, -7, and -9) modulate the physiology of the human cardiovascular, musculoskeletal, nervous, and immune systems. The regulatory capacity of this family of enzymes stems from their ability to shuttle between nuclear and cytoplasmic compartments in response to signal-driven post-translational modification. Here, we review the current knowledge of modifications that control spatial and temporal histone deacetylase functions by regulating subcellular localization, transcriptional functions, and cell cycle-dependent activity, ultimately impacting on human disease. We discuss the contribution of these modifications to cardiac and vascular hypertrophy, myoblast differentiation, neuronal cell survival, and neurodegenerative disorders.

Class II HDACs and neuronal regeneration

The vastly more superior regenerative capacity of the axons of peripheral nerves over central nervous system (CNS) neurons has been partly attributed to the former's intrinsic capacity to initiate and sustain the functionality of a new growth cone. Growth cone generation involves a myriad of processes that centers around the organization of microtubule bundles. Histone deacetylases (HDACs) modulate a wide range of key neuronal processes such as neural progenitor differentiation, learning and memory, neuronal death, and degeneration. HDAC inhibitors have been shown to be beneficial in attenuating neuronal death and promoting neurite outgrowth and axonal regeneration. Recent advances have provided insights on how manipulating HDAC activities, particularly the type II HDACs 5 and 6, which deacetylate tubulin, may benefit axonal regeneration. These advances are discussed herein.

Neuroepigenetic Regulation of Pathogenic Memories

Our unique collection of memories determines our individuality and shapes our future interactions with the world. Remarkable advances into the neurobiological basis of memory have identified key epigenetic mechanisms that support the stability of memory. Various forms of epigenetic regulation at the levels of DNA methylation, histone modification, and non-coding RNAs (ncRNAs) can modulate transcriptional and translational events required for memory processes. By changing the cellular profile in the brain's emotional, reward, and memory circuits, these epigenetic modifications have also been linked to perseverant, pathogenic memories. In this review, we will delve into the relevance of epigenetic dysregulation to pathogenic memory mechanisms by focusing on two neuropsychiatric disorders perpetuated by aberrant memory associations: substance use disorder (SUD) and post-traumatic stress disorder (PTSD). As our understanding improves, neuroepigenetic mechanisms may someday be harnessed to develop novel therapeutic targets for the treatment of these chronic, relapsing disorders.

Probing phosphorylation-dependent protein interactions within functional domains of histone deacetylase 5 (HDAC5)

Class IIa histone deacetylases (HDACs) are critical transcriptional regulators, shuttling between nuclear and cytoplasmic cellular compartments. Within the nucleus, these HDACs repress transcription as components of multiprotein complexes, such as the nuclear corepressor and beclin-6 corepressor (BCoR) complexes. Cytoplasmic relocalization relieves this transcriptional repressive function. Class IIa HDAC shuttling is controlled, in part, by phosphorylations flanking the nuclear localization signal (NLS). Furthermore, we have reported that phosphorylation within the NLS by the kinase Aurora B modulates the localization and function of the class IIa HDAC5 during mitosis. While we identified numerous additional HDAC5 phosphorylations, their regulatory functions remain unknown. Here, we studied phosphorylation sites within functional HDAC5 domains, including the deacetylation domain (DAC, Ser755), nuclear export signal (NES, Ser1108), and an acidic domain (AD, Ser611). We have generated phosphomutant cell lines to investigate how absence of phosphorylation at these sites impacts HDAC5 localization, enzymatic activity, and protein interactions. Combining molecular biology and quantitative MS, we have defined the interactions and HDAC5-containing complexes mediated by site-specific phosphorylation and quantified selected changes using parallel reaction monitoring. These results expand the current understanding of HDAC regulation, and the functions of this critical family of proteins within human cells.

Epigenetic signaling in psychiatric disorders

Psychiatric disorders are complex multifactorial illnesses involving chronic alterations in neural circuit structure and function. While genetic factors are important in the etiology of disorders such as depression and addiction, relatively high rates of discordance among identical twins clearly indicate the importance of additional mechanisms. Environmental factors such as stress or prior drug exposure are known to play a role in the onset of these illnesses. Such exposure to environmental insults induces stable changes in gene expression, neural circuit function, and ultimately behavior, and these maladaptations appear distinct between developmental and adult exposures. Increasing evidence indicates that these sustained abnormalities are maintained by epigenetic modifications in specific brain regions. Indeed, transcriptional dysregulation and associated aberrant epigenetic regulation is a unifying theme in psychiatric disorders. Aspects of depression and addiction can be modeled in animals by inducing disease-like states through environmental manipulations (e.g., chronic stress, drug administration). Understanding how environmental factors recruit the epigenetic machinery in animal models reveals new insight into disease mechanisms in humans.

