The histone methyltransferase G9a mediates stress-regulated alcohol drinking

The interplay between gut microbiome, epigenetics, and substance use disorders: from molecular to clinical perspectives

Substance use disorders (SUDs) involve a complex series of central and peripheral pathologies, leading to impairments in cognitive, behavioral, and physiological processes. Emerging evidence indicates a more significant role for the microbiome-gut-brain axis (MGBA) in SUDs than previously recognized. The MGBA is interconnected with various body systems by producing numerous metabolites, most importantly short-chain fatty acids (SCFAs), cytokines, and neurotransmitters. These mediators influence the human body's epigenome and transcriptome. While numerous epigenetic alterations in different brain regions have been reported in SUD models, the intricate relationship between SUDs and the MGBA suggests that the gut microbiome may partially contribute to the underlying mechanisms of SUDs. Promising results have been observed with gut microbiome-directed interventions in patients with SUDs, including prebiotics, probiotics, antibiotics, and fecal microbiota transplantation. Nonetheless, the long-term epigenetic effects of these interventions remain unexplored. Moreover, various confounding factors and study limitations have hindered the identification of molecular mechanisms and clinical applications of gut microbiome interventions in SUDs. In the present review, we will (i) provide a comprehensive discussion on how the gut microbiome influences SUDs, with an emphasis on epigenetic alterations; (ii) discuss the current evidence on the bidirectional relationship of gut microbiome and SUDs, highlighting potential targets for intervention; and (iii) review recent advances in gut microbiome-directed therapies, along with their limitations and future directions.

Heroin Regulates the Voltage-Gated Sodium Channel Auxiliary Subunit, SCN1b, to Modulate Nucleus Accumbens Medium Spiny Neuron Intrinsic Excitability and Cue-Induced Heroin Seeking

Self-administration of addictive substances like heroin can couple the rewarding/euphoric effects of the drug with drug-associated cues, and opioid cue reactivity contributes to relapse vulnerability in abstinent individuals recovering from an opioid use disorder (OUD). Opioids are reported to alter the intrinsic excitability of medium spiny neurons (MSNs) in the nucleus accumbens (NAc), a key brain reward region linked to drug seeking, but how opioids alter NAc MSN neuronal excitability and the impact of altered MSN excitability on relapse-like opioid seeking remain unclear. Here, we discovered that self-administered, but not experimenter-administered, heroin reduced NAc protein levels of the voltage-gated sodium channel auxiliary subunit, SCN1b, in male and female rats. Viral-mediated reduction of NAc SCN1b increased the intrinsic excitability of MSNs, but without altering glutamatergic and GABAergic synaptic transmission. While reducing NAc SCN1b levels had no effect on acquisition of heroin self-administration or extinction learning, we observed a significant increase in cue-reinstated heroin seeking, suggesting that NAc SCN1b normally limits cue-reinstated heroin seeking. We also observed that NAc SCN1b protein levels returned to baseline following heroin self-administration, home-cage abstinence, and extinction training, suggesting that the noted reduction of NAc SCN1b during acquisition of heroin self-administration likely enhances MSN excitability and the strength of heroin-cue associations formed during active heroin use. As such, enhancing NAc SCN1b function might mitigate opioid cue reactivity and a return to active drug use in individuals suffering from OUD.

The Histone Lysine Demethylase KDM7A Contributes to Reward Memory via Fscn1-Induced Synaptic Plasticity in the Medial Prefrontal Cortex

Lysine demethylase 7A (KDM7A) catalyzes the removal of dimethylation from histone H3 lysine 9 and lysine 27, both of which are associated with transcription repression. Previous study indicates that Kdm7a mRNA in the medial prefrontal cortex (mPFC) increases after drug exposure, yet its role in drug-related behaviors is largely unknown. In a morphine-conditioned place preference (CPP) paradigm, these findings reveal a specific increase of Kdm7a expression in the mPFC 7 days after drug withdrawal. Subsequently, these results demonstrate that knockdown of Kdm7a in the mPFC do not affect the acquisition of morphine-induced CPP, but it attenuate memory consolidation. To further explore Kdm7a-mediated transcriptomic changes, this work employs Nanopore direct RNA sequencing. Transcriptome profiling unveils several gene expression alterations impacted by KDM7A, which are enriched in relevant neural function categories. Notably, this work identifies and validates fascin actin-bundling protein 1 (Fscn1) as a downstream molecular target. Knockdown of Fscn1 has a similar impact on CPP to Kdm7a, along with corresponding decrease of dendritic spine density and neuronal activity in the mPFC. Additionally, silencing Kdm7a decreases enrichment of H3K9me2 and H3K27me2 at the Fscn1 promoter region, suggesting that KDM7A may act as a crucial regulator of transcriptional responses to morphine-related reward memory via Fscn1.

