Sex-biased effects on hippocampal circuit development by perinatal SERT expression in CA3 pyramidal neurons.

Neurodevelopmental disorders ranging from autism to intellectual disability display sex-biased prevalence and phenotypical presentations. Despite increasing knowledge about temporospatial cortical map development and genetic variants linked to neurodevelopmental disorders, when and how sex-biased neural circuit derailment may arise in diseased brain remain unknown. Here, we identify in mice that serotonin uptake transporter (SERT) in non-serotonergic neurons - hippocampal and prefrontal pyramidal neurons - confers sex-biased effects specifically during neural circuit development. A set of gradient-patterned CA3 pyramidal neurons transiently express SERT to clear extracellular serotonin, coinciding with hippocampal synaptic circuit establishment. Ablating pyramidal neuron SERT (SERTPyramidΔ) alters dendritic spine developmental trajectory in the hippocampus, and precipitates sex-biased impairments in long-term activity-dependent hippocampal synaptic plasticity and cognitive behaviors. Transcriptomic analyses identify sex-biased alterations in gene sets associated with autism, dendritic spine structure, synaptic function and male-specific enrichment of dysregulated genes in glial cells in early postnatal SERTPyramidΔ hippocampus. Our data suggest that SERT function in these pyramidal neurons underscores a temporal- and brain region-specific regulation of normal sex-dimorphic circuit development and a source for sex-biased vulnerability to cognitive and behavioral impairments. This article has an associated 'The people behind the papers' interview.

Histone serotonylation is a permissive modification that enhances TFIID binding to H3K4me3.

Chemical modifications of histones can mediate diverse DNA-templated processes, including gene transcription1-3. Here we provide evidence for a class of histone post-translational modification, serotonylation of glutamine, which occurs at position 5 (Q5ser) on histone H3 in organisms that produce serotonin (also known as 5-hydroxytryptamine (5-HT)). We demonstrate that tissue transglutaminase 2 can serotonylate histone H3 tri-methylated lysine 4 (H3K4me3)-marked nucleosomes, resulting in the presence of combinatorial H3K4me3Q5ser in vivo. H3K4me3Q5ser displays a ubiquitous pattern of tissue expression in mammals, with enrichment observed in brain and gut, two organ systems responsible for the bulk of 5-HT production. Genome-wide analyses of human serotonergic neurons, developing mouse brain and cultured serotonergic cells indicate that H3K4me3Q5ser nucleosomes are enriched in euchromatin, are sensitive to cellular differentiation and correlate with permissive gene expression, phenomena that are linked to the potentiation of TFIID4-6 interactions with H3K4me3. Cells that ectopically express a H3 mutant that cannot be serotonylated display significantly altered expression of H3K4me3Q5ser-target loci, which leads to deficits in differentiation. Taken together, these data identify a direct role for 5-HT, independent from its contributions to neurotransmission and cellular signalling, in the mediation of permissive gene expression.

TGM2-mediated histone transglutamination is dictated by steric accessibility.

Recent studies have identified serotonylation of glutamine-5 on histone H3 (H3Q5ser) as a novel posttranslational modification (PTM) associated with active transcription. While H3Q5ser is known to be installed by tissue transglutaminase 2 (TGM2), the substrate characteristics affecting deposition of the mark, at the level of both chromatin and individual nucleosomes, remain poorly understood. Here, we show that histone serotonylation is excluded from constitutive heterochromatic regions in mammalian cells. Biochemical studies reveal that the formation of higher-order chromatin structures associated with heterochromatin impose a steric barrier that is refractory to TGM2-mediated histone monoaminylation. A series of structure-activity relationship studies, including the use of DNA-barcoded nucleosome libraries, shows that steric hindrance also steers TGM2 activity at the nucleosome level, restricting monoaminylation to accessible sites within histone tails. Collectively, our data indicate that the activity of TGM2 on chromatin is dictated by substrate accessibility rather than by primary sequence determinants or by the existence of preexisting PTMs, as is the case for many other histone-modifying enzymes.

Nothing Is Yet Set in (Hi)stone: Novel Post-Translational Modifications Regulating Chromatin Function.

