Background: IBS is associated with microbial dysbiosis. It has been reported that populations of Bifidobacterium and Faecalibacterium are implicated in tryptophan (trp) metabolism and decreased in IBS. Kynurenic acid (KYNA), an important metabolite of tryptophan has an anti-nociceptive role by acting as an agonist on the anti-inflammatory GPR35 receptor. We have shown that fecal KYNA level is lower in IBS patients vs healthy subjects (HC). It is unknown whether IBS-D is associated with dysbiosis affecting KYNA levels resulting in visceral sensitivity (VH).
Hypothesis: IBS-D is associated with dysbiosis in KYNA-producing microbes, contributing to VH via a decrease in anti-nociceptive KYNA levels and a reduction in its agonist role on the anti-inflammatory GPR35 receptor.
Methods: Fecal KYNA levels were assessed with ELISA. Pain behavior was assessed in a validated IBS mouse model using visceral motor response to colorectal distension (VMR/CRD). Fecal supernatant from IBS-D patients (IBS-FS) was delivered intracolonically to naïve mice, and pain assessed using VMR/CRD. Epithelial barrier dysfunction was assessed ex vivo in colon biopsies using Transepithelial Electrical Resistance (TEER).
Results: Fecal KYNA was decreased 60% in IBS patients vs healthy controls (HC) (91.0±19.7 vs 232.1± 26.3 ng/mg, P<0.001). IBS-FS induced VH. KYNA or YE120, a potent GPR35 agonist (5 mg/kg, i.p., 1hr) reversed VH induced by IBS-FS. In contrast, 3-Hydroxykynurene (3HKA, 10 mg/kg, i.p., 1 hr), a downstream kynurenine metabolite, did not affect VH, suggesting that the analgesia effect of KYNA is ligand-specific. Administration of CID2745687, an antagonist of GPR35 prevented the effects of KYNA, supporting that the analgesic action of KYNA involves GPR35 receptors. In vivo studies revealed that a trp-deficient diet-induced VH and colon barrier dysfunction. Supplementation of trp-deficient diet with KYNA reversed VH and improved epithelial barrier function (n=4, p<0.05). Cultures of Bifidobacterium but not Faecalibacterim increased KYNA production in anaerobic culture conditions. Germ-free (GF) mice were reconstituted with human feces from IBS or HC. Fecal KYNA levels were significantly lower in the GF mice conventionalized with microbiota from patients with IBS-D compared to HC subjects, accompanied by a 3-fold increase in VH. Oral introduction of Bifidobacterium prevented VH in GF mice conventionalized with feces from IBS-D.
Summary: Novel observations include: i) fecal KYNA is reduced in IBS-D patients ii) KYNA may play an important role in the pathophysiology of IBS-D by modulating nociception via GPR35 receptor-mediated pathways; iii) decreased Bifidobacteria populations in IBS-D may play a role in the reduction of fecal KYNA; and iv) GPR35 receptor agonists may represent attractive therapeutic targets to treat IBS-D associated VH.

Shi-Yi Zhou

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