Retinal Degeneration Caused By Oxidized Low-density Lipoprotein Is Associated With The TLR-4/MyD88/NF-κB Pathway And Polarization Of Microglia In Mices

Background: Age-related macular degeneration（AMD）is the leading cause of visual impairment in individuals over 50 years old around the world,and the incidence of AMD is growing with aging.It is estimated that the number of patients with AMD will reach 300 million worldwide by year 2040 Microglia activation plays an important pathogenic role in the development of central nervous system and retinal diseases.In addition,activated microglia can polarize into two different phenotypes: M1 and M2.Many reports suggest that oxidized low-density lipoprotein（ox-LDL）-induced microglia activation is associated with the inflammatory and pathogenic mechanisms of AMD.In this experiment,we constructed a C57BL/6 mouse model,and an ARPE-19 cell model to investigate the activation of microglia by ox-LDL and its role in the inflammatory response to AMD.Purpose: To verify whether oxidized low-density lipoprotein（ox-LDL）activates microglia and polarizes microglia to M1 phenotype by activating Toll-like receptor-4（TLR-4）pathway,and to examine its effects on retinal function in mice.Methods: Two weeks after subretinal injection of 1μl of ox-LDL in male C57BL/6 mice,activation and polarization of retinal microglia were detected by immunofluorescence and q PCR.Photoreceptor apoptosis was assessed by TUNEL staining,ONL and INL thickness were assessed in vivo with OCT.Retinal function was evaluated with ERG.q PCR and Western blotting detected the expression of TLR-4 and My D88.In vitro,APRE-19 cells were treated with 100 μg/ml of ox-LDL for 24 hours,and the TUNEL staining was used to detect apoptosis,and western blot and q PCR to detect TLR-4 and My D88.Results: In vivo,the expression of TLR-4 and My D88 were upregulated after subretinal injection of ox-LDL,which indicated that TLR-4 receptor was activated by ox-LDL.In addition,ox-LDL elevated the expression of ionized calcium binding adapter molecule 1（Iba-1）,and nitric oxide synthase（i NOS）,indicating microglia were activated and polarized to the M1 phenotype.Meanwhile,TUNEL（+）cells in the ONL layer increased and the thickness of ONL/INL decreased,demonstrating apoptosis of photoreceptors.Finally,both ERG a-wave and b-wave amplitudes decreased confirming ox-LDL caused retinal dysfunction.In vitro,ox-LDL similarly induced the activation of TLR-4 and resulted in the apoptosis of human RPE cells.（4）Conclusions: ox-LDL caused retinal cell damage and dysfunction by activating the TLR-4/My D88 pathway,which consequently caused microglia activation and polarization to the M1 phenotype in mice.