Hypothesis / aims of study
The comorbidity of overactive bladder (OAB) and irritable bowel syndrome (IBS) presents a significant clinical challenge, yet the underlying neural and microbial mechanisms of the gut-bladder axis remain poorly understood. We hypothesized that enrichment of Akkermansia muciniphila drives this viscero-visceral crosstalk by shunting host tryptophan metabolism toward the serotonin (5-HT) pathway, thereby acting on upregulated colonic 5-HT3a receptors to sensitize dichotomizing dorsal root ganglion (DRG) neurons that co-innervate the colon and bladder. The aims were to delineate the complete causal pathway from this specific gut microbe to bladder dysfunction and to validate the gut-specific 5-HT3a receptor as a druggable therapeutic target.
Study design, materials and methods
A combined human and preclinical approach was employed. Twenty female OAB-IBS patients (meeting ICS OAB criteria and Rome IV IBS criteria) and 20 asymptomatic controls provided fecal samples for 16S rDNA sequencing, targeted metabolomics, and species-specific qPCR. A post-inflammatory trinitrobenzene-sulfonic acid (TNBS) colitis mouse model was established to recapitulate the comorbidity. Integrated techniques included dual retrograde neuronal tracing (Fast Blue for colon, DiI for bladder), fecal microbiota transplantation (FMT) using A. muciniphila or patient-derived microbiota after antibiotic depletion, urodynamic cystometry, voiding spot assay, von Frey test, colorectal distension with abdominal withdrawal reflex (AWR) scoring, and electromyography (EMG), whole-cell patch-clamp electrophysiology of lumbosacral DRG neurons, immunohistochemistry for c-Fos/TRPV1/5-HT3aR/PGP9.5, quantitative PCR, Western blot, and ELISA for tryptophan metabolites and neurotrophic factors. Two targeted interventions were performed on day 21 post-TNBS: pharmacological blockade of colonic 5-HT3a receptors via mesenteric artery injection of ondansetron or surgical severing of mesenteric nerves.
Results
Retrograde tracing confirmed dichotomized DRG neurons co-innervating colon and bladder. Both OAB-IBS patients and TNBS mice exhibited gut dysbiosis with significant Akkermansia muciniphila enrichment (LEfSe and qPCR), which positively correlated with OABSS, IBS-SSS, and visceral hypersensitivity scores. This occurred without bladder inflammation or urinary A. muciniphila colonization. TNBS mice developed sustained visceral and somatic hypersensitivity plus OAB phenotype (increased voiding/non-voiding contractions on cystometry) persisting to day 28. FMT with A. muciniphila or patient microbiota causally exacerbated visceral hypersensitivity, OAB phenotype, and DRG hyperexcitability (lower rheobase, higher action potential firing). Mechanistically, A. muciniphila shunted tryptophan metabolism toward 5-HT (elevated fecal/colonic 5-HT, upregulated Tph1, downregulated Slc6a4/Maoa), specifically upregulating colonic 5-HT3a receptors on extrinsic afferent nerve terminals. Crucially, ondansetron blockade or mesenteric nerve severing completely reversed bladder dysfunction, hypersensitivity, and DRG hyperexcitability without altering colonic inflammation.
Interpretation of results
These findings establish a novel, non-inflammatory, neurally mediated pathway wherein A. muciniphila enrichment drives functional gut-bladder comorbidity by promoting gut-derived serotonergic signaling that sensitizes shared pelvic afferent neurons via colonic 5-HT3a receptors, thereby inducing persistent OAB phenotype in the setting of IBS.