A metabolic constraint in the kynurenine pathway drives mucosal inflammation in IBD

Inflammatory bowel disease (IBD) is associated with perturbed metabolism of the essential amino acid tryptophan (Trp). Whether increased degradation of Trp directly fuels mucosal inflammation or acts as a compensatory attempt to restore cellular energy levels via de-novo nicotinamide adenine dinucleotide (NAD ⁺ ) synthesis is not understood. Employing a systems medicine approach on longitudinal IBD therapy intervention cohorts and targeted screening in preclinical IBD models, we discover that steady increases in Trp levels upon therapy success coincide with a rewiring of metabolic processes within the kynurenine pathway (KP). In detail, we identify that Trp catabolism in IBD is metabolically constrained at the level of quinolinate phosphorybosyltransferase (QPRT), leading to accumulation of quinolinic acid (Quin) and a decrease of NAD ⁺ . We further demonstrate that Trp degradation along the KP occurs locally in the inflamed intestinal mucosa and critically depends on janus kinase / signal transducers and activators of transcription (JAK/STAT) signalling. Subsequently, knockdown of QPRT in-vitro induces NAD ⁺ depletion and a pro-inflammatory state, which can largely be rescued by bypassing QPRT via other NAD ⁺ precursors. We hence propose a model of impaired de-novo NAD ⁺ synthesis from Trp in IBD. These findings point towards the replenishment of NAD ⁺ precursors as a novel therapeutic pathway in IBD.