Histone PTMs and Serotonin: Neuroepigenetic Insights

Histone posttranslational modifications (PTMs) represent a fundamental epigenetic mechanism governing gene expression regulation. Emerging research has revealed that specific neurotransmitters, including serotonin (5-HT), can significantly influence brain function by modulating these histone modifications. This review examines the molecular interplay between serotonin signaling and histone PTMs, and its consequent impact on epigenetic regulation within the central nervous system.

Histone Serotonylation: A Novel Neuroepigenetic Marker

Discovery and Definition

The histone modification H3Q5ser was first identified in 2019. This mark is formed when the neurotransmitter serotonin (5-HT) is covalently conjugated to the fifth glutamine residue (Q5) of histone H3 through the catalytic activity of transglutaminase 2 (TGM2). It represents a novel epigenetic mechanism wherein gene expression is regulated through alterations in chromatin architecture without changing the underlying DNA sequence. Notably, H3Q5ser is the first documented instance of an endogenous monoamine group being attached to a histone.

Molecular Mechanisms

• Catalytic Enzyme TGM2: The enzyme TGM2 functions as the primary "writer" for H3Q5ser, facilitating the calcium-dependent linkage of serotonin to the H3Q5 site. Recent evidence indicates TGM2 also possesses "eraser" and "exchanger" capabilities. It can dynamically remove or substitute monoamine groups (e.g., dopamine, histamine) from H3Q5 in response to changing intracellular monoamine levels.
• Synergy with H3K4me3: The H3Q5ser modification frequently co-occurs with the adjacent trimethylation of histone H3 at lysine 4 (H3K4me3). Although these two marks are established independently, H3Q5ser potentiates the affinity of the transcription factor TFIID for H3K4me3, thereby synergistically enhancing the initiation of gene transcription.
• Reader Protein WDR5: The chromatin-associated protein WDR5 acts as a specific "reader" for the H3Q5ser mark. Its binding is strengthened by the presence of H3Q5ser, which in turn promotes the recruitment of H3K4 methyltransferase complexes (e.g., MLL). This recruitment serves to stabilize the H3K4me3 mark by concurrently inhibiting the activity of demethylating enzymes like KDM5 and LSD1.
• Widely distributed: H3K4me3Q5ser signal is widely distributed in nerve tissue, heart, testis and peripheral blood mononuclear cells, suggesting that its function is not limited to neurons.

Nuclear Serotonin Transport Pathways

Serotonin enters the cytoplasm from the extracellular space via specific transmembrane transporters, including the serotonin transporter (SERT), organic ion transporters (OCTs), and the plasma membrane monoamine transporter (PMAT). Following cytoplasmic uptake, serotonin is subsequently shuttled into the nucleus.

Notably, the detection of robust H3Q5ser signals even in non-serotonergic cells suggests that serotonin-mediated nuclear modification is a widespread phenomenon, indicating a broad functional role beyond classical serotonergic pathways.

Regulation model of serotonin (Fu L et al., 2019)

Neural Function: From Gene Expression to Behavioral Regulation

Gene Transcription Licensing through Epigenetic Modification

During neuronal differentiation, H3Q5ser deposition at promoter regions of active genes (e.g., neurodevelopmental genes) facilitates chromatin relaxation, thereby promoting neural-specific transcriptional programs.

Signal Transduction: From Neurotransmitters to Epigenetic Instructions

The transglutaminase 2 (TGM2) enzyme responds to local serotonin concentrations in real time, dynamically modulating H3Q5 modifications (writing/erasing) to convert neural signals into epigenetic instructions.

Transcriptional Switching via Dual-Modification Recognition

Dual-modified histone marks (H3K4me3Q5ser) recruit WDR5 to circadian gene promoters (e.g., Per2), activating the CLOCK-BMAL1 complex in a time-dependent manner:

• Active phase (ZT16): Elevated WDR5 binding enhances wakefulness-associated gene expression
• Inactive phase (ZT20): Reduced H3Q5ser leads to WDR5 dissociation and transcriptional cessation

Behavioral Output Through Circadian Regulation

At the genomic level, this system regulates over 2500 rhythmic genes in the tuberomammillary nucleus (TMN), maintaining phase-specific expression (e.g., PER2 peaking at ZT16). Behavioral manifestations include:

• H3.3-Q5A mutation disrupts clock gene expression and causes aberrant locomotor rhythms
• Pharmacological inhibition (e.g., zolpidem) reduces H3Q5ser/WDR5 association, mimicking inactive-phase epigenetic states

Circadian Monoamine Regulation Mechanism

Histone H3 monoamination (H3Q5his/H3Q5ser) directly orchestrates circadian transcriptional oscillations through dynamic WDR5 recruitment, forming an epigenetic bridge between neurotransmitter signaling and behavioral rhythms.