Histone post-translational modifications (PTMs) have emerged as exciting mechanisms of biological regulation, impacting pathways related to cancer, immunity, brain function, and more. Over the past decade alone, several histone PTMs have been discovered, including acylation, lipidation, monoaminylation, and glycation, many of which appear to have crucial roles in nucleosome stability and transcriptional regulation. In this review, we discuss novel histone PTMs identified within the past 10 years, with an extended focus on enzymatic versus nonenzymatic mechanisms underlying modification and adduction. Furthermore, we consider how these novel histone PTMs might fit within the framework of a so-called 'histone code', emphasizing the physiological relevance of these PTMs in metabolism, development, and disease states.

Bioorthogonal Labeling and Enrichment of Histone Monoaminylation Reveal Its Accumulation and Regulatory Function in Cancer Cell Chromatin.

Histone monoaminylation (i.e., serotonylation and dopaminylation) is an emerging category of epigenetic mark occurring on the fifth glutamine (Q5) residue of H3 N-terminal tail, which plays significant roles in gene transcription. Current analysis of histone monoaminylation is mainly based on site-specific antibodies and mass spectrometry, which either lacks high resolution or is time-consuming. In this study, we report the development of chemical probes for bioorthogonal labeling and enrichment of histone serotonylation and dopaminylation. These probes were successfully applied for the monoaminylation analysis of in vitro biochemical assays, cells, and tissue samples. The enrichment of monoaminylated histones by the probes further confirmed the crosstalk between H3Q5 monoaminylation and H3K4 methylation. Finally, combining the ex vivo and in vitro analyses based on the developed probes, we have shown that both histone serotonylation and dopaminylation are highly enriched in tumor tissues that overexpress transglutaminase 2 (TGM2) and regulate the three-dimensional architecture of cellular chromatin.

Histone H3 dopaminylation in nucleus accumbens, but not medial prefrontal cortex, contributes to cocaine-seeking following prolonged abstinence.

Enduring patterns of epigenomic and transcriptional plasticity within the mesolimbic dopamine system contribute importantly to persistent behavioral adaptations that characterize substance use disorders (SUD). While drug addiction has long been thought of as a disorder of dopamine (DA) neurotransmission, therapeutic interventions targeting receptor mediated DA-signaling have not yet resulted in efficacious treatments. Our laboratory recently identified a non-canonical, neurotransmission-independent signaling moiety for DA in brain, termed dopaminylation, whereby DA itself acts as a donor source for the establishment of post-translational modifications (PTM) on substrate proteins (e.g., histone H3 at glutamine 5; H3Q5dop). In our previous studies, we demonstrated that H3Q5dop plays a critical role in the regulation of neuronal transcription and, when perturbed within monoaminergic neurons of the ventral tegmental area (VTA), critically contributes to pathological states, including relapse vulnerability to both psychostimulants (e.g., cocaine) and opiates (e.g., heroin). Importantly, H3Q5dop is also observed throughout the mesolimbic DA reward pathway (e.g., in nucleus accumbens/NAc and medial prefrontal cortex/mPFC, which receive DA input from VTA). As such, we investigated whether H3Q5dop may similarly be altered in its expression in response to drugs of abuse in these non-dopamine-producing regions. In rats undergoing extended abstinence from cocaine self-administration (SA), we observed both acute and prolonged accumulation of H3Q5dop in NAc, but not mPFC. Attenuation of H3Q5dop in NAc during drug abstinence reduced cocaine-seeking and affected cocaine-induced gene expression programs associated with altered dopamine signaling and neuronal function. These findings thus establish H3Q5dop in NAc, but not mPFC, as an important mediator of cocaine-induced behavioral and transcriptional plasticity during extended cocaine abstinence.

Histone H3Q5 serotonylation stabilizes H3K4 methylation and potentiates its readout.

