Diabetes-associated modifications in gut microbiota and tryptophan metabolism: implications for macrophage polarization and wound repair in mice.

BACKGROUND: Diabetic wound (DW) is a severe complication of diabetes with poor healing, linked to gut microbiota dysbiosis, metabolic imbalance, and macrophage polarization disorder. This study aimed to systematically explore the role of the gut-skin axis in DW. METHODS: A type 2 diabetes mellitus (T2DM) mouse model was established via high-fat diet feeding and streptozotocin injection. Wound healing was evaluated by histological and immunofluorescence analyses. Gut microbiota composition (16 S rRNA sequencing), serum non-targeted metabolomics (GC-MS), and targeted tryptophan metabolite detection (UPLC-MS/MS) in cecal contents and wound tissues were also performed. Macrophage polarization and cytokine levels were assessed by immunofluorescence and ELISA, respectively. RESULTS: T2DM mouse model showed delayed wound healing, reduced collagen deposition, impaired neovascularization, and persistent inflammation. 16 S rRNA sequencing revealed decreased gut α-diversity and significantly lower abundances of Lactobacillus johnsonii, Lactobacillus reuteri, and Lactobacillus sp. KC38 in T2DM mice. These Lactobacillus species were positively correlated with wound healing rate. Metabolomic analyses demonstrated suppressed tryptophan-indole metabolism in T2DM mice: cecal and wound levels of indole-3-propionic acid, indole-lactic acid, and other indole metabolites were reduced, accompanied by downregulated aromatic amino acid transaminase and phenyllactate dehydrogenase expression. T2DM mice also exhibited M1 macrophage persistence (elevated CD68 in wound and elevated IL-6, TNF-α, iNOS in serum) and M2 macrophage deficiency (reduced CD206 in wound and decreased IL-10, TGF-β, Arg-1 in serum), with impaired myofibroblast activity (α-SMA) and angiogenesis (CD31). Wound tryptophan-indole metabolites were positively correlated with healing-promoting indices and negatively correlated with pro-inflammatory markers. CONCLUSIONS: The reduction in gut Lactobacillus abundance induced by T2DM is associated with disrupted tryptophan-indole metabolism, which may impair macrophage polarization and wound healing. This study provides a systematic basis for microbiota-targeted therapies for diabetic wounds.