Glycogen synthase kinase-3: a key kinase in retinal neuron apoptosis in early diabetic retinopathy

Background Diabetes-related pathogenic factors can cause retinal ganglion cell (RGC) apoptosis, but the specific mechanism is not very clear. The aim of this study is to investigate the correlation between glycogen synthase kinase-3 (GSK-3) activation and retinal neuron apoptosis. Methods In an in vitro experiment, the number of apoptotic RGC-5 cells differentiated by staurosporine was evaluated via flow cytometry and nuclei staining using Hoechst 33258. GSK-3 phosphorylation and caspase-3 activation in RGC-5 cells after serum deprivation were determined using Western blotting. Mitochondrial membrane potential was detected using the dye 5,5',6,6'-tetrachloro-1,1',3,3'-tetrethyl benzimidalyl carbocyanine iodide, and reactive oxygen species (ROS) levels were measured with dihydroethidium. In an in vivo experiment, the number of apoptotic retinal neurons was evaluated via terminal transferase dUTP nick-end labeling (TUNEL), and GSK-3 phosphorylation was determined using Western blotting, in the retinal nerve epithelial tissue of rats in which diabetes was induced by intravenous tail-vein injection of streptozotocin for 4 weeks. Results The levels of phosphorylated Ser21/9 in GSK-3 alpha/beta and p-T308/S473-AKT were lower and the cleaved caspase-3 levels were higher in the serum-deprived model (P <0.05). Lithium chloride treatment was associated with a slower rate of apoptosis, increased mitochondrial membrane potential, and decreased ROS levels in differentiated RGC-5 cells (P <0.05). The level of blood glucose and the number of TUNEL-positive cells in the whole-mounted retinas were higher (P <0.01), and the levels of phosphorylated Ser21/9 in GSK-3 alpha/beta and body weight were lower (P <0.05). However, the thickness of the retinal nerve epithelial layer was not significantly less in diabetic rats compared with control group. Lithium chloride intravitreal injection increased the levels of phosphorylated Ser21/9 in GSK-3 alpha/beta and decreased TUNEL-positive cells in the whole-mounted retinas. Conclusion GSK-3 kinase is closely related to retinal neuron apoptosis, and the application of the GSK-3 inhibitor lithium chloride can reduce retinal neuron apoptosis in early diabetic retinopathy.