Serotonylation of glutamine 5 on histone H3 (H3Q5ser) was recently identified as a permissive posttranslational modification that coexists with adjacent lysine 4 trimethylation (H3K4me3). While the resulting dual modification, H3K4me3Q5ser, is enriched at regions of active gene expression in serotonergic neurons, the molecular outcome underlying H3K4me3-H3Q5ser crosstalk remains largely unexplored. Herein, we examine the impact of H3Q5ser on the readers, writers, and erasers of H3K4me3. All tested H3K4me3 readers retain binding to the H3K4me3Q5ser dual modification. Of note, the PHD finger of TAF3 favors H3K4me3Q5ser, and this binding preference is dependent on the Q5ser modification regardless of H3K4 methylation states. While the activity of the H3K4 methyltransferase, MLL1, is unaffected by H3Q5ser, the corresponding H3K4me3/2 erasers, KDM5B/C and LSD1, are profoundly inhibited by the presence of the mark. Collectively, this work suggests that adjacent H3Q5ser potentiates H3K4me3 function by either stabilizing H3K4me3 from dynamic turnover or enhancing its physical readout by downstream effectors, thereby potentially providing a mechanism for fine-tuning critical gene expression programs.

Cell-type-specific role for nucleus accumbens neuroligin-2 in depression and stress susceptibility.

Behavioral coping strategies are critical for active resilience to stress and depression; here we describe a role for neuroligin-2 (NLGN-2) in the nucleus accumbens (NAc). Neuroligins (NLGN) are a family of neuronal postsynaptic cell adhesion proteins that are constituents of the excitatory and inhibitory synapse. Importantly, NLGN-3 and NLGN-4 mutations are strongly implicated as candidates underlying the development of neuropsychiatric disorders with social disturbances such as autism, but the role of NLGN-2 in neuropsychiatric disease states is unclear. Here we show a reduction in NLGN-2 gene expression in the NAc of patients with major depressive disorder. Chronic social defeat stress in mice also decreases NLGN-2 selectively in dopamine D1-positive cells, but not dopamine D2-positive cells, within the NAc of stress-susceptible mice. Functional NLGN-2 knockdown produces bidirectional, cell-type-specific effects: knockdown in dopamine D1-positive cells promotes subordination and stress susceptibility, whereas knockdown in dopamine D2-positive cells mediates active defensive behavior. These findings establish a behavioral role for NAc NLGN-2 in stress and depression; provide a basis for targeted, cell-type specific therapy; and highlight the role of active behavioral coping mechanisms in stress susceptibility.

Epigenetic Regulation of Hippocampal Fosb Expression Controls Behavioral Responses to Cocaine.

Drug addiction results in part from maladaptive learning, including the formation of strong associations between the drug and the circumstances of consumption. However, drug-induced changes in gene expression underlying the saliency of these associations remain understudied. Consolidation of explicit memories occurs within the hippocampus, and we have shown that spatial learning induces expression of the transcription factor ΔFosB in hippocampus and that this induction is critical for learning. Drugs of abuse also upregulate ΔFosB in hippocampus, but the mechanism of its induction by cocaine and its role in hippocampus-dependent cocaine responses is unknown. We investigated differences in mouse dorsal and ventral hippocampal ΔFosB expression in response to chronic cocaine, because these regions appear to regulate distinct cocaine-related behaviors. We found that cocaine-mediated induction of ΔFosB was subregion-specific, and that ΔFosB transcriptional activity in both the dorsal and ventral hippocampus is necessary for cocaine conditioned place preference. Further, we characterize changes in histone modifications at the FosB promoter in hippocampus in response to chronic cocaine and found that locus-specific epigenetic modification is essential for FosB induction and multiple hippocampus-dependent behaviors, including cocaine place preference. Collectively, these findings suggest that exposure to cocaine induces histone modification at the hippocampal FosB gene promoter to cause ΔFosB induction critical for cocaine-related learning.SIGNIFICANCE STATEMENT Although cocaine addiction is driven in part by the formation of indelible associations between the drug and the environment, paraphernalia, and circumstances of use, and although this type of associative learning is dependent upon changes in gene expression in a brain region called the hippocampus, the mechanisms by which cocaine alters hippocampal gene expression to drive formation of these associations is poorly understood. Here, we demonstrate that chronic cocaine engages locus-specific changes in the epigenetic profile of the FosB gene in the hippocampus, and that these alterations are required for cocaine-dependent gene expression and cocaine-environment associations. This work provides novel insight into addiction etiology and potential inroads for therapeutic intervention in cocaine addiction.

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.

Translational Molecular Approaches in Substance Abuse Research.