Epigenetic mechanisms of drug addiction

Drug addiction involves potentially life-long behavioral abnormalities that are caused in vulnerable individuals by repeated exposure to a drug of abuse. The persistence of these behavioral changes suggests that long-lasting changes in gene expression, within particular regions of the brain, may contribute importantly to the addiction phenotype. Work over the past decade has demonstrated a crucial role for epigenetic mechanisms in driving lasting changes in gene expression in diverse tissues, including brain. This has prompted recent research aimed at characterizing the influence of epigenetic regulatory events in mediating the lasting effects of drugs of abuse on the brain in animal models of drug addiction. This review provides a progress report of this still early work in the field. As will be seen, there is robust evidence that repeated exposure to drugs of abuse induces changes within the brain's reward regions in three major modes of epigenetic regulation-histone modifications such as acetylation and methylation, DNA methylation, and non-coding RNAs. In several instances, it has been possible to demonstrate directly the contribution of such epigenetic changes to addiction-related behavioral abnormalities. Studies of epigenetic mechanisms of addiction are also providing an unprecedented view of the range of genes and non-genic regions that are affected by repeated drug exposure and the precise molecular basis of that regulation. Work is now needed to validate key aspects of this work in human addiction and evaluate the possibility of mining this information to develop new diagnostic tests and more effective treatments for addiction syndromes. This article is part of a Special Issue entitled 'NIDA 40th Anniversary Issue'.

Short and long access to cocaine self-administration activates tyrosine phosphatase STEP and attenuates GluN expression but differentially regulates GluA expression in the prefrontal cortex

RATIONALE

Dephosphorylation of extracellular signal-regulated kinase (ERK) and cyclic AMP response element binding protein (CREB) in the dorsomedial prefrontal cortex (dmPFC) at the end of short access (ShA) cocaine self-administration is implicated in cocaine seeking. However, what receptors and phosphatases mediate this effect and whether ERK/CREB and related phospho-proteins in the dmPFC react similarly during early withdrawal from long access (LgA) cocaine self-administration are unknown.

OBJECTIVES

The effects of ShA vs. LgA cocaine self-administration on the phosphorylation of protein phosphatase 2A (PP2A) and striatal-enriched protein tyrosine phosphatase (STEP), as well as GluN and GluA receptor subtype expression in the dmPFC during early withdrawal, were compared.

METHODS

Rats self-administered cocaine or received saline during 2- or 6-h daily sessions for 10-11 days. Two hours after the final session, the dmPFC was dissected out and processed for immunoblotting.

RESULTS

Similar to previous findings after ShA cocaine, phospho-ERK and phospho-CREB in the dmPFC were decreased after LgA cocaine. Cocaine elevated phospho-PP2A (deactivation) and decreased phospho-STEP (activation) in both ShA and LgA cocaine rats. GluN1, GluN2B, and phospho-GluN2B Tyr1472 in the dmPFC were decreased after ShA and LgA cocaine. Further, a significant reduction of GluA2, GluA1, and phospho-GluA1 Ser845 was found only in LgA rats.

CONCLUSIONS

Activation of phospho-STEP may underlie ERK and CREB dephosphorylation in the dmPFC as well as internalization and degradation of GluN complexes during early withdrawal from both ShA and LgA cocaine self-administration, whereas differential alteration of AMPA receptor subunits after ShA and LgA cocaine self-administration depends on cocaine intake.

Unique functional roles for class I and class II histone deacetylases in central nervous system development and function

Non-specific pharmacological inhibition of the histone deacetylase (HDAC) family of enzymes has largely beneficial effects in a variety of diverse contexts including cancer, cognitive function, and neurodegeneration. This review will discuss the role of individual HDAC isoforms in brain function during development and in the adult. Importantly class I and class II HDACs exhibit distinct cellular and subcellular expression patterns and utilize different signaling pathways to influence their substrates. Moreover, dissociable phenotypic outcomes emerge following manipulation of individual HDACs in the brain. To date, pharmacological inhibitors capable of targeting individual HDACs have proven difficult to develop, an obstacle that must be overcome to unlock the substantial clinical promise of manipulating endogenous HDAC isoforms in the central nervous system.

