Tryptamine Metabolism and Functionalization in Gut Commensal Bacteria Expand Human Tryptamine Signaling Responses

Gut microbes secrete specialized
small molecules that broadly influence
human physiology. Despite their potential significance, the variety
of functional small molecules known in the gut is relatively limited.
Here, we screened the supernatants from human fecal-derived bacterial
cultures to explore their agonist effects on the human G protein-coupled
receptors (GPCRs), melatonin receptor types 1A and 1B (MTNR1A, MTNR1B).
Chemical analysis of the supernatant-soluble molecules of Clostridium sporogenes, a prominent gut commensal
identified in the screen, led to the characterization of agonists
for these two melatonin receptors. Specifically, through bioactivity-assisted
isolation and characterization, we identified three small molecules, 1–3, including two previously uncharacterized
metabolites, which were synthesized to confirm their structures. While
the structure of 1 features a urea core symmetrically
disubstituted with tryptamine moieties, 2 and 3 harbor a monomeric tryptamine functionalized with methyl carbamate
and N-acetyl groups, respectively. These structural
characterization efforts illuminated downstream functional consequences
of tryptamine metabolism in C. sporogenes. Additional GPCR screening analyses revealed that 2 activates melatonin receptors and the purinergic P2RY11 receptor,
whereas 1 serves as an agonist for the semiorphan receptor
GPR55. Interestingly, 1 also exhibits significant inhibitory
activity against inflammatory soluble epoxide hydrolase with a half-maximal
inhibitory concentration of 420 nM. Single-cell RNA sequencing analysis
of the gut tissue from mice orally treated with 1 relative
to the solvent vehicle control revealed that 1 specifically
decreased the frequency of GPR55- and granzyme K-expressing effector-like
CD8 T cells in the intraepithelial lymphocyte population. Overall,
this study broadens our understanding of tryptamine-derived signaling
at the human–microbe interface.