Tryptophan metabolites are altered when Caco-2 cells are exposed to simulated microgravity

Microgravity, as experienced during spaceflights, can disrupt cellular function and metabolism, including critical pathways such as tryptophan metabolism. This study investigates how simulated microgravity influences the tryptophan pathway and serotonin production in intestinal Caco-2 cells under different experimental conditions. Cells were exposed to dextran sodium sulfate (DSS) and lipopolysaccharide (LPS), known pro-inflammatory agents, and co-cultured with human fecal samples to evaluate the impact of microbiota-derived factors. Using targeted liquid chromatography-tandem mass spectrometry (LC-MS/MS), significant alterations in tryptophan metabolites were observed under simulated microgravity. A significant increase in serotonin and quinolinic acid levels was observed when Caco-2 cells were exposed to simulated microgravity compared to those maintained under terrestrial conditions. Treating Caco-2 cells with DSS we observed an increase in serotonin, kynurenic acid and quinolinic acid levels in samples exposed to simulated microgravity compared to controls. Furthermore, exposure to simulated microgravity in conjunction with LPS treatment led to a significant increase in quinolinic acid levels. Additionally, in microgravity conditions, when Caco-2 cells were co-cultured with stool samples, an increase of serotonin and tryptophan levels was observed. All together, these findings suggest that microgravity, in combination with specific inflammation stimuli, can modulate serotonin synthesis in intestinal cells, potentially contributing to the alteration of gastrointestinal regulation during long term space missions.