Ian Maze

Histone Deacetylase 5 Epigenetically Controls Behavioral Adaptations to Chronic Emotional Stimuli

Previous work has identified alterations in histone acetylation in animal models of drug addiction and depression. However, the mechanisms which integrate drugs and stress with changes in chromatin structure remain unclear. Here, we identify the activity-dependent class II histone deacetylase, HDAC5, as a central integrator of these stimuli with changes in chromatin structure and gene expression. Chronic, but not acute, exposure to cocaine or stress decreases HDAC5 function in the nucleus accumbens (NAc), a major brain reward region, which allows for increased histone acetylation and transcription of HDAC5 target genes. This regulation is behaviorally important, as loss of HDAC5 causes hypersensitive responses to chronic, not acute, cocaine or stress. These findings suggest that proper balance of histone acetylation is a crucial factor in the saliency of a given stimulus and that disruption of this balance is involved in the transition from an acute adaptive response to a chronic psychiatric illness.

Essential Role of the Histone Methyltransferase G9a in Cocaine-induced Plasticity

Cocaine Addiction and Histone Methylation Long-lasting behavioral syndromes associated with chronic cocaine exposure may result from dysregulation of the global transcriptional machinery. Maze et al. (p. 213) observed that histone lysine methylation in the nucleus accumbens plays a critical role in mediating the regulation of gene expression in response to repeated cocaine self-administration. Chronic cocaine was linked to overall reductions in dimethylation of lysine 9 of histone 3 (H3K9) in this brain region. Repressing H3K9 after chronic cocaine administration facilitated reward-related changes in behavior. The authors identifed the methyltransferase G9a as an essential mediator and an important regulator of dendritic spine plasticity. Downregulation of G9a was linked to the transcription factor ΔFosB. Cocaine suppression of histone methylation in the nucleus accumbens mediates the drug’s ability to enhance reward. Cocaine-induced alterations in gene expression cause changes in neuronal morphology and behavior that may underlie cocaine addiction. In mice, we identified an essential role for histone 3 lysine 9 (H3K9) dimethylation and the lysine dimethyltransferase G9a in cocaine-induced structural and behavioral plasticity. Repeated cocaine administration reduced global levels of H3K9 dimethylation in the nucleus accumbens. This reduction in histone methylation was mediated through the repression of G9a in this brain region, which was regulated by the cocaine-induced transcription factor ∆FosB. Using conditional mutagenesis and viral-mediated gene transfer, we found that G9a down-regulation increased the dendritic spine plasticity of nucleus accumbens neurons and enhanced the preference for cocaine, thereby establishing a crucial role for histone methylation in the long-term actions of cocaine.

Antidepressant Actions of Histone Deacetylase Inhibitors

Persistent symptoms of depression suggest the involvement of stable molecular adaptations in brain, which may be reflected at the level of chromatin remodeling. We find that chronic social defeat stress in mice causes a transient decrease, followed by a persistent increase, in levels of acetylated histone H3 in the nucleus accumbens, an important limbic brain region. This persistent increase in H3 acetylation is associated with decreased levels of histone deacetylase 2 (HDAC2) in the nucleus accumbens. Similar effects were observed in the nucleus accumbens of depressed humans studied postmortem. These changes in H3 acetylation and HDAC2 expression mediate long-lasting positive neuronal adaptations, since infusion of HDAC inhibitors into the nucleus accumbens, which increases histone acetylation, exerts robust antidepressant-like effects in the social defeat paradigm and other behavioral assays. HDAC inhibitor [N-(2-aminophenyl)-4-[N-(pyridine-3-ylmethoxy-carbonyl)aminomethyl]benzamide (MS-275)] infusion also reverses the effects of chronic defeat stress on global patterns of gene expression in the nucleus accumbens, as determined by microarray analysis, with striking similarities to the effects of the standard antidepressant fluoxetine. Stress-regulated genes whose expression is normalized selectively by MS-275 may provide promising targets for the future development of novel antidepressant treatments. Together, these findings provide new insight into the underlying molecular mechanisms of depression and antidepressant action, and support the antidepressant potential of HDAC inhibitors and perhaps other agents that act at the level of chromatin structure.

