The Role of Fractalkine Signaling in Neuroprotection in the Primary Visual Cortex During Diabetes

Diabetic retinopathy (DR) is the leading cause of vision loss in working-aged adults worldwide. The hallmarks of DR include angiogenesis, blood vessel damage, pro-inflammatory cytokine production, and retinal ganglion cell (RGC) neurotoxicity, and the mechanism as to how DR develops remains poorly understood. Microglia, the resident macrophage of the central nervous system (CNS), may play a role in exacerbating inflammation during DR. Microglial activation is regulated by the CX3CR1 receptor which binds its ligand fractalkine (FKN) expressed by neurons in the CNS. Diabetic CX3CR1-KO mice and mice expressing the human variant hCX3CR1I249/M280 were more susceptible to RGC loss, reduction in axonal thickness, leakage of vascular-derived fibrinogen, and microglia activation. Little is known about the role of microglia and the impact of diabetes in the primary visual cortex (PVC), the visual processing center downstream of RGC neurons. The goal of this work is to understand the contribution of microglia to diabetic injury in the PVC. Neurotoxicity in the PVC during DR could cause further vision loss in diabetic patients. To understand how microglia contribute to diabetic injury in the PVC, microglia were pharmacologically depleted in diabetic mice using a colony-stimulating factor receptor 1 (CSF1R) antagonist PLX-5622. Immunofluorescent staining was performed to visualize neuronal cell loss, microglial activation, and microglial phagocytic activity in the PVC. The results of this study demonstrate that in CX3CR1-WT, CX3CR1-KO, and hCX3CR1I249/M280 mice, diabetes causes a decrease in neuronal cell density, morphological changes in microglia to appear more amoeboid, and an increase in microglial CD68 expression indicative of increased phagocytic activity. This was exacerbated in CX3CR1-KO and hCX3CR1I249/M280 mice compared to the wild-type control group. Treatment of diabetic mice with PLX-5622 reduced the level of CD68 expression as well as restored a resting/surveillant morphology in microglia. Loss of neuronal cell density in the diabetic PVC was not rescued by PLX-5622 treatment, suggesting that multiple factors contribute to neurodegeneration.