Development of a Chemical Biology Toolbox to Investigate Histone Monoaminylation

The dynamics of covalent post-translational modifications (PTMs) on histones are a key mechanism in epigenetic regulation. Histone PTMs (also known as the “histone code”) are dynamically introduced and removed by “writer” and “eraser” enzymes, while the recognition of these PTM makers by “reader” proteins controls the activation and suppression of specific genes, motivating downstream epigenetic effects. Histone monoaminylation (i.e., H3 serotonylation and dopaminylation) is a newly identified epigenetic marker that plays an important role in regulating neuronal transcription, both during development and in the adult brain. Transglutaminase 2 (TGM2) has been proved to serve as the writer enzyme for this emerging histone PTM, which installs serotonin or dopamine onto the N-terminal glutamine residue of H3 (i.e., H3Q5) through transamidation. However, the eraser and reader for H3Q5 monoaminylation still remain elusive. In our recent study, we applied chemical biology approaches to understand the dynamic control of histone monoaminylation and unexpectedly discovered that the installation, removal, and replacement of this modification are all mediated by a single enzyme, TGM2. The biochemical mechanism of this novel regulation is attributed to the formation of a reactive thioester complex between TGM2 and H3 that can be attacked by nucleophiles (such as diverse monoamine metabolites). Based on this unique enzymology, we identified an unreported histone monoaminylation, H3Q5 histaminylation, and found that this new epigenetic marker promotes neural rhythmicity through epigenetic regulations. In this research program, we will develop a series of chemical probes that can orthogonally label and enrich different histone monoaminylations. Utilizing these probes, we will identify new types of monoaminylations (especially the ones caused by gut microbiome-derived monoamines) both in vitro and in vivo. Thereafter, we will demonstrate the pathophysiological roles of these epigenetic makers. We will also design and synthesize photocrosslinker- containing monoaminylated peptides as chemical baits to covalently capture possible readers that recognize and bind the target monoaminylations. The epigenetic functions of these identified readers will be further validated both in vitro and in vivo. Finally, we will employ the chemical probes developed in this study to characterize the non-histone targets (such as transcription factors) of monoaminylations and elucidate their potential roles in epigenetics and chromatin biology. Together, these findings will expand the categories of histone code and open a new door towards understanding the interplay between monoamine metabolism (from either host cells or gut microbiome) and cell fate regulation.