Dnmt3a regulates emotional behavior and spine plasticity in the nucleus accumbens

Despite abundant expression of DNA methyltransferases (Dnmts) in brain, the regulation and behavioral role of DNA methylation remain poorly understood. We found that Dnmt3a expression was regulated in mouse nucleus accumbens (NAc) by chronic cocaine use and chronic social defeat stress. Moreover, NAc-specific manipulations that block DNA methylation potentiated cocaine reward and exerted antidepressant-like effects, whereas NAc-specific Dnmt3a overexpression attenuated cocaine reward and was pro-depressant. On a cellular level, we found that chronic cocaine use selectively increased thin dendritic spines on NAc neurons and that DNA methylation was both necessary and sufficient to mediate these effects. These data establish the importance of Dnmt3a in the NAc in regulating cellular and behavioral plasticity to emotional stimuli.

Genome-wide Analysis of Chromatin Regulation by Cocaine Reveals a Role for Sirtuins

Changes in gene expression contribute to the long-lasting regulation of the brains reward circuitry seen in drug addiction; however, the specific genes regulated and the transcriptional mechanisms underlying such regulation remain poorly understood. Here, we used chromatin immunoprecipitation coupled with promoter microarray analysis to characterize genome-wide chromatin changes in the mouse nucleus accumbens, a crucial brain reward region, after repeated cocaine administration. Our findings reveal several interesting principles of gene regulation by cocaine and of the role of DeltaFosB and CREB, two prominent cocaine-induced transcription factors, in this brain region. The findings also provide comprehensive insight into the molecular pathways regulated by cocaine-including a new role for sirtuins (Sirt1 and Sirt2)-which are induced in the nucleus accumbens by cocaine and, in turn, dramatically enhance the behavioral effects of the drug.

Nuclear factor kB signaling regulates neuronal morphology and cocaine reward

Although chronic cocaine-induced changes in dendritic spines on nucleus accumbens (NAc) neurons have been correlated with behavioral sensitization, the molecular pathways governing these structural changes, and their resulting behavioral effects, are poorly understood. The transcription factor, nuclear factor κ B (NFκB), is rapidly activated by diverse stimuli and regulates expression of many genes known to maintain cell structure. Therefore, we evaluated the role of NFκB in regulating cocaine-induced dendritic spine changes on medium spiny neurons of the NAc and the rewarding effects of cocaine. We show that chronic cocaine induces NFκB-dependent transcription in the NAc of NFκB-Lac transgenic mice. This induction of NFκB activity is accompanied by increased expression of several NFκB genes, the promoters of which show chromatin modifications after chronic cocaine exposure consistent with their transcriptional activation. To study the functional significance of this induction, we used viral-mediated gene transfer to express either a constitutively active or dominant-negative mutant of Inhibitor of κ B kinase (IKKca or IKKdn), which normally activates NFκB signaling, in the NAc. We found that activation of NFκB by IKKca increases the number of dendritic spines on NAc neurons, whereas inhibition of NFκB by IKKdn decreases basal dendritic spine number and blocks the increase in dendritic spines after chronic cocaine. Moreover, inhibition of NFκB blocks the rewarding effects of cocaine and the ability of previous cocaine exposure to increase an animals preference for cocaine. Together, these studies establish a direct role for NFκB pathways in the NAc to regulate structural and behavioral plasticity to cocaine.

Every amino acid matters: essential contributions of histone variants to mammalian development and disease

Despite a conserved role for histones as general DNA packaging agents, it is now clear that another key function of these proteins is to confer variations in chromatin structure to ensure dynamic patterns of transcriptional regulation in eukaryotes. The incorporation of histone variants is particularly important to this process. Recent knockdown and knockout studies in various cellular systems, as well as direct mutational evidence from human cancers, now suggest a crucial role for histone variant regulation in processes as diverse as differentiation and proliferation, meiosis and nuclear reprogramming. In this Review, we provide an overview of histone variants in the context of their unique functions during mammalian germ cell and embryonic development, and examine the consequences of aberrant histone variant regulation in human disease.