Adaptations in glutathione-based redox protein signaling pathways and alcohol drinking across species

Alcohol use disorder (AUD) is the most prevalent substance use disorder but there is incomplete knowledge of the underlying molecular etiology. Here, we examined the cytosolic proteome from the nucleus accumbens core (NAcC) of ethanol drinking rhesus macaques to identify ethanol-sensitive signaling proteins. The targets were subsequently investigated using bioinformatics, genetic, and pharmacological manipulations in mouse models of ethanol drinking. Of the 1000+ cytosolic proteins identified in our screen, 50 proteins differed significantly between control and ethanol drinking macaques. Gene Ontology analysis of the differentially expressed proteins identified enrichment in pathways regulating metabolic processes and proteasome activity. Because the family of Glutathione S-transferases (GSTs) was enriched in these pathways, validation studies targeted GSTs using bioinformatics and genetically diverse mouse models. Gstp1 and Gstm2 were identified in Quantitative Trait Loci and published gene sets for ethanol-related phenotypes (e.g., ethanol preference, conditioned taste aversion, differential expression), and recombinant inbred strains that inherited the C57BL/6J allele at the Gstp2 interval consumed higher amounts of ethanol than those that inherited the DBA/2J allele. Genetic deletion of Gstp1/2 led to increased ethanol consumption without altering ethanol metabolism or sucrose preference. Administration of the pharmacologic activator of Gstp1/2, carnosic acid, decreased voluntary ethanol drinking. Proteomic analysis of the NAcC cytosolic of heavy drinking macaques that were validated in mouse models indicate a role for glutathione-mediated redox regulation in ethanol-related neurobiology and the potential of pharmacological interventions targeting this system to modify excessive ethanol drinking.

Olfactory dysfunction decreased local field potential in the reward system and increased EtOH consumption in mice

The relationship between olfactory dysfunction and alcohol intake is unobvious. Chronic alcohol intake results in reduced olfactory acuity and olfactory discrimination and addiction in humans. However, alcohol is a beverage with distinctive odors, which usually works as a cue to induce addictive memories and craving behavior. Whether olfactory impairment increase or decrease alcohol consumption remains an important but unclear issue. In this study, we measured ethanol (EtOH) consumption in the two-bottle choice EtOH drinking test, two bottle choice EtOH/sucrose drinking test and the drinking in the dark (DID) test during the olfactory loss. We also recorded local field potentials (LFPs) from the brain reward system, the ventral tegmental area (VTA), nucleus accumbens (NAc), and piriform cortex (Pir) one and four weeks after the induction of olfactory epithelium lesions using zinc sulfate (ZnSO4) in mice. The results showed that the EtOH consumption and preference were increased during the period of olfactory dysfunction. 1 week after the olfactory injury, LFP powers in the reward system at low- and high-gamma bands decreased significantly, coherence between the Pir and the reward system was also decrease. 4 weeks after the ZnSO4 treatment, LFP powers were reversed, but the coherence between VTA and NAc was decreased, indicating lasting effects post-recovery. This study demonstrates that olfactory dysfunction increased EtOH consumption in mice, which was accompanied by decreased LFP power and coherence in the reward system, which suggest that olfactory deficits changed activities in the reward system and could alter reward-seeking behaviors, which provide insights into the neurobiology of alcohol addiction.

Cocaine taking and craving produce distinct transcriptional profiles in dopamine neurons

Dopamine (DA) signaling plays an essential role in reward valence attribution and in encoding the reinforcing properties of natural and artificial rewards. The adaptive responses from midbrain dopamine neurons to artificial rewards such as drugs of abuse are therefore important for understanding the development of substance use disorders. Drug-induced changes in gene expression are one such adaptation that can determine the activity of dopamine signaling in projection regions of the brain reward system. One of the major challenges to obtaining this understanding involves the complex cellular makeup of the brain, where each neuron population can be defined by a distinct transcriptional profile. To bridge this gap, we have adapted a virus-based method for labeling and capture of dopamine nuclei, coupled with nuclear RNA-sequencing, to study the transcriptional adaptations, specifically, of dopamine neurons in the ventral tegmental area (VTA) during cocaine taking and cocaine craving, using a mouse model of cocaine intravenous self-administration (IVSA). Our results show significant changes in gene expression across non-drug operant training, cocaine taking, and cocaine craving, highlighted by an enrichment of repressive epigenetic modifying enzyme gene expression during cocaine craving. Immunohistochemical validation further revealed an increase of H3K9me3 deposition in DA neurons during cocaine craving. These results demonstrate that cocaine-induced transcriptional adaptations in dopamine neurons vary by phase of self-administration and underscore the utility of this approach for identifying relevant phase-specific molecular targets to study the behavioral course of substance use disorders.