Excessive abuse of psychoactive substances is one of the leading contributors to morbidity and mortality worldwide. In this book chapter, we review translational research strategies that are applied in the pursuit of new and more effective therapeutics for substance use disorder (SUD). The complex, multidimensional nature of psychiatric disorders like SUD presents difficult challenges to investigators. While animal models are critical for outlining the mechanistic relationships between defined behaviors and genetic and/or molecular changes, the heterogeneous pathophysiology of brain diseases is uniquely human, necessitating the use of human studies and translational research schemes. Translational research describes a cross-species approach in which findings from human patient-based data can be used to guide molecular genetic investigations in preclinical animal models in order to delineate the mechanisms of reward circuitry changes in the addicted state. Results from animal studies can then inform clinical investigations toward the development of novel treatments for SUD. Here we describe the strategies that are used to identify and functionally validate genetic variants in the human genome which may contribute to increased risk for SUD, starting from early candidate gene approaches to more recent genome-wide association studies. We will next examine studies aimed at understanding how transcriptional and epigenetic dysregulation in SUD can persistently alter cellular function in the disease state. In our discussion, we then focus on examples from the literature illustrating molecular genetic methodologies that have been applied to studies of different substances of abuse - from alcohol and nicotine to stimulants and opioids - in order to exemplify how these approaches can both delineate the underlying molecular systems driving drug addiction and provide insights into the genetic basis of SUD.

Aberrant H3.3 dynamics in NAc promote vulnerability to depressive-like behavior.

Human major depressive disorder (MDD), along with related mood disorders, is among the world's greatest public health concerns; however, its pathophysiology remains poorly understood. Persistent changes in gene expression are known to promote physiological aberrations implicated in MDD. More recently, histone mechanisms affecting cell type- and regional-specific chromatin structures have also been shown to contribute to transcriptional programs related to depressive behaviors, as well as responses to antidepressants. Although much emphasis has been placed in recent years on roles for histone posttranslational modifications and chromatin-remodeling events in the etiology of MDD, it has become increasingly clear that replication-independent histone variants (e.g., H3.3), which differ in primary amino acid sequence from their canonical counterparts, similarly play critical roles in the regulation of activity-dependent neuronal transcription, synaptic connectivity, and behavioral plasticity. Here, we demonstrate a role for increased H3.3 dynamics in the nucleus accumbens (NAc)-a key limbic brain reward region-in the regulation of aberrant social stress-mediated gene expression and the precipitation of depressive-like behaviors in mice. We find that molecular blockade of these dynamics promotes resilience to chronic social stress and results in a partial renormalization of stress-associated transcriptional patterns in the NAc. In sum, our findings establish H3.3 dynamics as a critical, and previously undocumented, regulator of mood and suggest that future therapies aimed at modulating striatal histone dynamics may potentiate beneficial behavioral adaptations to negative emotional stimuli.

Histone arginine methylation in cocaine action in the nucleus accumbens.

Repeated cocaine exposure regulates transcriptional regulation within the nucleus accumbens (NAc), and epigenetic mechanisms-such as histone acetylation and methylation on Lys residues-have been linked to these lasting actions of cocaine. In contrast to Lys methylation, the role of histone Arg (R) methylation remains underexplored in addiction models. Here we show that protein-R-methyltransferase-6 (PRMT6) and its associated histone mark, asymmetric dimethylation of R2 on histone H3 (H3R2me2a), are decreased in the NAc of mice and rats after repeated cocaine exposure, including self-administration, and in the NAc of cocaine-addicted humans. Such PRMT6 down-regulation occurs selectively in NAc medium spiny neurons (MSNs) expressing dopamine D2 receptors (D2-MSNs), with opposite regulation occurring in D1-MSNs, and serves to protect against cocaine-induced addictive-like behavioral abnormalities. Using ChIP-seq, we identified Src kinase signaling inhibitor 1 (Srcin1; also referred to as p140Cap) as a key gene target for reduced H3R2me2a binding, and found that consequent Srcin1 induction in the NAc decreases Src signaling, cocaine reward, and the motivation to self-administer cocaine. Taken together, these findings suggest that suppression of Src signaling in NAc D2-MSNs, via PRMT6 and H3R2me2a down-regulation, functions as a homeostatic brake to restrain cocaine action, and provide novel candidates for the development of treatments for cocaine addiction.