Diurnal Oscillation Dynamics

• H3Q5his/H3Q5ser: Exhibit antiphasic rhythms (H3Q5his peaks at ZT16; H3Q5ser declines by ZT20)
• WDR5 dissociation: Reduced H3Q5ser causes WDR5 chromatin ejection, terminating transcription
• H3K4me2/3: Peak during inactive phase (ZT0–4), opposing monoamination timing
• WDR5 recruitment: Maximally enriched at ZT16 at clock genes (e.g., *Per1/2*), driving transcriptional activation

Core Mechanism

• H3K4me3-Q5his/Q5ser → WDR5 recruitment → enhanced CLOCK–BMAL1 binding → rhythmic gene activation (e.g., Per family).
• WDR5 selectively targets transcriptionally permissive loci (e.g., Clock) while avoiding repressive regions.

Pharmacological Validation

Hypnotic agents (e.g., zolpidem) reduce H3Q5ser levels, replicating the epigenetic and behavioral states characteristic of the inactive phase (Zheng Q et al., 2025).

H3Q5 monoaminylations causally contribute to transcriptional and behavioural rhythmicity (Zheng Q et al., 2025)

Serotonin's Central Role in Astrocytic Function

1. Signaling Cascade

Deletion of the SLC22A3 gene reduces serotonin uptake in astrocytes, leading to decreased H3Q5ser modification. This epigenetic change downregulates expression of the GABA synthase MAOB, ultimately impairing GABA release.

2. Key Experimental Evidence

Cell-specific Deficit: SLC22A3 knockout selectively diminishes astrocytic (non-neuronal) serotonin, resulting in an 18% reduction of H3Q5ser modification (confirmed via immunostaining).

Epigenetic–Transcriptional Coupling: Among downregulated genes, 51% (including MAOB) exhibit synchronous loss of H3Q5ser marks (validated by integrated RNA-seq and ChIP-seq).

Functional Impairment:

• MAOB protein levels decrease by 31.5%, reducing astrocytic GABA synthesis.
• Tonic GABA currents decline by 86.71% (neuron-activity dependent). This deficit persists (75.24% reduction) under TTX blockade, confirming the astrocytic origin of the release impairment.

3. Pathophysiological Implications

• Circuit Dysregulation: Impaired GABA release disrupts inhibitory microcircuits in the olfactory bulb.
• Disease Relevance: This mechanism may contribute to anxiety disorders, epilepsy (via disinhibition), and olfactory dysfunction.
• Core Insight: Serotonin directly regulates astrocytic GABAergic signaling through H3Q5ser-mediated epigenetic mechanisms, revealing a novel glia-specific pathway for neuromodulation (Sardar D et al., 2023).

Central Neural Mechanisms of Histone Serotonylation (H3Q5ser)

1. Placenta-Brain Developmental Axis: An Epigenetic Nexus

The serotonin transporter (SERT) mediates maternal serotonin (5-HT) uptake in the placenta, facilitating H3Q5ser epigenetic modifications that regulate key metabolic genes (e.g., GLUT1). This process directly influences fetal nutrient supply. Concurrently, H3Q5ser enrichment at neurodevelopmental gene promoters (e.g., BDNF, FOXP2) in fetal brain tissue activates transcriptional programs governing neuronal migration, synaptogenesis, and cortical differentiation.

2. Neurodevelopmental Consequences of SERT Disruption

• Placental Dysfunction: SERT knockout reduces H3Q5ser levels, dysregulating nutrient transporter genes and compromising energy provision to the developing brain.
• Neural Impairment: Disruption of neurodevelopmental gene networks leads to aberrant neuronal migration, reduced synaptic plasticity, and increased susceptibility to neurodevelopmental disorders such as autism spectrum disorder (ASD) and intellectual disability.

3. Dual Mechanisms of Maternal-Fetal Signaling

• Direct Transmission: Maternal 5-HT crosses the placental barrier, directly modulating fetal neural precursor cell differentiation.
• Indirect Epigenetic Reprogramming: H3Q5ser-mediated remodeling of the placental transcriptome stimulates neurotrophin release (e.g., IGF2), activating the PI3K-AKT pathway in the fetus to promote neural development.

What role histone PTM plays in developmental disorders can be consulted "Histone PTMs in Developmental Disorders: Epigenetic Errors and Disease Pathways".

Understanding histone ptms in neurobiology can be consulted "Histone PTMs in Neurobiology: From Memory to Neurodegeneration".

Conclusion

H3Q5ser functions as a critical epigenetic switch at the maternal-fetal interface, orchestrating fetal neurodevelopment. Its dysregulation represents a foundational mechanism underlying the etiology of neurodevelopmental disorders (Chan JC et al., 2024).