Possible contributions of a novel form of synaptic plasticity in Aplysia to reward, memory, and their dysfunctions in mammalian brain

Recent studies in Aplysia have identified a new variation of synaptic plasticity in which modulatory transmitters enhance spontaneous release of glutamate, which then acts on postsynaptic receptors to recruit mechanisms of intermediate- and long-term plasticity. In this review I suggest the hypothesis that similar plasticity occurs in mammals, where it may contribute to reward, memory, and their dysfunctions in several psychiatric disorders. In Aplysia, spontaneous release is enhanced by activation of presynaptic serotonin receptors, but presynaptic D1 dopamine receptors or nicotinic acetylcholine receptors could play a similar role in mammals. Those receptors enhance spontaneous release of glutamate in hippocampus, entorhinal cortex, prefrontal cortex, ventral tegmental area, and nucleus accumbens. In all of those brain areas, glutamate can activate postsynaptic receptors to elevate Ca²⁺ and engage mechanisms of early-phase long-term potentiation (LTP), including AMPA receptor insertion, and of late-phase LTP, including protein synthesis and growth. Thus, presynaptic receptors and spontaneous release may contribute to postsynaptic mechanisms of plasticity in brain regions involved in reward and memory, and could play roles in disorders that affect plasticity in those regions, including addiction, Alzheimer’s disease, schizophrenia, and attention deficit hyperactivity disorder (ADHD).

A Tet-on system for DRD1-expressing cells

Cells expressing the dopamine D1 receptor (DRD1) have significant functional roles in diverse physiological processes including locomotion and drug addiction. The present work presents a novel in vivo DRD1-Bacterial Artificial Chromosome (BAC) Tet-on system allowing for the inducible activation of tet-operated transgenes specifically within DRD1-expressing cells of transgenic mice. It is shown that the DRD1-rtTA BAC-driven expression of a tet-operated reporter is under tight regulation by doxycycline and is restricted to DRD1-expressing brain regions. The model will be a useful research tool in studies of movement and reward and associated pathologies such as Parkinson’s disease and addiction.

Behavioral effects of cocaine mediated by nitric oxide-GAPDH transcriptional signaling

Cocaine's behavioral-stimulant effects derive from potentiation of synaptic signaling by dopamine and serotonin leading to transcriptional alterations in postsynaptic cells. We report that a signaling cascade involving nitric oxide (NO) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) mediates cocaine's transcriptional and behavioral actions. Lower, behavioral-stimulant doses enhance the cAMP response element-binding (CREB) signaling system, while higher, neurotoxic doses stimulate the p53 cytotoxic system. The drug CGP3466B, which potently and selectively blocks GAPDH nitrosylation and GAPDH-Siah binding, prevents these actions as well as behavioral effects of cocaine providing a strategy for anticocaine therapy.

Epigenetic mechanisms of drug addiction

Epigenetic regulation can mediate long-lasting changes in gene expression, which makes it an attractive mechanism for the stable behavioral abnormalities that characterize drug addiction. Recent research has unveiled numerous types of epigenetic modifications within the brain's reward circuitry in animal models of drug addiction. In this review, we summarize the latest advances in the field, focusing on histone modifications, DNA methylation, and noncoding RNAs. We also highlight several areas for future research. Unraveling the highly complex epigenetic mechanisms of addiction is adding to our understanding of this syndrome and has the potential to trigger novel approaches for better diagnosis and therapy.

