miR-21-5p Alleviates Retinal Ischemia–Reperfusion Injury by Inhibiting M1 Polarization of Microglia via Suppression of STAT3 Signaling

Background/Objectives: Retinal ischemia–reperfusion (I/R) injury is a common mechanism in glaucoma, diabetic retinopathy, and retinal vein occlusion, leading to progressive loss of retinal ganglion cells (RGCs). This study investigates the regulatory role of miR-21-5p and its interaction with Signal Transducer and Activator of Transcription 3 (STAT3) in retinal I/R injury. Methods: An acute intraocular hypertension (AIH) rat model was used to induce retinal I/R. The interaction between miR-21-5p and STAT3 was examined by dual-luciferase reporter assays. miR-21-5p and STAT3 expression were quantified by qRT-PCR and Western blotting. Retinal morphology, microglial polarization, and RGC survival were assessed by H&E staining and immunofluorescence. In vitro, microglia and RGCs were subjected to oxygen–glucose deprivation/reperfusion (OGD/R), and microglial-conditioned media (MCM) were applied to RGCs. Results: (1) miR-21-5p ameliorated AIH-induced retinal damage in vivo. (2) Overexpression of miR-21-5p inhibits M1 polarization of RM cultured in vitro. (3) MCM from miR-21-5p-overexpressing microglia attenuated OGD/R-induced RGC death. (4) miR-21-5p downregulates STAT3 expression to inhibit RM M1 polarization. (5) miR-21-5p down-regulation of STAT3 levels inhibits M1 polarization and reduces apoptosis of RGCs in retinal microglia of AIH rats. Conclusions: miR-21-5p alleviates retinal I/R injury by restraining microglial M1 polarization through direct repression of STAT3, thereby promoting RGC survival. These findings identify the miR-21-5p/STAT3 axis as a potential therapeutic target for ischemic retinal diseases.