Abstract Background Tryptophan metabolism is severely disrupted in IBD. Pro-inflammatory cytokines activate the first rate-limiting enzyme IDO1 leading to tryptophan degradation through the kynurenine pathway for de novo NAD+ synthesis. It has been previously shown that QPRT—the key enzyme converting quinolinic acid to NAD+—is markedly downregulated in IBD and correlates with poor prognosis. Single cell seq data show that QPRT is mainly expressed in intestinal fibroblasts but significantly reduced under inflammatory conditions. This study aims to elucidate the pathological role of fibroblast-specific QPRT downregulation during intestinal inflammation. Methods Transcriptomic changes were analyzed by RNA sequencing. siRNA was performed in CCD18 colonic fibroblast to mediate QPRT knockdown. Quantitative RT-PCR and Western blot were employed to measure mRNA and protein expression. Tryptophan metabolism was quantified via LC-MS. Intracellular NAD levels were assessed by NAD-glo assay. Seahorse assay was conducted to evaluate mitochondrial function. Mitochondrial morphology was examined by transmission electron microscopy. Cytosolic fractions were isolated to assess mitochondrial RNA leakage by qPCR, and immunofluorescence staining was performed to visualize cytosolic mtRNA distribution. Transepithelial electrical resistance (TEER) was measured to assess epithelial barrier function. Results Meta-analysis of mucosal scRNA seq data from HC, CD and UC patients showed that QPRT expression was significantly downregulated in fibroblast of inflamed IBD patients (CD and UC, compared to non-inflamed). Knockdown of QPRT in intestinal fibroblasts led to spontaneous activation of type I interferon and proinflammatory cytokine expression. Mechanistically, QPRT loss caused NAD+ depletion and accumulation of quinolinic acid, resulting in mitochondrial oxidative damage, mitochondrial permeability transition pore opening, and cytosolic release of mitochondrial double-stranded RNA. Consequently, the RIG-I–MAVS signaling pathway and downstream interferon responses were activated. Furthermore, QPRT-deficient fibroblasts exhibited an exaggerated response to cytokine stimulation, amplifying IDO-associated inflammatory signaling. In coculture with human colonic monolayer organoids, these fibroblasts induced epithelial inflammation, impaired proliferation, and disrupted barrier integrity Conclusion Our findings reveal that QPRT downregulation in intestinal fibroblasts acts as a metabolic–immune link driving spontaneous and amplified inflammatory responses through NAD+ deficiency and quinolinic acid–induced mitochondrial stress. This fibroblast-specific metabolic disruption compromises epithelial homeostasis, highlighting the value of QPRT as a potential therapeutic target in IBD Conflict of interest: Wu, Qicong: No conflict of interest Wang, Shihan: No conflict of interest Shima, Kensuke: No conflict of interest Welz, Lina: No conflict of interest Soluch, Ryszard: No conflict of interest Hattermann-Koch, Kirsten: No conflict of interest Schreiber, Stefan Wolfgang: No conflict of interest Meyer, Thomas F.: No conflict of interest Rosenstiel, Philip: No conflict of interest Aden, Konrad: No conflict of interest
Wu et al. (Thu,) studied this question.