Production of Histamine by Diverse Gut Bacteria Can Activate Intestinal HRH1 and Promote Calcium Mobilization
Authors: Engevik Kristen , Edens Rachel , Engevik Amy , Engevik Melinda , Horvath Thomas 
Published: May 2024

Background: Histamine is a biogenic amine that impacts several physiological processes, including allergic reactions, neurotransmitter activity and immune regulation. Bacteria uptake L-histidine and convert it into histamine via the activity of histidine decarboxylase enzymes. Histamine can also be produced by host cells such as enterochromaffn-like cells, mast cells and neurons. We hypothesized that a diverse group of bacteria would be capable of generating histamine. We predicted that histamine producing bacteria in the gut would activate intestinal epithelial histamine receptors and promote gut health. Methods and Results: To investigate how widespread histamine production is across bacterial species, we examined 102,018 annotated genomes in the Integrated Microbiome Genome Database. We found 3,679 bacterial genomes (3.6%) possessed histidine decarboxylase. We found histidine decarboxylase in bacteria belonged to 9 phyla: Bacillota, Bacteroidota, Actinomycetota, Pseudomonadota, Lentisphaerota, Fusobacteriota, Armatimonadota, Cyanobacteriota, and Verrucomicrobiota. Although the majority of the identified bacteria were aquatic and terrestrial in origin, ~14% of the bacteria were from the human gut microbiota. We confirmed the ability of several gut bacteria to generate histamine in a chemically defined bacterial growth medium using liquid chromatography-tandem mass spectrometry (LC-MS/MS)-based targeted metabolomics. Briefly, histamine and histidine quantification is performed using authentic analytical and stable-label deuterated internal standard compounds that are used to produce calibration curves that span a linear dynamic range of 0.977-1,000 ng/mL. We found higher concentrations of histamine produced by gut-associated Klebsiella aerogenes, Fusobacterium varium, Clostridium perfringens, Limosilactobacillus reuteri and Morganella morganii. To address the question if bacterial-derived histamine could activate intestinal epithelial histamine receptors in the gut, we examined histamine receptors in publicly available single cell RNAseq datasets. We found that HRH1 was expressed in all cell types of the small and large intestine. We did not observe any expression of HRH2, HRH3 or HRH4 in the gut epithelium. We confirmed this finding in human intestinal organoids derived from duodenum, jejunum, ileum and colon. HRH1 activation in airway epithelial cells is known to signal through MAPK and promote pro-inflammatory cytokine release. Using GCaMP6S transduced organoids, we found that multiple concentrations of histamine stimulated robust calcium mobilization. However, histamine did not alter the levels of phosphorylated MAPK members p38, JNK, ERK, CREB or AKT and did not stimulate the release of any of the 18 epithelial specific cytokines we examined. Conclusions: Our work reveals that gut bacteria are capable of generating histamine, and histamine in the gut can activate epithelial HRH1 to drive calcium signaling and other downstream signals. K01 DK123195 (M Engevik) K01 DK121869 (A Engevik F32 DK130288 (K Engevik) APS Postdoctoral Fellowship (K Engevik). This is the full abstract presented at the American Physiology Summit 2024 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.