HDAC3 is a negative regulator of cocaine-context-associated memory formation

Cocaine-induced neuroplasticity mediated by histone acetylating and deacetylating enzymes may contribute to addiction-like behaviors. For example, overexpression of histone deacetylases (HDACs) 4 or 5 in the nucleus accumbens suppresses cocaine-induced conditioned place preference (CPP) acquisition in mice. HDAC4 and HDAC5 are known to interact with HDAC3, but the role of HDAC3 in cocaine-induced behaviors has never been examined. In this study, we address the hypothesis that HDAC3 is a negative regulator of cocaine-context-associated memory formation in mice. We examined the role of HDAC3 during the conditioning phase of CPP, when the mouse has the opportunity to form an associative memory between the cocaine-paired context and the subjective effects of cocaine. To address this hypothesis, Hdac3(flox/flox) and Hdac3(+/+) mice (generated from a C57BL/6 background) were infused into the nucleus accumbens with adeno-associated virus expressing Cre recombinase to create focal, homozygous Hdac3 deletions. Hdac3(flox/flox) mice exhibit significantly enhanced CPP acquisition, which is correlated with increased gene expression during the consolidation phase of acquisition. Increased gene expression of c-Fos and Nr4a2 is correlated with decreased HDAC3 occupancy and increased histone H4 lysine 8 acetylation at their promoters. The results from this study demonstrate that HDAC3 negatively regulates cocaine-induced CPP acquisition.

Dephosphorylation at a conserved SP motif governs cAMP sensitivity and nuclear localization of class IIa histone deacetylases

Histone deacetylase 4 (HDAC4) and its paralogs, HDAC5, -7, and -9 (all members of class IIa), possess multiple phosphorylation sites crucial for 14-3-3 binding and subsequent nuclear export. cAMP signaling stimulates nuclear import of HDAC4 and HDAC5, but the underlying mechanisms remain to be elucidated. Here we show that cAMP potentiates nuclear localization of HDAC9. Mutation of an SP motif conserved in HDAC4, -5, and -9 prevents cAMP-stimulated nuclear localization. Unexpectedly, this treatment inhibits phosphorylation at the SP motif, indicating an inverse relationship between the phosphorylation event and nuclear import. Consistent with this, leptomycin B-induced nuclear import and adrenocorticotropic hormone (ACTH) treatment result in the dephosphorylation at the motif. Moreover, the modification synergizes with phosphorylation at a nearby site, and similar kinetics was observed for both phosphorylation events during myoblast and adipocyte differentiation. These results thus unravel a previously unrecognized mechanism whereby cAMP promotes dephosphorylation and differentially regulates multisite phosphorylation and the nuclear localization of class IIa HDACs.

The role of histone acetylation in cocaine-induced neural plasticity and behavior

How do drugs of abuse, such as cocaine, cause stable changes in neural plasticity that in turn drive long-term changes in behavior? What kind of mechanism can underlie such stable changes in neural plasticity? One prime candidate mechanism is epigenetic mechanisms of chromatin regulation. Chromatin regulation has been shown to generate short-term and long-term molecular memory within an individual cell. They have also been shown to underlie cell fate decisions (or cellular memory). Now, there is accumulating evidence that in the CNS, these same mechanisms may be pivotal for drug-induced changes in gene expression and ultimately long-term behavioral changes. As these mechanisms are also being found to be fundamental for learning and memory, an exciting new possibility is the extinction of drug-seeking behavior by manipulation of epigenetic mechanisms. In this review, we critically discuss the evidence demonstrating a key role for chromatin regulation via histone acetylation in cocaine action.

Activation of tachykinin, neurokinin 3 receptors affects chromatin structure and gene expression by means of histone acetylation

The tachykinin, neurokinin 3 receptor (NK3R) is a g-protein coupled receptor that is broadly distributed in the nervous system and exerts its diverse physiological actions through multiple signaling pathways. Despite the role of the receptor system in a range of biological functions, the effects of NK3R activation on chromatin dynamics and gene expression have received limited attention. The present work determined the effects of senktide, a selective NK3R agonist, on chromatin organization, acetylation, and gene expression, using qRT-PCR, in a hypothalamic cell line (CLU 209) that expresses the NK3R. Senktide (1 nM, 10nM) caused a relaxation of chromatin, an increase in global acetylation of histone H3 and H4, and an increase in the expression of a common set of genes involved in cell signaling, cell growth, and synaptic plasticity. Pretreatment with histone acetyltransferase (HAT) inhibitor (garcinol and 2-methylene y-butylactone), that inhibits p300, p300/CREB binding protein (CBP) associated factor (PCAF), and GCN 5, prevented the senktide-induced increase in expression of most, but not all, of the genes upregulated in response to 1 nM and 10nM senktide. Treatment with 100 nM had the opposite effect: a reduction in chromatin relaxation and decreased acetylation. The expression of four genes was significantly decreased and the HAT inhibitor had a limited effect in blocking the upregulation of genes in response to 100 nM senktide. Activation of the NK3R appears to recruit multiple pathways, including acetylation, and possibly histone deactylases, histone methylases, or DNA methylases to affect chromatin structure and gene expression.

