Endocrine Abstracts

The majority of the body’s 5-HT (serotonin) is produced from enterochromaffin cells (ECs) of the intestinal epithelium. 5-HT has important roles within the gastrointestinal tract in the modulation of motility, secretion and inflammation, while also signalling satiety and discomfort to the central nervous system. The factors regulating release of 5-HT from human small intestinal ECs have not been clearly elucidated: although circulating 5-HT levels typically increase after a meal and EC-derived 5-HT is a critical postprandial satiety signal, the importance of direct nutrient stimulation of ECs vs paracrine regulation by other gut hormones – including GLP-1 – remains debated. To investigate the mechanisms of 5-HT release from ECs, organoids established from human duodenum, ileum and rectum were CRISPR-Cas9-modified to fluorescently label tryptophan hydroxylase 1 (TPH1) expressing ECs. Bulk RNA sequencing was performed on human duodenal ECs purified by fluorescence-activated cell sorting and this dataset was considered in parallel with single cell RNA sequencing of human small intestinal enteroendocrine cells, isolated based on chromogranin A expression. The long-chain fatty acid receptor FFAR1 was notably absent in duodenal ECs but most other nutrient-sensing receptors were significantly enriched, including the amino acid responsive GPR142, the short-chain fatty acid receptor FFAR2, the monoacylglycerol receptor GPR119 and the bile acid receptor GPBAR1. Receptors for several gut hormones (including GIP, insulin-like 5 and somatostatin) were also significantly enriched in duodenal ECs, with GLP1R detectable at lower levels. Consistent with transcriptomic data, single cell calcium and cyclic AMP imaging suggests a stimulatory role for several short-chain fatty acids, the aromatic amino acid tryptophan and adrenergic agonists in duodenal EC subpopulations. Ongoing work aims to measure 5-HT secretion from human duodenal organoids in response to these potential regulators of EC function. Collectively this data will provide novel insights into the physiological control of 5-HT release from human small intestine.