Promoting Metastasis of Hepatic Neuroendocrine Prostate Cancer

1. Neurotransmitter-Driven Epigenetic Modifications

Neuroendocrine tumor cells (e.g., NEPC) secrete serotonin (5-HT), which is internalized into target cells (such as neutrophils) via the serotonin transporter (SERT). Within the nucleus, transglutaminase 2 (TGM2) catalyzes histone serotonylation at H3Q5 (H3Q5ser), while peptidylarginine deiminase 4 (PAD4) concurrently mediates histone citrullination (H3cit). These synergistic modifications promote chromatin decompaction and facilitate transcriptional activation.

2. The Neuro-Epigenetic-Immune Axis

H3Q5ser and H3cit induce chromatin decondensation, leading to the formation of neutrophil extracellular traps (NETs). These NETs entrap circulating tumor cells and establish a pro-metastatic niche in the liver, thereby accelerating the development of macroscopic metastases.

3. Bidirectional Neural Regulation

Tumor-associated nerves release serotonin, which activates intracellular signaling cascades via neurotransmitter receptors and amplifies H3Q5ser modification. This epigenetic change further upregulates neuropeptide genes, such as BDNF, creating a feedforward loop that reinforces neuro-tumor crosstalk and promotes metastatic progression (Tang DG 2025).

Serotonin promotes NEPC metastasis in the liver via posttranslational modification of H3 in neutrophils and increases NET formation (Tang DG 2025).

Therapeutic Perspectives: Novel Strategies for Targeting Modification Pathways

TGM2 Inhibition

Pharmacological inhibition of transglutaminase 2 (TGM2) activity effectively blocks aberrant H3Q5ser deposition. The inhibitor GK921, for instance, demonstrates significant antitumor efficacy in liver cancer models without inducing myeloid toxicity.

WDR5 Interaction Blockade

Small molecules designed to disrupt the binding interface between WDR5 and H3Q5ser offer a strategy for selective suppression of oncogene transcription.

Neuromodulatory Approaches

Fine-tuning TGM2’s bidirectional activity—such as stabilizing its circadian oscillations—holds therapeutic potential for managing insomnia and mood disorders.

Future Directions

• Dynamic Modification Monitoring: Developing live-cell imaging platforms to resolve spatiotemporal dynamics of H3Q5ser in real time.
• Novel Monoamine Modifications: Investigating whether other monoamines (e.g., epinephrine) engage in analogous epigenetic regulatory mechanisms.
• Precision Therapeutics: Designing individualized interventions targeting TGM2 or reader proteins based on patient-specific epigenetic signatures.

Conclusion

Histone serotonylation (H3Q5ser) unveils a direct mechanism of neurotransmitter-driven gene regulation, reshaping our understanding of neurodevelopment, behavioral control, and pathogenesis. Therapeutic targeting of this pathway promises transformative advances in treating cancers and neuropsychiatric disorders.

References

1. Fu L, Zhang L. Serotonylation: A novel histone H3 marker. Signal Transduct Target Ther. 2019 May 10;4:15.
2. Al-Kachak A, Maze I. Post-translational modifications of histone proteins by monoamine neurotransmitters. Curr Opin Chem Biol. 2023 Jun;74:102302.
3. Zheng Q, Weekley BH, Vinson DA, Zhao S, Bastle RM, Thompson RE, Stransky S, Ramakrishnan A, Cunningham AM, Dutta S, Chan JC, Di Salvo G, Chen M, Zhang N, Wu J, Fulton SL, Kong L, Wang H, Zhang B, Vostal L, Upad A, Dierdorff L, Shen L, Molina H, Sidoli S, Muir TW, Li H, David Y, Maze I. Bidirectional histone monoaminylation dynamics regulate neural rhythmicity. Nature. 2025 Jan;637(8047):974-982.
4. Sardar D, Cheng YT, Woo J, Choi DJ, Lee ZF, Kwon W, Chen HC, Lozzi B, Cervantes A, Rajendran K, Huang TW, Jain A, Arenkiel BR, Maze I, Deneen B. Induction of astrocytic Slc22a3 regulates sensory processing through histone serotonylation. Science. 2023 Jun 16;380(6650):eade0027.
5. Chan JC, Alenina N, Cunningham AM, Ramakrishnan A, Shen L, Bader M, Maze I. Serotonin Transporter-dependent Histone Serotonylation in Placenta Contributes to the Neurodevelopmental Transcriptome. J Mol Biol. 2024 Apr 1;436(7):168454.
6. Tang DG. Serotonin sets up neutrophil extracellular traps to promote neuroendocrine prostate cancer metastasis in the liver. J Clin Invest. 2025 Apr 15;135(8):e191687.