ABSTRACT: Diabetic retinopathy, a microvascular disease characterized by irreparable vascular damage, neurodegeneration and neuroinflammation, is a leading complication of diabetes mellitus. Medical interventions slow the progression of disease. However, current therapies do not specifically target microglia, a cell type implicated in mediating disease development. Microglia-mediated inflammation in the diabetic retina is regulated via CX3CR1-FKN signaling, where FKN serves as a dampening signal for microglial activation. Studying this signaling axis is important as polymorphic variants of CX3CR1 are found in 25% of the human population, hCX3CR1I249/M280, resulting in a receptor with lower binding affinity for FKN. Furthermore, disrupted CX3CR1-FKN signaling in CX3CR1-KO and FKN-KO mice leads to exacerbated microglial activation, robust neuronal cell loss and substantial vascular damage in the diabetic retina. Thus, studies to characterize the effects of hCX3CR1I249/M280-expression in microglia-mediated inflammation in the diseased retina are potentially clinically relevant to identify microglia-specific therapies. Our results show that hCX3CR1I249/M280 mice are significantly more susceptible to microgliosis and production of Cxcl10 and TNFα under acute inflammatory conditions. This pathology is exacerbated under diabetic conditions and coincides with robust neuronal loss in comparison to CX3CR1-WT mice. Therefore, to further investigate the role of hCX3CR1I249/M280-expression in microglial responses, we pharmacologically depleted microglia using PLX-5622, a CSF-1R antagonist. PLX-5622 treatment led to a robust (~70%) reduction in Iba1+ microglia in all non-diabetic and diabetic mice. CSF-1R antagonism in diabetic CX3CR1-WT prevented TUJ1+ axonal loss, angiogenesis and fibrinogen deposition. In contrast, PLX-5622 microglia depletion in CX3CR1-KO and hCX3CR1I249/M280 mice did not alleviate TUJ1+ axonal loss or angiogenesis. Interestingly, PLX-5622 treatment reduced fibrinogen deposition in CX3CR1-KO mice but not in hCX3CR1I249/M280 mice, revealing that hCX3CR1I249/M280 receptor variant mice behave differently in terms of vascular pathology compared to CX3CR1-KOs. mRNAseq gene expression analysis in CX3CR1-WT retinal isolates revealed that PLX-5622-induced microglia depletion and repopulation induced a downregulation in genes associated with microglial activation and phagocytosis, B2m, Cx3cr1, and Trem2, and complement-associated synaptic pruning, C1qa, C1qb, and C1qc. Furthermore, mRNAseq analysis of PLX-5622 treated CX3CR1-WT retinas showed lower fold changes in genes encoding proinflammatory mediators (Cxcl10, Ccl2, Il6, Cxcl1, Selp, Il12b, Tnf, Cxcl2, Icam1 and Vcam1) in comparison to diabetic + normal chow mice.