ΔFosB Mediates Epigenetic Desensitization of the c-fos Gene After Chronic Amphetamine Exposure

The molecular mechanisms underlying the transition from recreational drug use to chronic addiction remain poorly understood. One molecule implicated in this process is ΔFosB, a transcription factor that accumulates in striatum after repeated drug exposure and mediates sensitized behavioral responses to psychostimulants and other drugs of abuse. The downstream transcriptional mechanisms by which ΔFosB regulates drug-induced behaviors are incompletely understood. We reported previously the chromatin remodeling mechanisms by which ΔFosB activates the expression of certain genes; however, the mechanisms underlying ΔFosB-mediated gene repression remain unknown. Here, we identify c-fos, an immediate early gene rapidly induced in striatum after acute psychostimulant exposure, as a novel downstream target that is repressed chronically by ΔFosB. We show that accumulation of ΔFosB in striatum after chronic amphetamine treatment desensitizes c-fos mRNA induction to a subsequent drug dose. ΔFosB desensitizes c-fos expression by recruiting histone deacetylase 1 (HDAC1) to the c-fos gene promoter, which, in turn, deacetylates surrounding histones and attenuates gene activity. Accordingly, local knock-out of HDAC1 in striatum abolishes amphetamine-induced desensitization of the c-fos gene. In concert, chronic amphetamine increases histone H3 methylation on the c-fos promoter, a chromatin modification also known to repress gene activity, as well as expression levels of the H3 histone methyltransferase, KMT1A (lysine methyltransferase 1A, formerly SUV39H1). This study reveals a novel epigenetic pathway through which ΔFosB mediates distinct transcriptional programs that may ultimately alter behavioral plasticity to chronic amphetamine exposure.

The epigenetic landscape of addiction

Drug‐induced alterations in gene expression throughout the reward circuitry of the brain are likely components of the persistence of the drug‐addicted state. Recent studies examining the molecular mechanisms controlling drug‐induced transcriptional, behavioral, and synaptic plasticity have indicated a direct role for chromatin remodeling in the regulation and stability of drug‐mediated neuronal gene programs, and the subsequent promulgation of addictive behaviors. In this review, we discuss recent advances in our understanding of chromatin phenomena—or epigenetics, by one definition—that contribute to drug addiction, with the hope that such mechanistic insights may aid in the development of novel therapeutics for future treatments of addiction.

Distinct Patterns of ΔFosB Induction in Brain by Drugs of Abuse

The transcription factor ΔFosB accumulates and persists in brain in response to chronic stimulation. This accumulation after chronic exposure to drugs of abuse has been demonstrated previously by Western blot most dramatically in striatal regions, including dorsal striatum (caudate/putamen) and nucleus accumbens. In the present study, we used immunohistochemistry to define with greater anatomical precision the induction of ΔFosB throughout the rodent brain after chronic drug treatment. We also extended previous research involving cocaine, morphine, and nicotine to two additional drugs of abuse, ethanol and Δ9‐tetrahydrocannabinol (Δ9‐THC, the active ingredient in marijuana). We show here that chronic, but not acute, administration of each of four drugs of abuse, cocaine, morphine, ethanol, and Δ9‐THC, robustly induces ΔFosB in nucleus accumbens, although different patterns in the core vs. shell subregions of this nucleus were apparent for the different drugs. The drugs also differed in their degree of ΔFosB induction in dorsal striatum. In addition, all four drugs induced ΔFosB in prefrontal cortex, with the greatest effects observed with cocaine and ethanol, and all of the drugs induced ΔFosB to a small extent in amygdala. Furthermore, all drugs induced ΔFosB in the hippocampus, and, with the exception of ethanol, most of this induction was seen in the dentate. Lower levels of ΔFosB induction were seen in other brain areas in response to a particular drug treatment. These findings provide further evidence that induction of ΔFosB in nucleus accumbens is a common action of virtually all drugs of abuse and that, beyond nucleus accumbens, each drug induces ΔFosB in a region‐specific manner in brain. Synapse 358–369, 2008.