Histone turnover and chromatin accessibility: Critical mediators of neurological development, plasticity, and disease.

In postmitotic neurons, nucleosomal turnover was long considered to be a static process that is inconsequential to transcription. However, our recent studies in human and rodent brain indicate that replication-independent (RI) nucleosomal turnover, which requires the histone variant H3.3, is dynamic throughout life and is necessary for activity-dependent gene expression, synaptic connectivity, and cognition. H3.3 turnover also facilitates cellular lineage specification and plays a role in suppressing the expression of heterochromatic repetitive elements, including mutagenic transposable sequences, in mouse embryonic stem cells. In this essay, we review mechanisms and functions for RI nucleosomal turnover in brain and present the hypothesis that defects in histone dynamics may represent a common mechanism underlying neurological aging and disease.

ATRX tolerates activity-dependent histone H3 methyl/phos switching to maintain repetitive element silencing in neurons.

ATRX (the alpha thalassemia/mental retardation syndrome X-linked protein) is a member of the switch2/sucrose nonfermentable2 (SWI2/SNF2) family of chromatin-remodeling proteins and primarily functions at heterochromatic loci via its recognition of "repressive" histone modifications [e.g., histone H3 lysine 9 tri-methylation (H3K9me3)]. Despite significant roles for ATRX during normal neural development, as well as its relationship to human disease, ATRX function in the central nervous system is not well understood. Here, we describe ATRX's ability to recognize an activity-dependent combinatorial histone modification, histone H3 lysine 9 tri-methylation/serine 10 phosphorylation (H3K9me3S10ph), in postmitotic neurons. In neurons, this "methyl/phos" switch occurs exclusively after periods of stimulation and is highly enriched at heterochromatic repeats associated with centromeres. Using a multifaceted approach, we reveal that H3K9me3S10ph-bound Atrx represses noncoding transcription of centromeric minor satellite sequences during instances of heightened activity. Our results indicate an essential interaction between ATRX and a previously uncharacterized histone modification in the central nervous system and suggest a potential role for abnormal repetitive element transcription in pathological states manifested by ATRX dysfunction.

Cocaine dynamically regulates heterochromatin and repetitive element unsilencing in nucleus accumbens.

Repeated cocaine exposure induces persistent alterations in genome-wide transcriptional regulatory networks, chromatin remodeling activity and, ultimately, gene expression profiles in the brain's reward circuitry. Virtually all previous investigations have centered on drug-mediated effects occurring throughout active euchromatic regions of the genome, with very little known concerning the impact of cocaine exposure on the regulation and maintenance of heterochromatin in adult brain. Here, we report that cocaine dramatically and dynamically alters heterochromatic histone H3 lysine 9 trimethylation (H3K9me3) in the nucleus accumbens (NAc), a key brain reward region. Furthermore, we demonstrate that repeated cocaine exposure causes persistent decreases in heterochromatization in this brain region, suggesting a potential role for heterochromatic regulation in the long-term actions of cocaine. To identify precise genomic loci affected by these alterations, chromatin immunoprecipitation followed by massively parallel DNA sequencing (ChIP-Seq) was performed on NAc. ChIP-Seq analyses confirmed the existence of the H3K9me3 mark mainly within intergenic regions of the genome and identified specific patterns of cocaine-induced H3K9me3 regulation at repetitive genomic sequences. Cocaine-mediated decreases in H3K9me3 enrichment at specific genomic repeats [e.g., long interspersed nuclear element (LINE)-1 repeats] were further confirmed by the increased expression of LINE-1 retrotransposon-associated repetitive elements in NAc. Such increases likely reflect global patterns of genomic destabilization in this brain region after repeated cocaine administration and open the door for future investigations into the epigenetic and genetic basis of drug addiction.

Serotonin Transporter-dependent Histone Serotonylation in Placenta Contributes to the Neurodevelopmental Transcriptome.

