Bone Marrow Mesenchymal Stem Cell‐Derived Exosomal Let‐7b‐5p Reduces High Glucose‐Induced Microglial Activation and Inflammation Through TLR4/ATF4

Background and Objective: Diabetic retinopathy (DR) is a leading cause of vision loss in patients with diabetes mellitus (DM), and its pathogenesis is closely associated with aberrant microglial activation. Although bone marrow mesenchymal stem cell-derived exosomes (BMSC-Exo) and the miRNAs that they carry show therapeutic potential for DR, the specific roles and molecular mechanisms through which let-7b-5p regulates microglial activation after it is delivered by BMSC-Exo remain unclear. This study aimed to elucidate the function and underlying mechanism of BMSC-Exo let-7b-5p in DR. Methods: A DR mouse model was established by intraperitoneal injection of streptozotocin (STZ), and BV-2 microglia were stimulated with high glucose (HG) to induce activation in vitro. The morphological characteristics of the BMSC-Exo were identified using transmission electron microscopy (TEM). Protein and gene expression levels, as well as microglial activation, were assessed by Western blot, RT-qPCR, and immunofluorescence, respectively. Retinal tissue damage and apoptosis were evaluated using HE staining and TUNEL assays. Results: BMSC-Exo treatment significantly suppressed the expression of activation markers (Iba1 and TSPO) and inflammatory cytokines (TNF-α, IL-1β, and IL-6) in HG-induced BV-2 cells and DR mouse retinas while alleviating retinal tissue damage and apoptosis. Bioinformatics analysis revealed the downregulation of let-7b-5p in DR. Functional experiments demonstrated that let-7b-5p overexpression enhanced the inhibitory effects of BMSC-Exo on microglial activation, inflammation, and retinal damage, whereas let-7b-5p knockdown attenuated these therapeutic benefits. Mechanistically, BMSC-Exo let-7b-5p inhibited excessive microglial activation and inflammatory responses by targeting the TLR4/ATF4 signaling pathway. Conclusion: BMSC-Exo deliver let-7b-5p to suppress the TLR4/ATF4 pathway, thereby mitigating microglial activation and inflammation and ultimately delaying DR progression. These findings support the potential of this novel therapeutic strategy for targeted DR treatment.