Mechanism of Inosine from Lactiplantibacillus plantarum MWFLp-182-Treated Mice Model in Alleviating D-Galactose-Induced HT-22 Cell Injury via Oxidative and Inflammatory Pathways

Gut microbial metabolites play a crucial role in modulating cognitive function. In a previous animal study, oral administration of Lactiplantibacillus plantarum MWFLp-182 (L. plantarum MWFLp-182) significantly increased inosine levels in both the serum and feces of D-galactose (D-gal)-induced mice, which was accompanied by improved cognitive performance. Building on this finding, we further investigated the neuroprotective mechanisms of inosine derived from L. plantarum MWFLp-182 in alleviating D-gal-induced neuronal damage in HT-22 cells. Reverse transcription-quantitative PCR (RT-qPCR) was used to analyze the addition of inosine (250 μg/mL, 500 μg/mL), which considerably reduces oxidative stress induced by D-gal (20 mg/mL), on the regulation of mRNA expression of the nuclear factor erythroid 2-related factor (Nrf2)/hemeoxygenase 1 (HO-1) signaling pathway factors. Compared to the D-gal group, the inosine-treated group exhibited a 4.3-fold and 8.7-fold increase in HO-1 and Nrf2 levels, respectively. Furthermore, inosine alleviates neuroinflammation by modulating the mRNA expression of the Toll-like receptor 4 (TLR4)/myeloid differentiation primary response protein 88 (MyD88)/nuclear factor kappa B (NF-κB) signaling pathway. Compared to the D-gal group, the inosine-treated group showed reductions of 41.75%, 28.29%, and 32.17% in TLR4, MyD88, and NF-κB levels, respectively. Moreover, immunofluorescence staining revealed that inosine exhibits anti-apoptotic properties by enhancing the levels of neurotrophic factors, including nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF), while simultaneously lowering the expression of the pro-apoptotic protein bcl-2-associated X (Bax). These findings suggest that inosine, a differentially expressed metabolite identified in a probiotic-intervention mouse model, alleviates D-gal-induced neuronal damage in HT-22 cells by modulating oxidative, inflammatory, and apoptotic pathways, providing mechanistic insights into the neuroprotective effects of this metabolite.