Acute β-adrenergic activation triggers nuclear import of histone deacetylase 5 and delays G(q)-induced transcriptional activation

During hemodynamic stress, catecholamines and neurohumoral stimuli may induce co-activation of G(q)-coupled receptors and β-adrenergic receptors (β-AR), leading to cardiac remodeling. Dynamic regulation of histone deacetylase 5 (HDAC5), a transcriptional repressor, is crucial during stress signaling due to its role in epigenetic control of fetal gene markers. Little is known about its regulation during acute and chronic β-AR stimulation and its cross-interaction with G(q) signaling in adult cardiac myocytes. Here, we evaluate the potential cross-talk between G(q)-driven and β-AR mediated signaling at the level of nucleocytoplasmic shuttling of HDAC5. We show the translocation of GFP-tagged wild type HDAC5 or mutants (S279A and S279D) in response to β-AR or G(q) agonists. Isoproterenol (ISO) or PKA activation results in strong nuclear accumulation of HDAC5 in contrast to nuclear export driven by Ca(2+)-calmodulin protein kinase II and protein kinase D. Moreover, nuclear accumulation of HDAC5 under acute ISO/PKA signaling is dependent on phosphorylation of Ser-279 and can block subsequent G(q)-mediated nuclear HDAC5 export. Intriguingly, the attenuation of G(q)-induced export is abolished after chronic PKA activation, yet nuclear HDAC5 remains elevated. Last, the effect of chronic β-AR signaling on HDAC5 translocation was examined in adult myocytes from a rabbit model of heart failure, where ISO-induced nuclear import is ablated, but G(q)-agonist mediated export is preserved. Acute β-AR/PKA activation protects against hypertrophic signaling by delaying G(q)-mediated transcriptional activation. This serves as a key physiological control switch before allowing genetic reprogramming via HDAC5 nuclear export during more severe stress, such as heart failure.

SMRT-mediated co-shuttling enables export of class IIa HDACs independent of their CaM kinase phosphorylation sites

The Class IIa histone deacetylases (HDAC)4 and HDAC5 play a role in neuronal survival and behavioral adaptation in the CNS. Phosphorylation at 2/3 N-terminal sites promote their nuclear export. We investigated whether non-canonical signaling routes to Class IIa HDAC export exist because of their association with the co-repressor Silencing Mediator Of Retinoic And Thyroid Hormone Receptors (SMRT). We found that, while HDAC5 and HDAC4 mutants lacking their N-terminal phosphorylation sites (HDAC4^MUT, HDAC5^MUT) are constitutively nuclear, co-expression with SMRT renders them exportable by signals that trigger SMRT export, such as synaptic activity, HDAC inhibition, and Brain Derived Neurotrophic Factor (BDNF) signaling. We found that SMRT's repression domain 3 (RD3) is critical for co-shuttling of HDAC5^MUT, consistent with the role for this domain in Class IIa HDAC association. In the context of BDNF signaling, we found that HDAC5^WT, which was more cytoplasmic than HDAC5^MUT, accumulated in the nucleus after BDNF treatment. However, co-expression of SMRT blocked BDNF-induced HDAC5^WT import in a RD3-dependent manner. In effect, SMRT-mediated HDAC5^WT export was opposing the BDNF-induced HDAC5 nuclear accumulation observed in SMRT's absence. Thus, SMRT's presence may render Class IIa HDACs exportable by a wider range of signals than those which simply promote direct phosphorylation.

Neuroplasticity in addiction: cellular and transcriptional perspectives

Drug addiction is a chronic, relapsing brain disorder which consists of compulsive patterns of drug-seeking and taking that occurs at the expense of other activities. The transition from casual to compulsive drug use and the enduring propensity to relapse is thought to be underpinned by long-lasting neuroadaptations in specific brain circuitry, analogous to those that underlie long-term memory formation. Research spanning the last two decades has made great progress in identifying cellular and molecular mechanisms that contribute to drug-induced changes in plasticity and behavior. Alterations in synaptic transmission within the mesocorticolimbic and corticostriatal pathways, and changes in the transcriptional potential of cells by epigenetic mechanisms are two important means by which drugs of abuse can induce lasting changes in behavior. In this review we provide a summary of more recent research that has furthered our understanding of drug-induced neuroplastic changes both at the level of the synapse, and on a transcriptional level, and how these changes may relate to the human disease of addiction.

