Watch out epigenomic evil-doers, histone H3 has a new sidekick to help it keep gene regulation in shape. By day, serotonin can be found in its traditional role as a neurotransmitter; being released by cells of the nervous system and binding to specific receptors to impact a wide range of functions, from cognition to gut motility. But a new study from the lab of Ian Maze (Mount Sinai School of Medicine, New York) suggests that serotonin may have a secret life as a histone post-translational modification.
Working off previous evidence that found traces of serotonin in the nucleus, the authors of this study found that:
- Labelled serotonin (5-HT) can be endogenously added to histone H3 proteins in living cells
- This process, called serotonylation, relies on the transglutaminase 2 (TGM2) enzyme, and won’t occur if TGM2 is mutated or inhibited
- Serotonylation happens at the 5thamino acid, glutamine, of the H3 histone tail (H3Q5ser) as identified by targeted liquid chromatography-tandem mass spectrometry (LC-MS/MS)
- H3Q5ser is found almost exclusively along with its neighboring methylation modification, H3K4me3, in tissues that produce 5-HT, like the brain and colon
Since histone modifications, like H3K4me3 are important regulators of gene expression, the researchers turned to cultured 5-HT-producing neurons to study if the new mark had an epigenetic calling:
- ChIP-seq found that the co-occurrence of the two marks (H3K4me3Q5ser) increases as cells begin to produce serotonin, especially at the promoters of genes that are activated during differentiation from pluripotent cells (hPSCs) to 5-HT neurons
- In RN46A-B14 cells, another 5-HT producing neuronal line, most promoters that had H3K4me3 ChIP-seq peaks before differentiation gain Q5ser after maturation
- More than half of promoters that have both permissive H3K4me3 and repressive H3K27me3 marks gain serotonylation after differentiation, which suggests that the mark might play a role in bypassing bivalency
- Transfecting cells with a lentivirus containing mutant H3, which cannot to be serotonylated, prevents the expression of genes that are needed for normal maturation and leaves the cells with shorter, immature projections
- Combining synthetic H3 tails with nuclear protein lysates, and using LC-MS/MS, the researchers identified a large group of proteins that have different binding affinities for H3K4me3 alone vs H3K4me3Q5ser, including 15 members of the TATA-box binding protein and associated factors (TFIID) family
- ChIP-seq for TFIID found that these transcription factors are more likely to bind to genes activated during differentiation and that these genes overlap with genes enriched for H3K4me3Q5ser
All together, it looks like serotonylation might help H3K4me3 activate gene expression in cells that either produce or have access to extravesicular serotonin.
Senior author Ian Maze shares, “Our findings represent a dramatic divergence from the current dogma, which works primarily on the premise that neurotransmitters like serotonin and dopamine act solely through the activation of their membrane receptors in the brain to regulate brain cell activity. We found actions of these brain chemicals that are independent of neurotransmission but critically important to their overall signaling, suggesting that our current understanding of these molecules is incomplete and requires further investigation. The serendipitous discovery that serotonin can be chemically attached to histone proteins in the nucleus of cells to regulate gene expression raises many exciting questions, most of which are yet to be answered. While this new discovery raises a whole new crop of questions that will need to be answered through more research, what is clear is that this work promises to fundamentally change the way the field interprets the biological activities of serotonin, and possibly other monoamines as well, phenomena that may contribute significantly to our understanding of human disease and inform new targets for therapeutic treatment.”
So, stay tuned, maybe we’ll see other neurotransmitters play roles in the sequels!
No need to stand in line at a theatre, you can read about the H3K4me3Q5ser origin story, in Nature, March 2019