Histone Methyltransferase G9a Plays an Essential Role on Nicotine Preference in Zebrafish

Psychostimulants regulate behavioral responses in zebrafish via epigenetic mechanisms. We have previously shown that DNA methylation and histone deacetylase (HDAC) inhibition abolish nicotine-induced conditioned place preference (CPP) but little is known about the role of histone methylation in addictive-like behaviors. To assess the influence of histone methylation on nicotine-CPP, zebrafish were treated with a histone (H3) lysine-9 (K9) dimethyltransferase G9a/GLP inhibitor, BIX-01294 (BIX), which was administered before conditioning sessions. We observed a dual effect of the inhibitor BIX: at high doses inhibited while at low doses potentiated nicotine reward. Transcriptional expression of α6 and α7 subunits of the nicotinic acetylcholine receptor and of G9a, DNA methyl transferase-3, and HDAC-1 was upregulated in zebrafish with positive scores for nicotine-CPP. Changes in relative levels of these mRNA molecules reflected the effects of BIX on nicotine reward. BIX treatment per sé did not affect transcriptional levels of epigenetic enzymes that regulate trimethylation or demethylation of H3. BIX reduced H3K9me2 protein levels in a dose-dependent manner in key structures of the reward pathway. Thus, our findings indicated that different doses of BIX differentially affect nicotine CPP via strong or weak inhibition of G9a/GLP activity. Additionally, we found that the lysine demethylase inhibitor daminozide abolished nicotine-CPP and drug seeking. Our data demonstrate that H3 methylation catalyzed by G9a/GLP is involved in nicotine-CPP induction. Dimethylation of K9 at H3 is an important epigenetic modification that should be considered as a potential therapeutic target to treat nicotine reward and perhaps other drug addictions.

Chronic alcohol-induced long-lasting working memory deficits are associated with altered histone H3K9 dimethylation in the prefrontal cortex

Introduction

Epigenetic modifications have emerged as key contributors to the enduring behavioral, molecular and epigenetic neuroadaptations during withdrawal from chronic alcohol exposure. The present study investigated the long-term consequences of chronic alcohol exposure on spatial working memory (WM) and associated changes of transcriptionally repressive histone H3 lysine 9 dimethylation (H3K9me2) in the prefrontal cortex (PFC).

Methods

Male C57BL/6 mice were allowed free access to either 12% (v/v) ethanol for 5 months followed by a 3-week abstinence period or water. Spatial WM was assessed through the spontaneous alternation T-maze test. Alcoholic and water mice received daily injections of GABAB agonist baclofen or saline during alcohol fading and early withdrawal. Global levels of histone modifications were determined by immunohistochemistry.

Results

Withdrawal mice displayed WM impairments along with reduced prefrontal H3K9me2 levels, compared to water-drinking mice. The withdrawal-induced decrease of H3K9me2 occurred concomitantly with increased level of permissive H3K9 acetylation (H3K9ac) in the PFC. Baclofen treatment rescued withdrawal-related WM deficits and fully restored prefrontal H3K9me2 and H3K9ac. Alcohol withdrawal induced brain region-specific changes of H3K9me2 and H3K9ac after testing, with significant decreases of both histone marks in the dorsal hippocampus and no changes in the amygdala and dorsal striatum. Furthermore, the magnitude of H3K9me2 in the PFC, but not the hippocampus, significantly and positively correlated with individual WM performances. No correlation was observed between H3K9ac and behavioral performance. Results also indicate that pre-testing intraperitoneal injection of UNC0642, a selective inhibitor of histone methyltransferase G9a responsible for H3K9me2, led to WM impairments in water-drinking and withdrawal-baclofen mice. Collectively, our results demonstrate that alcohol withdrawal induced brain-region specific alterations of H3K9me2 and H3K9ac, an effect that persisted for at least three weeks after cessation of chronic alcohol intake.

Conclusion

The findings suggest a role for long-lasting decreased H3K9me2 specifically in the PFC in the persistent WM impairments related to alcohol withdrawal.

Epigenetic Effects of Addictive Drugs in the Nucleus Accumbens

Substance use induces long-lasting behavioral changes and drug craving. Increasing evidence suggests that epigenetic gene regulation contributes to the development and expression of these long-lasting behavioral alterations. Here we systematically review extensive evidence from rodent models of drug-induced changes in epigenetic regulation and epigenetic regulator proteins. We focus on histone acetylation and histone methylation in a brain region important for drug-related behaviors: the nucleus accumbens. We also discuss how experimentally altering these epigenetic regulators via systemically administered compounds or nucleus accumbens-specific manipulations demonstrate the importance of these proteins in the behavioral effects of drugs and suggest potential therapeutic value to treat people with substance use disorder. Finally, we discuss limitations and future directions for the field of epigenetic studies in the behavioral effects of addictive drugs and suggest how to use these insights to develop efficacious treatments.