Early Diabetes Induces Changes in Mitochondrial Physiology of Inner Retinal Neurons

Diabetic retinopathy, a leading cause of vision loss, was considered as a solely vascular disorder but some recent studies suggest that retinal neurons may be affected much before the appearance of vascular lesions. However, the cellular processes involved in diabetes-induced degeneration of retinal neurons are poorly understood. Calcium (Ca²⁺) signaling plays a key role in normal functioning of neurons, and its dysregulation may lead to degeneration of neurons. Mitochondria are crucial components involved in the regulation of intracellular Ca²⁺ signaling. In this study, we have investigated the effects of diabetes on Ca²⁺ signaling in retinal neurons. The study was performed in rat retinal neurons cultured in high glucose condition (HGC) for 7-14 days and in acutely prepared retinal slices isolated from diabetic rats. When Ca²⁺ influx was induced by depolarization of neurons with 60 mM KCl in HGC neurons, the Ca²⁺ rise was sustained for a much longer duration as compared to controls, suggesting perturbation of Ca²⁺ buffering. In addition, HGC neurons also showed notably enhanced Ca²⁺ load in the mitochondria, which was accompanied by depolarization of mitochondrial membrane and enhanced reactive oxygen species formation. Similar results were obtained in acutely prepared retinal slices from control and diabetic rats. The depolarization of mitochondrial membrane was more pronounced in the neurons of the inner nuclear layer of diabetic rats. The physiological changes in mitochondria were observed as early as 9 weeks post diabetes induction. Thus, we report here that the intracellular Ca²⁺ signaling and mitochondrial function in retinal neurons are altered at an early stage of diabetes.