Fig 2: The effects of honokiol on mRNA and protein expression of hippocampal indoleamine 2,3-dioxygenase (IDO) and NF-?B. (A) IDO1 mRNA expression; (B) NF-?B mRNA expression; (C) IDO protein expression; (D) NF-?B protein expression; (E) The western blot bands. Data are expressed as mean ± SEM (n = 8). ### p < 0.001, ## p < 0.01, # p < 0.05 LPS vs. control; *** p < 0.001, ** p < 0.01 LPS + Honokiol vs. LPS.

Fig 3: Hyperactivated indoleamine 2,3-dioxygenase 1 in colonocytes mediated the gut dysbiosis-induced dysregulation of peripheral Trp-Kyn metabolism. a Volcano plot showing the HFD-altered transcriptomes in colonocytes (n = 3 for each group). b Gene set enrichment analysis (GSEA) showing enrichment of the Trp metabolism gene sets (left) and heatmap of involved genes (right) (n = 3 for each group). c Fluorescent immunostaining of indoleamine 2,3-dioxygenase 1 (IDO1) (green) in mouse colon sections. Nuclei were counterstained with DAPI (blue) (n = 9 slices from 3 mice). d Enriched gene sets of lipopolysaccharide (LPS)-induced inflammation (left) and the heatmap of involved genes (right) (n = 3 for each group). e HFD enhanced the LPS concentration in the colonic contents (n = 8 for each group). f Escherichia coli (E. coli) indicator strains were isolated from the feces of HFD mice and enriched in vitro for bacterial transplantation to confirm the causal role of Proteobacteria expansion in HFD-induced dysregulation of Kyn pathway (C-E.coli). To determine the central role of colonic IDO1 in E.coli-mediated upregulation of the Kyn pathway, mice were administrated with palmatine to eliminate colonic IDO1 before receiving E.coli (C-Pal). g E. coli transplantation enhanced LPS concentration in the colonic contents (n = 8 for each group). h IDO1 activity in the colon (n = 9 slices from 3 mice). i E. coli transplantation upregulated the serum Kyn pathway, which was reversed by palmatine pre-treatment (n = 6 for each group). Data are represented as mean ± SD. In e, g, and i, data are normalized to Chow. In c and e, p-values were determined by independent samples t-test. In g–i, statistical significance was assessed by ANOVA with Dunnett’s multiple comparison test. NS not significant, * p-value = 0.05, ** p-value = 0.01, *** p-value = 0.001

Fig 4: Schematic diagram of the mechanism underlying HFD-dysregulated peripheral Trp-Kyn metabolism. a In the healthy gut, the gut microbiota is dominated by obligate anaerobic bacteria that convert fiber into fermentation products, such as butyrate, to maintain colonocyte metabolism. b Sustained HFD reduced colonic butyrate concentrations and enhanced ß-oxidation of long- and very long-chain fatty acids in colonic cells, which impaired mitochondrial bioenergetics and caused a metabolic reprogramming of colonic cells, allowing Proteobacteria to thrive. Bacteria-derived LPS stimulated colonic immune responses to upregulate the IDO1-mediated Kyn pathway

Fig 5: Colonic mitochondrial dysfunction-induced gut dysbiosis disturbed systemic Trp-Kyn metabolism. a Standard diet-fed mice were administrated with GW9662 to examine the relationship between colonic mitochondrial dysfunction and Trp metabolism (C-GW). To confirm the beneficial effect of remodeling colonic mitochondrial bioenergetics on systemic Trp-Kyn metabolism, butyrate was provided to HFD-fed mice in drinking water (H-Buy). b Colonic epithelial oxygenation was determined by pimonidazole (red). Nuclei were counterstained with DAPI (blue) (n = 9 slices from 3 mice). Concentrations of ATP (c) and nitrate (d) in colonic tissue (n = 8 for each group). e PCoA showed the similarity of gut microbiota determined by 16S profiling (n = 8 for each group; PERMANOVA by Adonis). f Relative abundance of the bacterial phylum. g Relative abundance of phylum Proteobacteria, family Enterobacteriaceae, and genus Desulfovibrio and the ratio of Firmicutes/Bacteroidetes. h Fluorescent immunostaining of IDO1 (green) in mouse colon sections. Nuclei were counterstained with DAPI (blue) (n = 9 slices from 3 mice). i PCA score plot showing serum metabolic profiles (n = 6 for each group; PERMANOVA by Adonis). j Serum metabolites involved in the Kyn pathway (n = 6 for each group). Data are represented as mean ± SD. In c, d, and j, data are normalized to Chow or HFD respectively. In b, the p-value was determined by the Wilcoxon test. Other statistical procedures were performed by independent samples t-test. NS not significant, * p-value = 0.05, ** p-value = 0.01, *** p-value = 0.001