Saikosaponin A restores the IDO1-driven gut-testis kynurenine axis to alleviate oligozoospermia.

Busulfan (Bus)-induced oligozoospermia still lacks a disease-modifying therapy, and its pathogenesis has been largely attributed to germ-cell DNA damage. Emerging evidence indicates that microbiota-derived metabolites are key determinants of spermatogenic failure. Saikosaponin A (SSA), a major triterpenoid from Bupleurum, has never been evaluated in male infertility. Consequently, its regulatory role in the gut microbiota-metabolite axis and causal efficacy remain completely undefined. To determine whether SSA rescues Bus-induced oligozoospermia via the gut microbiota-metabolite axis, and to uncover a novel pathogenic mechanism of Bus, beyond the known germ-cell DNA damage pathway, revealing a gut microbiota-metabolite-mediated regulatory axis. Bus-induced oligozoospermia was established in male C57BL/6 J mice and followed by SSA treatment. Sperm count, testis index, and histology were assessed; spermatogenic proteins were quantified by Western blot. Microbiota and metabolites were profiled via 16S rDNA sequencing; serum metabolomics; and fecal microbiota transplantation (FMT) from SSA donors. Testicular transcriptome sequencing identified differentially expressed pathways. Indole-3-carboxaldehyde (ICA) administration and Kynurenine (Kyn) supplementation were performed in parallel. Indoleamine 2,3-dioxygenase 1 (IDO1) protein level and activity were measured by Western blot and ELISA. ICA-IDO1 interaction was verified by molecular docking and surface plasmon resonance (SPR). The l-tryptophan/L-kynurenine ratio was determined by targeted liquid chromatography-mass spectrometry (LC-MS/MS). SSA restored sperm count, testis index, and tubular architecture while increasing DDX4, DAZL, and SYCP1/3. It reversed Bus-induced Lactobacillus expansion, decreased colonic ICA, relieved ICA-mediated IDO1 inhibition, thereby restoring colonic IDO1 activity, and elevated Kyn. FMT from SSA donors reproduced these protective effects. Transcriptomics showed up-regulation of AKT-mediated targets: pro-growth (CCND1), antioxidant (NRF2), and anti-apoptotic (Bcl-2), with down-regulation of pro-apoptotic genes. Mechanistically, SSA reshaped the gut microbiota, lowered colonic ICA, relieved ICA-mediated IDO1 inhibition, and restored Kyn-dependent testicular antioxidant and anti-apoptotic signaling. By reshaping the gut microbiota, reducing colonic ICA and relieving ICA-mediated IDO1 inhibition, SSA restored Kyn-driven testicular antioxidant and anti-apoptotic signaling. This suggests a microbiota-directed, non-hormonal candidate preclinical approach for Bus-induced oligozoospermia; the gut microbiota-ICA-IDO1-Kyn axis offers a framework awaiting human validation.