Altered fecal and serum aromatic amino acids and transporters in small intestinal inflammation

Inflammatory Bowel Diseases (IBD) are characterized by chronic inflammation of the gastrointestinal tract. Emerging metabolomic studies show aromatic amino acids (AA) L-phenylalanine (Phe) and L-tryptophan (Trp) levels are increased in the feces of patients with active Crohn’s Disease (CD) and decreased in the serum of patients with active CD and Ulcerative Colitis (UC) when compared to healthy controls or patients in remission. Trp is the precursor to serotonin (5-HT), a neurotransmitter that regulates mood in the brain and gut homeostasis. The altered amino acid profile in IBD may contribute to malnutrition, oxidative stress, disruption of tight junctions, and pro-inflammatory signals such as increased nitric oxide synthases. A decrease in transport proteins, including the serotonin transporter (SERT, SLC6A4) involved in the NaCl-dependent reuptake of 5-HT, was also implicated in the pathophysiology of IBD. However, the mechanism underlying the altered aromatic amino acid profile in IBD remains unknown. We hypothesized that alteration of Phe and Trp in the feces and serum occurs by a decrease in their absorption via amino acid transporters located on the apical and/or basolateral sides of enterocytes. Methods: SAMP/YitFC (SAMP; spontaneous ileitis mouse model; M/F, 20wks), AKR/J (SAMP controls; M/F, 20wks), C57BL/6 (M, 6-10wks), TNF ΔARE (chronic ileitis via elevated TNF levels; M, 6-10wks), and SERT KO mice (M, 8-12wks) were used. Fecal phenylalanine levels were determined using a phenylalanine detection kit (Sigma, MAK005). Serum amino acid levels from SERT KO mice were analyzed by LC-MS/MS. Results: Fecal phenylalanine levels were significantly increased in SAMP mice compared to the AKR control. The basolateral AA transporter MCT10 (SLC16A10) mRNA levels in the ileum were decreased by 36% (p<0.05) as compared to AKR with no changes in the apical AA transporter BOAT1 (SLC6A19).TNF ΔARE mice had a reduction in mRNA levels of both AA transporters, BOAT1 and MCT10, in the ileum (52%, p<0.005, and 34% decrease, p<0.01, respectively, compared to WT). There was no change in AA transporter LAT1 (SLC7A5) in the ileum of SAMP and TNF ΔARE mice compared to their controls. SAMP and TNF ΔARE mice also exhibited significant decreases in the ileal SERT mRNA. We utilized SERT KO mice to examine if decreased SERT is implicated in altered amino acid levels. SERT deletion resulted in a significant decrease of Phe and Trp in serum by LC-MS/MS, as well as a significant reduction in mRNA levels of MCT10 by 36% (p<0.0328) compared to WT, and no change in BOAT1 and LAT1 in the ileum. Conclusion: These data demonstrate that intestinal inflammation and/or SERT deficiency results in an altered aromatic amino acid profile, possibly via the downregulation of MCT10 in the ileum. NIH/NIDDK and Crohn's and Colitis Foundation of America This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.