Decoding serotonin in endometriosis: unveiling its role in disease pathogenesis via the gut–reproductive microbiota axis

STUDY QUESTION: How can the potential mechanisms and targets of endometriosis be explored through multi-omics and multi-location approaches? SUMMARY ANSWER: This exploration of the gut-reproductive axis in patients with endometriosis found that serotonin is elevated in endometriosis and promotes disease progression through enhanced cell proliferation and inflammation. WHAT IS KNOWN ALREADY: Endometriosis is a common inflammatory disease. Recent studies indicate that peripheral serotonin, which is regulated by the gut microbiota, can promote the progression of irritable bowel syndrome and various cancers. STUDY DESIGN, SIZE, DURATION: This cross-sectional study enrolled 22 endometriosis patients and 22 control patients with uterine fibroids (surgical cases, October 2022-June 2023). Samples of vaginal secretions, endometrial tissue, peritoneal lavage fluid, feces, and ectopic lesions were collected from both groups. For validation, serum samples were added from 20 additional endometriosis patients and 20 healthy reproductive-age volunteers. PARTICIPANTS/MATERIALS, SETTING, METHODS: This study employed 16S rRNA gene sequencing to analyze the microbiota in the vagina, endometrial tissue, peritoneal fluid, and feces of patients with endometriosis and control groups, complemented by untargeted metabolomic analysis of peritoneal fluid. The results identified serotonin as a key metabolite and revealed specific bacterial species, shared between the reproductive and gastrointestinal tracts of endometriosis patients, which were significantly correlated with serotonin levels. Mendelian randomization analysis was conducted to explore the relationship between serotonin, these bacterial species, and endometriosis. Serum serotonin levels in endometriosis patients, BALB/C mouse models, and their respective controls were measured using ELISA. Immunohistochemistry and fluorescence staining were used to detect the expression of serotonin and its receptors in both ectopic and normal endometrium. The effects of serotonin on the biological behavior of various endometriosis cell models, including proliferation, migration, invasion, and apoptosis, were investigated using CCK8 assay, wound healing test, Transwell assay, apoptosis detection, ELISA, transcriptomics, and qPCR. The impact of serotonin on BALB/C mouse models was evaluated using H&E staining, flow cytometry, and ELISA. MAIN RESULTS AND THE ROLE OF CHANCE: We identified a significant enrichment of Akkermansia muciniphila (a bacterium shared by the gut and reproductive tract) in endometriosis patients, which positively correlated with peritoneal serotonin levels; Mendelian randomization analysis linked both to elevated endometriosis risk. Serotonin levels were elevated in patients' serum (using mouse models) and in ectopic endometrium, in comparison to those of controls. In vitro, serotonin boosted endometriosis cell proliferation, migration, invasion, and inflammation, with upregulated IL-17/NF-κB pathways. In mice, serotonin treatment increased lesion growth, cell proliferation, and inflammation. LARGE SCALE DATA: N/A. LIMITATIONS, REASONS FOR CAUTION: (a) The relatively limited sample size, together with potential imbalance in endometriosis ASRM stage distribution and cesarean section rates, may restrict the generalizability of our findings. In addition, due to the requirement for peritoneal lavage fluid collection, the control group could not consist of entirely healthy women, which may have resulted in a more conservative estimation of group differences. Serum sex hormone levels were not assessed; however, strict inclusion criteria and uniform surgical timing were applied to minimize hormonal confounding. Future studies incorporating cycle-phase-standardized hormone measurements may provide additional insights. (b) Dietary information was not collected in this study, despite the known influence of diet on gut microbiota composition and serotonin metabolism. (c) The direct causal relationship between Akkermansia muciniphila and elevated serotonin levels remains to be established and warrants further validation using germ-free mouse models or fecal microbiota transplantation approaches. (d) The precise mechanisms by which the gut-reproductive tract microbiota axis regulates local and systemic serotonin synthesis remain unclear and require further investigation. WIDER IMPLICATIONS OF THE FINDINGS: Our study is the first to utilize a multi-omics approach combined with a joint analysis of the female gut-reproductive tract axis across multiple loci, revealing and validating a significant increase in serotonin levels in patients with endometriosis. This change may be regulated by the gut-reproductive microbiota axis. These findings provide new insights into the pathogenesis of endometriosis and identify potential targets for prevention and treatment. STUDY FUNDING/COMPETING INTEREST(S): This study was funded by the Jilin Provincial Key Laboratory of Precision Infectious Diseases (Grant No. 20200601011JC), Key Laboratory of Health and Family Planning Commission of Jilin Province (Grant No. 3D5200117426). The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. TRIAL REGISTRATION NUMBER: ChiCTR2300077490.