Brain development requires appropriate regulation of serotonin (5-HT) signaling from distinct tissue sources across embryogenesis. At the maternal-fetal interface, the placenta is thought to be an important contributor of offspring brain 5-HT and is critical to overall fetal health. Yet, how placental 5-HT is acquired, and the mechanisms through which 5-HT influences placental functions, are not well understood. Recently, our group identified a novel epigenetic role for 5-HT, in which 5-HT can be added to histone proteins to regulate transcription, a process called H3 serotonylation. Here, we show that H3 serotonylation undergoes dynamic regulation during placental development, corresponding to gene expression changes that are known to influence key metabolic processes. Using transgenic mice, we demonstrate that placental H3 serotonylation is dependent on 5-HT uptake by the serotonin transporter (SERT/SLC6A4). SERT deletion robustly reduces enrichment of H3 serotonylation across the placental genome, and disrupts neurodevelopmental gene networks in early embryonic brain tissues. Thus, these findings suggest a novel role for H3 serotonylation in coordinating placental transcription at the intersection of maternal physiology and offspring brain development.

Early-life stress alters chromatin modifications in VTA to prime stress sensitivity.

Early-life stress increases sensitivity to subsequent stress, which has been observed among humans, other animals, at the level of cellular activity, and at the level of gene expression. However, the molecular mechanisms underlying such long-lasting sensitivity are poorly understood. We tested the hypothesis that persistent changes in transcription and transcriptional potential were maintained at the level of the epigenome, through changes in chromatin. We used a combination of bottom-up mass spectrometry, viral-mediated epigenome-editing, behavioral quantification, and RNA-sequencing in a mouse model of early-life stress, focusing on the ventral tegmental area (VTA), a brain region critically implicated in motivation, reward learning, stress response, and mood and drug disorders. We find that early-life stress in mice alters histone dynamics in VTA and that a majority of these modifications are associated with an open chromatin state that would predict active, primed, or poised gene expression, including enriched histone-3 lysine-4 methylation and the H3K4 monomethylase Setd7. Mimicking ELS through over-expression of Setd7 and enrichment of H3K4me1 in VTA recapitulates ELS-induced behavioral and transcriptional hypersensitivity to future stress. These findings enrich our understanding of the epigenetic mechanisms linking early-life environmental experiences to long-term alterations in stress reactivity within the brain's reward circuitry, with implications for understanding and potentially treating mood and anxiety disorders in humans.

Dissociable contributions of the amygdala and ventral hippocampus to stress-induced changes in defensive behavior.

Stress can have profound consequences on mental health. While much is known about the neural circuits supporting associative memories of stressful events, our understanding of the circuits underlying the non-associative impacts of stress, such as heightened stress sensitivity and anxiety-related behavior, is limited. Here, we demonstrate that the ventral hippocampus (vHC) and basolateral amygdala (BLA) support distinct non-associative behavioral changes following stress. Inhibiting stress-induced protein synthesis in the BLA blocked subsequent increases in stress sensitivity but not anxiety-related behaviors. Conversely, inhibiting stress-induced protein synthesis in the vHC blocked subsequent increases in anxiety-related behavior but not stress sensitivity. Inhibiting neuronal activity in the BLA and vHC during the assessment of stress sensitivity or anxiety-related behavior recapitulated these structures' dissociable contributions to defensive behavior. Lastly, blocking the associative memory of a stressor had no impact on stress-induced changes in anxiety-related behavior. These findings highlight that multiple memory systems support the long-lasting effects of stress.

Bioorthogonal labeling and enrichment of histone monoaminylation reveal its accumulation and regulatory function in cancer cell chromatin.

Histone monoaminylation ( i . e ., serotonylation and dopaminylation) is an emerging category of epigenetic mark occurring on the fifth glutamine (Q5) residue of H3 N-terminal tail, which plays significant roles in gene transcription. Current analysis of histone monoaminylation is mainly based on site-specific antibodies and mass spectrometry, which either lacks high resolution or is time-consuming. In this study, we report the development of chemical probes for bioorthogonal labeling and enrichment of histone serotonylation and dopaminylation. These probes were successfully applied for the monoaminylation analysis of in vitro biochemical assays, cells, and tissue samples. The enrichment of monoaminylated histones by the probes further confirmed the crosstalk between H3Q5 monoaminylation and H3K4 methylation. Finally, combining the ex vivo and in vitro analyses based on the developed probes, we have shown that both histone serotonylation and dopaminylation are highly enriched in tumor tissues that overexpress transglutaminase 2 (TGM2) and regulate the three-dimensional architecture of cellular chromatin.