Abstract
Purpose :
The apolipoprotein E4 (APOE4) allele is associated with an increased risk of Alzheimer’s disease and a decreased risk of glaucoma and age-related macular degeneration, but the underlying mechanisms remain poorly understood. Prior research has demonstrated that Apoe is a critical regulator of the microglial neurodegenerative (MGnD/DAM) molecular signature in disease. The goal of this study was to investigate the effect of human APOE alleles, APOE3 and APOE4, on microglial molecular signature and retinal ganglion cell (RGC) survival in glaucoma in order to test the hypothesis that the APOE4 allele is protective in glaucoma.
Methods :
We investigated the microglial molecular signature in humanized APOE3 and APOE4 mice using the microbead (MB) glaucoma model. Retinal microglia were isolated one month after MB injection as CD11b+/Ly6C-/FCRLS+ cells and underwent Smart-Seq2 RNA sequencing. RGC survival was assessed in MB-injected APOE3 and APOE4 humanized mice using Brn3a+ cell body and PPD axon counts. As Galectin-3 (Lgals3) was found to be one of the strongly differentially expressed genes between APOE3 and APOE4 microglia, RGC survival was also evaluated in MB-injected Lgals3-/- mice and in wildtype mice intravitreally treated with Galectin-3 inhibitor TD139. Human retinal postmortem samples were genotyped for APOE and immunostained for Iba1 and Galectin-3.
Results :
We have found that MB-injected APOE3 animals experience significant RGC loss and their microglia transition to MGnD molecular phenotype, characterized by upregulation of inflammatory cytokines, complement, and Lgals3. In contrast, MB-injected APOE4 animals were protected from RGC degeneration, and their microglia did not upregulate MGnD genes despite IOP elevation. Genetic and pharmacologic targeting of Galectin-3, the downstream effector of Apoe, also ameliorated RGC degeneration. Finally, Galectin-3 was upregulated in microglia in human APOE3 glaucoma samples, and its expression was attenuated in APOE4 glaucomatous retinas.
Conclusions :
In this study we demonstrate that APOE4 impairs the response of neurodegenerative microglia and protects RGCs from glaucomatous degeneration. These results provide an explanation as to why the APOE4 allele is associated with a decreased risk of glaucoma and show that the APOE-Galectin-3 signaling pathway can be targeted to develop novel neuroprotective therapies for this blinding disease.
This abstract was presented at the 2022 ARVO Annual Meeting, held in Denver, CO, May 1-4, 2022, and virtually.