Study Of Kynurenine Pathway Metabolism In Patients With Diarrhea-type Irritable Bowel Syndrome

Irritable bowel syndrome(IBS)is often combined with mental disorders which is associated with 5-hydroxy tryptamine(5-HT)metabolic changes.While,as a precursor of 5-HT,about 95% of tryptophan(Trp)is mainly catabolized by the kynurenine pathway(KP).In the low-inflammation state of IBS intestinal mucosa,the indoleamine 2,3-dioxygenase 1(IDO1)can promote the Trp to the KP and produce the kynurenine(KYN),which further produces neurotoxic metabolite quinolinic acid(QUIN)and neuroprotective metabolite kynurenic acid(KYNA).The imbalance of these two compounds can lead to overexcitation of intestinal neurons and increased release of glutamate,a excitatory neurotransmitter,which can act on the N-methyl D-aspartate receptor(NMDAR).It can induce different changes in the expression of NMDAR in different subtypes.As the most subtype with the strongest affinity of NMDAR,NMDAR2 B can mediate intestinal hyperkinesia and pain hypersensitivity,but its changes and whose significance are not clear.Therefore,this study aims to investigate the serum level of Trp,KYN,QUIN and KYNA in diarrhea-type IBS(IBS-D)patients and the expression of NMDAR2 B in the intestinal mucosa to discuss whether there is an imbalance between neurotoxic metabolite and neuroprotective metabolite in IBS-D patients and whether it will influence the expression of NMDAR2 B in enteric nervous system,and further explore the relationship between abnormal kynurenine pathway metabolism and intestinal symptoms and mood disorders in patients with IBS-D.Objective: To demonstrate whether the kynurenine pathway is activated in IBS-D patients,and whether the neurotoxic metabolite QUIN is out of balance with the neuroprotective metabolite KYNA,and further explore whether the imbalance of kynurenine metabolism leads to an increase in NMDAR2 B expression in the enteric nervous system.This in turn leads to intestinal symptoms and mood disorders in patients with IBS-D.Methods: We selected fifteen IBS-D patients as test group and fifteen healthy volunteers as control group from the Department of Gastroenterology and the physical examination center of Hebei General Hospital,and they accepted IBS symptom severity scale(IBS-SSS)score,Self-rating Depression Scale(SDS)and Self-rating Anxiety Scale(SAS)anxiety and depression scores.All enrolled patients underwent colonoscopy,and ileum and colonic mucosa specimens were collected from them in the premise of signing an informed consent form.The expression of NMDAR2 B in intestinal mucosa was detected by immunofluorescence,and fasting serum was collected to detect the content of Trp,KYN,KYNA and QUIN by high performance liquid chromatography tandem mass spectrometry(HPLC-MS/MS).Results:1.There was no statistical difference in serum Trp content between IBS-D group and healthy control group(P>0.05).2.The serum levels of KYN and QUIN in IBS-D group were higher than those of healthy control group(P<0.05),while KYNA was lower than that of healthy control group(P<0.05).3.The serum KYN/Trp ratio of IBS-D group was higher than that of healthy control group(P<0.05),and the KYNA/KYN ratio was lower than that of healthy control group(P<0.05),while there was no difference of statistics in the QUIN/KYN ratio between IBS-D group and healthy control group(P>0.05).The serum QUIN/KYNA ratio of IBS-D group was higher than that of healthy control group(P<0.05).4.The IBS-SSS symptom score,SAS and SDS scores in the IBS-D group were higher than those in the healthy control group(P<0.05).There were no correlation between IBS-SSS symptom score and serum KYN/Trp and QUIN/KYN ratios(P>0.05),but positive correlation with QUIN/KYNA ratio(P<0.05),and they were negatively correlated with KYNA/KYN ratio(P<0.05).There were no correlation between SAS score and serum KYN/Trp and QUIN/KYN ratios(P>0.05),and positive correlation with QUIN/KYNA ratio(P<0.05),and negative correlation with KYNA/KYN ratio(P<0.05).And SDS score were positively correlated with serum KYN/Trp and QUIN/KYNA ratios(P<0.05),and negatively correlated with KYNA/KYN ratio(P<0.05),however,it had no correlation with QUIN/KYN ratio(P>0.05).5.The expression of NMDAR2 B in ileum and colonic mucosa of IBS-D group was higher than that of healthy control group(P<0.05),and the expression of NMDAR2 B in colonic mucosa was higher than that in ileum in both IBS-D group and healthy control group(P<0.05).The expression of NMDAR2 B in ileal mucosa of IBS-D group was not correlated with serum KYNA and QUIN levels(P>0.05),but the expression of NMDAR2 B in colonic mucosa was negatively correlated with serum KYNA level(P<0.05),and positively correlated with serum QUIN level(P<0.05),and both the expression of NMDAR2 B in ileum and colonic mucosa were significantly positively correlated with QUIN/KYNA ratio(P<0.05).The expression of NMDAR2 B in ileum and colonic mucosa of IBS-D group was positively correlated with IBS-SSS symptom score(P<0.05),and also positively correlated with SAS and SDS scores(P<0.05).Conclusions:1.Compared with healthy people,the absorption of Trp in IBS-D patients is normal.2.The kynurenine pathway of IBS-D patients was activated.The production of neurotoxic metabolites QUIN was increased,while the production of neuroprotective metabolite KYNA was decreased.3.The ratio of KYN/Trp was increased in IBS-D patients,indicating that the IDO1 enzyme activity was enhanced,while the KYNA/KYN ratio was decreased,and indicating that the enzyme activity of KAT was decreased.4.The ratio of QUIN/KYNA in patients with IBS-D was elevated,indicating an imbalance in KYN metabolites in IBS-D patients,which was biased towards neurotoxic metabolite.5.The imbalance between neuroprotective metabolites and neurotoxic metabolites was involved in the development of intestinal symptoms and mood disorders in patients with IBS-D.6.The expression of NMDAR2 B in colonic mucosa of IBS-D patients and healthy controls were higher than that of ileum,and the expression of NMDAR2 B in IBS-D intestinal mucosa was higher than that of healthy controls.In patients with IBS-D,serum KYNA was decreased while QUIN was increased,the synergistic effect of these two aspects may increase the expression of NMDAR2 B,which may be involved in intestinal symptoms and mood disorders of IBS-D.