Aurora B-dependent regulation of class IIa histone deacetylases by mitotic nuclear localization signal phosphorylation

Class IIa histone deacetylases (HDACs 4/5/7/9) are transcriptional regulators with critical roles in cardiac disease and cancer. HDAC inhibitors are promising anticancer agents, and although they are known to disrupt mitotic progression, the underlying mechanisms of mitotic regulation by HDACs are not fully understood. Here we provide the first identification of histone deacetylases as substrates of Aurora B kinase (AurB). Our study identifies class IIa HDACs as a novel family of AurB targets and provides the first evidence that HDACs are temporally and spatially regulated by phosphorylation during the cell cycle. We define the precise site of AurB-mediated phosphorylation as a conserved serine within the nuclear localization signals of HDAC4, HDAC5, and HDAC9 at Ser265, Ser278, and Ser242, respectively. We establish that AurB interacts with these HDACs in vivo, and that this association increases upon disruption of 14-3-3 binding. We observe colocalization of endogenous, phosphorylated HDACs with AurB at the mitotic midzone in late anaphase and the midbody during cytokinesis, complemented by a reduction in HDAC interactions with components of the nuclear corepressor complex. We propose that AurB-dependent phosphorylation of HDACs induces sequestration within a phosphorylation gradient at the midzone, maintaining separation from re-forming nuclei and contributing to transcriptional control.

Post-insult valproic acid-regulated microRNAs: potential targets for cerebral ischemia

Stroke is a devastating brain injury that is a leading cause of adult disability with limited treatment options. Using a rat model of middle cerebral artery occlusion (MCAO) to induce cerebral ischemia, we profiled microRNAs (miRNAs), small non-protein coding RNAs, in the ischemic cortex. Many miRNAs were confirmed by qPCR to be robustly upregulated 24 hours following MCAO surgery including miR-155, miR-297a, miR-466f, miR-466h, and miR-1224. In addition, we treated MCAO rats with valproic acid (VPA), a mood stabilizer and histone deacetylase inhibitor. This post-insult treatment was shown to improve neurological deficits and motor performance following MCAO. To provide mechanistic insight into the potential targets and pathways that may underlie these benefits, we profiled miRNAs regulated following this VPA treatment. Two promising post-insult VPA-regulated candidates were miR-331 and miR-885-3p. miR-331 was also regulated by VPA pre-treatment in rat cortical neuronal cultures subjected to oxygen-glucose deprivation, an in vitro ischemic model. The predicted targets of these miRNAs analyzed by Ingenuity Pathway Analysis (IPA) identified networks involved in hematological system development, cell death, and nervous system development. These predicted networks were further filtered using IPA and showed significant associations with neurological diseases including movement disorders, neurodegenerative disorders, damage to cerebral cortex, and seizure disorders among others. Collectively, these data support common disease mechanisms that may be under miRNA control and provide exciting directions for further investigations aimed at elucidating the miRNA mechanisms and targets that may yield new therapies for neurological disorders.

Regulated shuttling of the histone deacetylase HDAC5 to the nucleus may put a brake on cocaine addiction

The histone deacetylase HDAC5 has been shown to regulate behavioral adaptations to cocaine. In this issue of Neuron, Taniguchi et al. (2012) describe a cAMP-dependent signaling pathway that regulates nuclear accumulation of HDAC5, suggesting a mechanism to couple cocaine with changes in HDAC5 function.

Molecular brake pad hypothesis: pulling off the brakes for emotional memory

Under basal conditions histone deacetylases(HDACs) and their associated co-repressor complexes serve as molecular 'brake pads' to prevent the gene expression required for long-term memory formation. Following a learning event, HDACs and their co-repressor complexes are removed from a subset of specific gene promoters, allowing the histone acetylation and active gene expression required for long-term memory formation.Inhibition of HDACs increases histone acetylation,extends gene expression profiles, and allows for the formation of persistent long-term memories for training events that are otherwise forgotten. We propose that emotionally salient experiences have utilized this system to form strong and persistent memories for behaviorally significant events. Consequently, the presence or absence of HDACs at a selection of specific gene promoters could serve as a critical barrier for permitting the formation of long-term memories.